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2014-08-28
|
The Gaia-ESO Survey: the chemical structure of the Galactic discs from the first internal data release
|
Most high-resolution spectroscopic studies of the Galactic discs were mostly
confined to objects in the solar vicinity. Here we aim at enlarging the volume
in which individual chemical abundances are used to characterise both discs,
using the first internal data release of the Gaia-ESO survey. We derive and
discuss the abundances of eight elements (Mg, Al, Si, Ca, Ti, Fe, Cr, Ni, and
Y). The trends of these elemental abundances with iron are very similar to
those in the solar neighbourhood. We find a natural division between alpha-rich
and alpha-poor stars, best seen in the bimodality of the [Mg/M] distributions
in bins of metallicity, which we attribute to thick- and thin-disc sequences,
respectively. With the possible exception of Al, the observed dispersion around
the trends is well described by the expected errors, leaving little room for
astrophysical dispersion. Using previously derived distances from Recio-Blanco
et al. (2014b), we further find that the thick-disc is more extended vertically
and is more centrally concentrated towards the inner Galaxy than the thin-disc,
which indicates a shorter scale-length. We derive the radial and vertical
gradients in metallicity, iron, four alpha-element abundances, and Al for the
two populations, taking into account the identified correlation between R_GC
and |Z|. Radial metallicity gradient is found in the thin disc. The positive
radial individual [alpha/M] gradients found are at variance from the gradients
observed in the RAVE survey. The thin disc also hosts a negative vertical
metallicity gradient, accompanied by positive individual [alpha/M] and [Al/M]
gradients. The thick-disc, presents no radial metallicity gradient, a shallower
vertical metallicity gradient than the thin-disc, an alpha-elements-to-iron
radial gradient in the opposite sense than that of the thin disc, and positive
vertical individual [alpha/M] and [Al/M] gradients.
|
1408.6687v1
|
2015-09-17
|
The XMM deep survey in the CDF-S. IX. An X-ray outflow in a luminous obscured quasar at z~1.6
|
In active galactic nuclei (AGN)-galaxy co-evolution models, AGN winds and
outflows are often invoked to explain why super-massive black holes and
galaxies stop growing efficiently at a certain phase of their lives. They are
commonly referred to as the leading actors of feedback processes. Evidence of
ultra-fast (v>0.05c) outflows in the innermost regions of AGN has been
collected in the past decade by sensitive X-ray observations for sizable
samples of AGN, mostly at low redshift. Here we present ultra-deep XMM-Newton
and Chandra spectral data of an obscured (Nh~2x10^{23} cm^-2), intrinsically
luminous (L2-10keV~4x10^{44} erg/s) quasar (named PID352) at z~1.6 (derived
from the X-ray spectral analysis) in the Chandra Deep Field-South. The source
is characterized by an iron emission and absorption line complex at observed
energies of E~2-3 keV. While the emission line is interpreted as being due to
neutral iron (consistent with the presence of cold absorption), the absorption
feature is due to highly ionized iron transitions (FeXXV, FeXXVI) with an
outflowing velocity of 0.14^{+0.02}_{-0.06}c, as derived from photoionization
models. The mass outflow rate - ~2 Msun/yr - is similar to the source accretion
rate, and the derived mechanical energy rate is ~9.5x10^{44} erg/s,
corresponding to 9% of the source bolometric luminosity. PID352 represents one
of the few cases where indications of X-ray outflowing gas have been observed
at high redshift thus far. This wind is powerful enough to provide feedback on
the host galaxy.
|
1509.05413v1
|
2016-12-08
|
The distribution of radioactive $^{44}$Ti in Cassiopeia A
|
The distribution of elements produced in the inner-most layers of a supernova
explosion is a key diagnostic for studying the collapse of massive stars. Here
we present the results of a 2.4 Ms \textit{NuSTAR} observing campaign aimed at
studying the supernova remnant Cassiopeia A (Cas A). We perform
spatially-resolved spectroscopic analyses of the $^{44}$Ti ejecta which we use
to determine the Doppler shift and thus the three-dimensional (3D) velocities
of the $^{44}$Ti ejecta. We find an initial $^{44}$Ti mass of 1.54 $\pm$ 0.21
$\times 10^{-4}$ M$_{\odot}$ which has a present day average momentum direction
of 340$^{\circ}$ $\pm$ 15$^{\circ}$ projected on to the plane of the sky
(measured clockwise from Celestial North) and tilted by 58$^{\circ}$ $\pm$
20$^{\circ}$ into the plane of the sky away from the observer, roughly opposite
to the inferred direction of motion of the central compact object. We find some
$^{44}$Ti ejecta that are clearly interior to the reverse shock and some that
are clearly exterior to the reverse shock. Where we observe $^{44}$Ti ejecta
exterior to the reverse shock we also see shock-heated iron; however, there are
regions where we see iron but do not observe $^{44}$Ti. This suggests that the
local conditions of the supernova shock during explosive nucleosynthesis varied
enough to suppress the production of $^{44}$Ti in some regions by at least a
factor of two, even in regions that are assumed to be the result of processes
like $\alpha$-rich freezeout that should produce both iron and titanium.
|
1612.02774v1
|
2020-06-06
|
EMPRESS. II. Highly Fe-Enriched Metal-poor Galaxies with $\sim 1.0$ (Fe/O)$_\odot$ and $0.02$ (O/H)$_\odot$ : Possible Traces of Super Massive ($>300 M_{\odot}$) Stars in Early Galaxies
|
We present element abundance ratios and ionizing radiation of local young
low-mass (~$10^{6}$ M_sun) extremely metal poor galaxies (EMPGs) with a 2%
solar oxygen abundance (O/H)_sun and a high specific star-formation rate
(sSFR~300 Gyr$^{-1}$), and other (extremely) metal poor galaxies, which are
compiled from Extremely Metal-Poor Representatives Explored by the Subaru
Survey (EMPRESS) and the literature. Weak emission lines such as [FeIII]4658
and HeII4686 are detected in very deep optical spectra of the EMPGs taken with
8m-class telescopes including Keck and Subaru (Kojima et al. 2019, Izotov et
al. 2018), enabling us to derive element abundance ratios with photoionization
models. We find that neon- and argon-to-oxygen ratios are comparable to those
of known local dwarf galaxies, and that the nitrogen-to-oxygen abundance ratios
(N/O) are lower than 20% (N/O)_sun consistent with the low oxygen abundance.
However, the iron-to-oxygen abundance ratios (Fe/O) of the EMPGs are generally
high; the EMPGs with the 2%-solar oxygen abundance show high Fe/O ratios of
~90-140% (Fe/O)_sun, which are unlikely explained by suggested scenarios of
Type Ia supernova iron productions, iron's dust depletion, and metal-poor gas
inflow onto previously metal-riched galaxies with solar abundances. Moreover,
these EMPGs have very high HeII4686/H$\beta$ ratios of ~1/40, which are not
reproduced by existing models of high-mass X-ray binaries whose progenitor
stellar masses are less than 120 M_sun. Comparing stellar-nucleosynthesis and
photoionization models with a comprehensive sample of EMPGs identified by this
and previous EMPG studies, we propose that both the high Fe/O ratios and the
high HeII4686/H$\beta$ ratios are explained by the past existence of super
massive ($>$300 M_sun) stars, which may evolve into intermediate-mass black
holes ($\gtrsim$100 M_sun).
|
2006.03831v2
|
2021-07-02
|
On the use of field RR Lyrae as Galactic probes: IV. New insights into and around the Oosterhoff dichotomy
|
We discuss the largest and most homogeneous spectroscopic dataset of field RR
Lyrae variables (RRLs) available to date. We estimated abundances using both
high-resolution and low-resolution ({\Delta S} method) spectra for fundamental
(RRab) and first overtone (RRc) RRLs. The iron abundances for 7,941 RRLs were
supplemented with similar literature estimates available, ending up with 9,015
RRLs (6,150 RRab, 2,865 RRc). The metallicity distribution shows a mean value
of <[Fe/H]> = -1.51\pm0.01, and {\sigma}(standard deviation)= 0.41 dex with a
long metal-poor tail approaching [Fe/H] = -3 and a sharp metal-rich tail
approaching solar iron abundance. The RRab variables are more metal-rich
(<[Fe/H]>ab = -1.48\pm0.01, {\sigma} = 0.41 dex) than RRc variables (<[Fe/H]>c
= -1.58\pm0.01, {\sigma} = 0.40 dex). The relative fraction of RRab variables
in the Bailey diagram (visual amplitude vs period) located along the
short-period (more metal-rich) and the long-period (more metal-poor) sequences
are 80% and 20\%, while RRc variables display an opposite trend, namely 30\%
and 70\%. We found that the pulsation period of both RRab and RRc variables
steadily decreases when moving from the metal-poor to the metal-rich regime.
The visual amplitude shows the same trend, but RRc amplitudes are almost two
times more sensitive than RRab amplitudes to metallicity. We also investigated
the dependence of the population ratio (Nc/Ntot) of field RRLs on the
metallicity and we found that the distribution is more complex than in globular
clusters. The population ratio steadily increases from ~0.25 to ~0.36 in the
metal-poor regime, it decreases from ~0.36 to ~0.18 for -1.8 < [Fe/H] < -0.9
and it increases to a value of ~0.3 approaching solar iron abundance.
|
2107.00919v1
|
2018-02-13
|
Disentangling superconducting and magnetic orders in NaFe_1-xNi_xAs using muon spin rotation
|
Muon spin rotation and relaxation studies have been performed on a "111"
family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was
characterized by obtaining the temperature and doping dependences of the local
ordered magnetic moment size and the volume fraction of the magnetically
ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long
range magnetically ordered state is observed, while for higher x than 0.4 %
magnetic order becomes more disordered and is completely suppressed for x = 1.5
%. The magnetic volume fraction continuously decreases with increasing x. The
combination of magnetic and superconducting volumes implies that a
spatially-overlapping coexistence of magnetism and superconductivity spans a
large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction
of both the ordered moment size and the volume fraction is observed below the
superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron
pnictides in which one of these two parameters exhibits a reduction below TC,
but not both. The suppression of magnetic order is further enhanced with
increased Ni doping, leading to a reentrant non-magnetic state below T_C for x
= 1.3 %. The reentrant behavior indicates an interplay between
antiferromagnetism and superconductivity involving competition for the same
electrons. These observations are consistent with the sign-changing s-wave
superconducting state, which is expected to appear on the verge of microscopic
coexistence and phase separation with magnetism. We also present a universal
linear relationship between the local ordered moment size and the
antiferromagnetic ordering temperature TN across a variety of iron-based
superconductors. We argue that this linear relationship is consistent with an
itinerant-electron approach, in which Fermi surface nesting drives
antiferromagnetic ordering.
|
1802.04458v1
|
2022-06-24
|
The origin and evolution of the normal Type Ia SN 2018aoz with infant-phase reddening and excess emission
|
SN~2018aoz is a Type Ia SN with a $B$-band plateau and excess emission in the
infant-phase light curves $\lesssim$ 1 day after first light, evidencing an
over-density of surface iron-peak elements as shown in our previous study.
Here, we advance the constraints on the nature and origin of SN~2018aoz based
on its evolution until the nebular phase. Near-peak spectroscopic features show
the SN is intermediate between two subtypes of normal Type Ia: Core-Normal and
Broad-Line. The excess emission could have contributions from the radioactive
decay of surface iron-peak elements as well as ejecta interaction with either
the binary companion or a small torus of circumstellar material. Nebular-phase
limits on H$\alpha$ and He~I favour a white dwarf companion, consistent with
the small companion size constrained by the low early SN luminosity, while the
absence of [O~I] and He~I disfavours a violent merger of the progenitor. Of the
two main explosion mechanisms proposed to explain the distribution of surface
iron-peak elements in SN~2018aoz, the asymmetric Chandrasekhar-mass explosion
is less consistent with the progenitor constraints and the observed blueshifts
of nebular-phase [Fe~II] and [Ni~II]. The helium-shell double-detonation
explosion is compatible with the observed lack of C spectral features, but
current 1-D models are incompatible with the infant-phase excess emission,
$B_{\rm max}-V_{\rm max}$ color, and absence of nebular-phase [Ca~II]. Although
the explosion processes of SN~2018aoz still need to be more precisely
understood, the same processes could produce a significant fraction of Type Ia
SNe that appear normal after $\sim$ 1 day.
|
2206.12437v1
|
2022-10-31
|
A JWST Near- and Mid-Infrared Nebular Spectrum of the Type Ia Supernova 2021aefx
|
We present JWST near- and mid-infrared spectroscopic observations of the
nearby normal Type Ia supernova SN 2021aefx in the nebular phase at $+255$ days
past maximum light. Our Near Infrared Spectrograph (NIRSpec) and Mid Infrared
Instrument (MIRI) observations, combined with ground-based optical data from
the South African Large Telescope (SALT), constitute the first complete optical
$+$ NIR $+$ MIR nebular SN Ia spectrum covering 0.3$-$14 $\mu$m. This spectrum
unveils the previously unobserved 2.5$-$5 $\mu$m region, revealing strong
nebular iron and stable nickel emission, indicative of high-density burning
that can constrain the progenitor mass. The data show a significant improvement
in sensitivity and resolution compared to previous Spitzer MIR data. We
identify numerous NIR and MIR nebular emission lines from iron-group elements
and as well as lines from the intermediate-mass element argon. The argon lines
extend to higher velocities than the iron-group elements, suggesting stratified
ejecta that are a hallmark of delayed-detonation or double-detonation SN Ia
models. We present fits to simple geometric line profiles to features beyond
1.2 $\mu$m and find that most lines are consistent with Gaussian or spherical
emission distributions, while the [Ar III] 8.99 $\mu$m line has a distinctively
flat-topped profile indicating a thick spherical shell of emission. Using our
line profile fits, we investigate the emissivity structure of SN 2021aefx and
measure kinematic properties. Continued observations of SN 2021aefx and other
SNe Ia with JWST will be transformative to the study of SN Ia composition,
ionization structure, density, and temperature, and will provide important
constraints on SN Ia progenitor and explosion models.
|
2211.00038v2
|
1995-10-11
|
Is the X-ray spectrum of the Seyfert 2 galaxy NGC5252 intrinsically flat?
|
The first X-ray observation of the Seyfert 2 galaxy NGC5252 is reported. ASCA
collected $\sim 4000$ photons/detector enough to perform an accurate spectral
analysis of this source. The luminosity of NGC5252 is L$_{\rm X}$(0.7-10 keV)
$\simeq 2.6 \times 10^{43}$ erg s$^{-1}$, typical of a Seyfert 1 galaxy. A
simple description of the spectrum with a single power law is ruled out by the
SIS data which shows a strong soft excess at E $\lsimeq$ 1.2 keV. The spectrum
is best fitted by models assuming either partial covering of the central source
or scattering of the X-ray continuum. The best-fit partial covering model
results in a flat ($\Gamma \simeq 1.45 \pm 0.2$) power law continuum emitted by
a source almost completely covered (at $\sim$ 94-97\%) by neutral matter
($N_{\rm H} \simeq (4.3 \pm 0.6) \times 10^{22}$ cm$^{-2}$). The detected iron
line is remarkably weaker (EW $\sim 90 \pm 60$ eV) than normally found from
Seyfert 2 galaxies. In an effort to interpret the observed flat spectrum with
an intrinsically steep power law as predicted by unified models, we also
included in the models neutral reflection, ionized absorption (warm absorber),
and non-uniform cold absorption (dual-absorber). We find that the observed flat
continum {\bf and} weakness of the iron line poses a problem for neutral
reflection models, whatever the assumed geometry is. The use of an ionized
absorber (scattering + warm-absorber model) instead of a neutral absorber seems
not justified with the present data. The ``dual-absorber'' model, representing
nonuniform (density) absorption along the line of sight, may provide an
alternative explanation for the flatness of the spectrum, with no constraints
on the Fe K emission line. Another plausible interpretation of the present data
is that the X-ray spectrum of NGC5252 is truly intrinsically flat. Its strong
|
9510064v1
|
1996-04-10
|
The Hubble constant from $^{56}$Co-powered Nebular Candles
|
Type Ia supernovae (SNe Ia), produced by the thermonuclear explosion of white
dwarf (WD) stars, are used here to derive extragalactic distances and an
estimate of the Hubble constant from their emission signatures at late phases
({\it Nebular SNe Ia Method}, NSM). The method, first developed in
Ruiz--Lapuente \& Lucy (1992), makes use here of an improved modeling of the
forbidden line emission at late phases. Hydrodynamic models of the explosion of
WDs of different masses, both sub--Chandrasekhar and Chandrasekhar, provide the
basis for comparison with the observations. It is shown that it is possible to
probe the overall density structure of the ejecta and the mass of the exploding
WD by the effect that the electron density profile has in shaping the forbidden
line emission of the iron ions, and that a robust diagnostic of the mass of the
exploding WD can be obtained. Cosmic distance scale can thus be related to
basic diagnostics of excitation of iron lines. Once the most adequate model is
selected, comparison of the predicted line emission at these phases with the
observed spectra gives an internal estimate of both the reddening and the
distance to the SNe Ia. The results presented here favor denser models than
those corresponding to sub--Chandrasekhar explosions. From a sample of seven
SNe Ia in Leo, Virgo, Fornax and beyond, a value of the Hubble constant $H_{0}
= 68 \ \pm 6\ (stat) \pm 7\ (syst)\ km\ s^{-1}\ Mpc^{-1}$ is derived. The depth
of the Virgo cluster is found to be large, ranging from 13 to 23 Mpc at least.
If NGC 4526 traces well the core of the Virgo Cluster, then the latter is
located at $16\pm 2 \ Mpc$. The galaxy NGC 3267 in Leo appears to be located at
9.8 $\pm$ 1.5 Mpc.
|
9604044v1
|
1996-06-27
|
ASCA observations of Seyfert 1 galaxies: II. Relativistic Iron K-alpha emission
|
We present evidence for widespread relativistic effects in the central
regions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxies
observed by \asca, 14 show an iron K$\alpha$ line which is is resolved, with
mean width $\sigma_{\rm K\alpha}=0.43\pm 0.12$~keV for a gaussian profile (Full
Width at Half Maximum, FWHM$\sim 50,000$~km s$^{-1}$). However, many of the
line profiles are asymmetric. A strong red wing is indicative of gravitational
redshifts close to a central black hole and accretion disk models provide an
excellent description of the data. The peak energy of the line is 6.4~keV,
indicating that it arises by fluorescence in near-neutral material. Our fits
imply a low inclination for the disk in these Seyfert 1 galaxies, with a mean
of 30\deg, consistent with orientation-dependent unification schemes.
Differences in the line profiles from source-to-source imply slight variations
in geometry, which cannot be accounted for solely by inclination. In most
cases, we require that the line emission arises from a range of radii. Although
a small contribution to the emission from a region other than the disk is not
ruled out, it is not generally required and has little effect on our
conclusions regarding the disk line. Our data are fit equally well with
rotating (Kerr) and non-rotating (Schwarzschild) black hole models. We find a
mean spectral index in the 3-10 keV range of $<\Gamma_{3-10}>=1.91\pm 0.07$
after accounting for the effects of reflection. Such observations probe the
innermost regions of AGN, and arguably provide the best evidence yet obtained
for the existence of super-massive black holes in the centers of active
galaxies.
|
9606169v2
|
1998-04-27
|
A Strange Star Model for Pulsars
|
It is suggested in this paper that the `bare' strange star might be not bare,
and there could be a magnetosphere around it. As a strange star might be an
intensely magnetized rotator, the induced unipolar electric field would be
large enough to construct a magnetosphere around the strange matter core.
This kind of magnetosphere is very similar to that of the rotating magnetized
neutron stars discussed by many authors. A magnetosphere will be established
very soon through pair production by gamma-B or two photon processes after a
strange star was born in a supernova explosion. It is emphasized that the fact
that the strange star surface can not supply charged particles does not stop
the formation of a space charge separated magnetosphere around the bare strange
star. An accretion crust is quite difficult to come into being around an
isolated strange star. Therefore the observed radio signals of an
rotation-powered pulsar may come from a bare strange stars rather than a
neutron stars or a strange star with an accretion crust. The idea, that the
radio pulsars are the strange stars without crusts, is supported by some
observations. The fact, that the iron emission lines have been observed in many
X-ray pulsars but never been reported in X-ray emission of radio pulsars, is
difficult to be understood if the radio pulsars are the neutron stars or
strange star with crust where the surface binding energy of iron ions is too
low to avoid a ion free-flow from the surface.
|
9804278v1
|
1998-10-07
|
Stellar Metallicities and SNIa Rates in the Early-type Galaxy NGC5846 from ROSAT and ASCA Observations
|
In this paper we analyze the diffuse X-ray coronae surrounding the elliptical
galaxy NGC5846, combining measurements from two observatories, ROSAT and ASCA.
We map the gas temperature distribution and find a central cool region within
an approximately isothermal gas halo extending to a radius of about 50 kpc, and
evidence for a temperature decrease at larger radii. With a radially falling
temperature profile, the total mass converges to 9.6+/-1.0 10^12 Msun at ~230
kpc radius. Using the spectroscopic measurements, we also derive radial
distributions for the heavy elements silicon and iron and find that the
abundances of both decrease with galaxy radius. The mass ratio of Si to Fe lies
between the theoretical predictions for element production in SN Ia and SN II,
suggesting an important role for SN Ia, as well as SN II, for gas enrichment in
ellipticals. Using the SN Ia yield of Si, we set an upper limit of 0.012 SNU
for the SN Ia rate at radii >50 kpc, which is independent of possible
uncertainties in the iron L-shell modeling. We compare our observations with
the theoretical predictions for the chemical evolution of ellipticals, taken
from Matteucci & Gibson (1995). We conclude that the metal content in stars, if
explained by the star formation duration, requires a significant decline in the
duration of star formation with galaxy radius, ranging from ~1 Gyr at the
center to ~0.01 Gyr at 100 kpc radius. Alternatively, the decline in
metallicity with galaxy radius may be caused by a similar drop with radius in
the efficiency of star formation. Based on the Si and Fe measurements presented
in this paper, we conclude that the latter scenario is preferred, unless a
dependence of the SN Ia rate on stellar metallicity is invoked. (Abridged).
|
9810107v1
|
1999-02-25
|
Chemical Evolution on the Scale of Clusters of Galaxies, and Beyond
|
Clusters of galaxies allow a direct estimate of the metallicity and metal
production yield on the largest scale so far. The ratio of the total iron mass
in the ICM to the total optical luminosity of the cluster (the iron
mass-to-light-ratio) is the same for all clusters which ICM is hotter than
$\sim 2$ keV, and the elemental proportions (i.e. the [$\alpha$/Fe] ratio)
appear to be solar. From these evidences it is argued that both the IMF as well
the relative contributions of SN types are likely to be universal. Constraints
on the past SN activity in galaxy clusters are then derived, and support is
given to the notion that the average SNIa rate was much higher in the past,
i.e. at least 10 times more than currently observed in local ellopticals. It is
also argued that cluster metallicity ($\sim 1/3$ solar) should be taken as
representative of the low-z universe as a whole. There is now compelling
evidence that the bulk of stars in cluster as well as in field ellipticals and
bulges formed at high redshifts ($z\gsim 3$). Since such stars account for at
least $\sim 30%$ of the baryons now locked into stars, it is argued that at
least 30% of stars and metals formed before $z\simeq 3$. As a consequence, the
metallicity of the universe at z=3 is predicted to be $\sim 1/10$ solar. This
requires the cosmic star formation rate to run at least flat from $z\sim 1$ to
$\sim 5$, which appears to agree with the most recent derect determinations of
the star formation rate in Lyman-break galaxies.
|
9902361v2
|
1999-04-12
|
Presupernova Evolution of Rotating Massive Stars I: Numerical Method and Evolution of the Internal Stellar Structure
|
The evolution of rotating stars with zero-age main sequence (ZAMS) masses in
the range 8 to 25 M_sun is followed through all stages of stable evolution. The
initial angular momentum is chosen such that the star's equatorial rotational
velocity on the ZAMS ranges from zero to ~ 70 % of break-up. Redistribution of
angular momentum and chemical species are then followed as a consequence of
rotationally induced circulation and instablities. The effects of the
centrifugal force on the stellar structure are included. Uncertain mixing
efficiencies are gauged by observations. We find, as noted in previous work,
that rotation increases the helium core masses and enriches the stellar
envelopes with products of hydrogen burning. We determine, for the first time,
the angular momentum distribution in typical presupernova stars along with
their detailed chemical structure. Angular momentum loss due to (non-magnetic)
stellar winds and the redistribution of angular momentum during core hydrogen
burning are of crucial importance for the specific angular momentum of the
core. Neglecting magnetic fields, we find angular momentum transport from the
core to the envelope to be unimportant after core helium burning. We obtain
specific angular momenta for the iron core and overlaying material of
1E16...1E17 erg s. These values are insensitive to the initial angular
momentum. They are small enough to avoid triaxial deformations of the iron core
before it collapses, but could lead to neutron stars which rotate close to
break-up. They are also in the range required for the collapsar model of
gamma-ray bursts. The apparent discrepancy with the measured rotation rates of
young pulsars is discussed.
|
9904132v2
|
1999-07-12
|
The Steep Spectrum Quasar PG1404+226 with ASCA, HST and ROSAT
|
(abridged) We present and discuss our observations of the NL quasar
PG1404+226 with ASCA and HST, and a re-analysis of our earlier observations
with ROSAT. The soft X-ray spectrum is very steep and displays an absorption
feature (edge or line at ~1.1 keV). We have applied a variety of models to the
ASCA and ROSAT spectra without finding a completely satisfactory fit, and the
identification of the edge remains uncertain. A satisfactory fit of the ASCA
spectrum assuming that the edge is produced by highly ionized iron (using the
code absori in XSPEC) is obtained with an overabundance of iron by a factor >
25 compared to solar, a suggestion supported by the extremely high equivalent
width of the Fe K_alpha line at 6.4 keV. A warm absorber model fitting the
absorption feature with NeVII-NeX edges and assuming a peculiar oxygen/neon
abundance ratio is consistent with the ROSAT data but not the ASCA data.
Finally, it is also possible that the observed edge is caused by a OVIII or
OVII edge or line, blueshifted by z_abs=0.2 to 0.5 depending on the specific
identification, as has been suggested previously for 2 other NL quasars, but
there are no other features in the UV and X-ray spectra in support of this
suggestion. Two systems of UV absorption lines, one nearly at rest in the
source frame, the other blueshifted by ~1900 km/s are identified in the HST/FOS
spectra.
Photoionization models indicate that the UV absorption and the ~1 keV
absorption are probably caused by absorbers with different physical conditions.
PG1404+226 is one more case of AGN where both UV and X-ray absorption features
are detected, thereby increasing further the significance of the previously
noted statistical association of the two types of absorbers.
|
9907150v1
|
1999-10-11
|
ASCA and ROSAT observations of nearby cluster cooling flows
|
We present a detailed analysis of the X-ray properties of the cooling flows
in a sample of nearby, X-ray bright clusters of galaxies using high-quality
ASCA spectra and ROSAT X-ray images. We demonstrate the need for multiphase
models to consistently explain the spectral and imaging X-ray data for the
clusters. The mass deposition rates of the cooling flows, independently
determined from the ASCA spectra and ROSAT images, exhibit reasonable
agreement. We confirm the presence of intrinsic X-ray absorption in the
clusters using a variety of spectral models. We also report detections of
extended $100\mu$m infrared emission, spatially coincident with the cooling
flows, in several of the systems studied. The observed infrared fluxes and flux
limits are in good agreement with the predicted values due to reprocessed X-ray
emission from the cooling flows. We present precise measurements of the
abundances of iron, magnesium, silicon and sulphur in the central regions of
the Virgo and Centaurus clusters. Our results firmly favour models in which a
high mass fraction (70-80 per cent) of the iron in the X-ray gas in these
regions is due to Type Ia supernovae. Finally, we present a series of methods
which may be used to measure the ages of cooling flows from the X-ray data. The
results for the present sample of clusters indicate ages of between 2.5 and 7
Gyr. If the ages of cooling flows are primarily set by subcluster merger
events, then our results suggest that in the largest clusters, mergers with
subclusters with masses of approximately 30 per cent of the final cluster mass
are likely to disrupt cooling flows.
|
9910188v2
|
1999-11-03
|
Hard X-ray Emission From Low Mass X-ray Binaries
|
We report on Rossi X-ray Timing Explorer observations of four type I X-ray
bursters; namely 1E1724-3045, GS1826-238, SLX1735-269 and KS1731-260. The first
three were in a low state (LS) whereas KS1731-260 was in a high state. The LS
sources have very similar power spectra, displaying high frequency noise up to
\~200 Hz. For KS1731-260, its power spectrum is dominated by noise at
frequencies below 20 Hz. In addition a quasi-periodic oscillation at 1200 Hz is
detected. The 1-200 keV LS spectra are all consistent with resulting from
thermal Comptonization with an electron temperature (kTe) around 25-30 keV. For
KS1731-260, the spectrum is also dominated by Comptonization, but with kTe
around 3 keV and no significant hard X-ray emission. With the exception of
GS1826-238, there is an underlying soft component. For all sources, we have
detected an iron line at 6.4 keV. A reflection component is present in the
spectra of GS1826-238 and SLX1735-269. We suggest a model in which the region
of main energy release, where hard X-rays are produced would be an optically
thin boundary layer merged with an Advection Dominated Accretion Flow (ADAF).
The soft component observed probably represents the unscattered emission from
the accretion disk of variable inner radius. When the accretion rate increases,
the inner disk radius shrinks, the strength of the reflected component and
associated iron line increase, and the Comptonization region cools off in
response to an increased cooling flux from the accretion disk and from the
reprocessed/reflected component. Finally, in the light of these observations,
we discuss extensively the various criteria recently proposed to distinguish
between non-quiescent accreting black holes and neutron stars.
|
9911042v1
|
2000-02-10
|
Atomic data from the Iron Project.XLIII. Transition probabilities for Fe V
|
An extensive set of dipole-allowed, intercombination, and forbidden
transition probabilities for Fe V is presented. The Breit-Pauli R-matrix (BPRM)
method is used to calculate 1.46 x 10^6 oscillator strengths for the allowed
and intercombination E1 transitions among 3,865 fine-structure levels dominated
by configuration complexes with n <= 10 and l <= 9. These data are complemented
by an atomic structure configuration interaction (CI) calculation using the
SUPERSTRUCTURE program for 362 relativistic quadrupole (E2) and magnetic dipole
(M1) transitions among 65 low-lying levels dominated by the 3d^4 and 3d^ 4s
configurations. Procedures have been developed for the identification of the
large number of fine-structure levels and transitions obtained through the BPRM
calculations. The target ion Fe VI is represented by an eigenfunction expansion
of 19 fine-structure levels of 3d^3 and a set of correlation configurations. Fe
V bound levels are obtained with angular and spin symmetries SL\pi and J\pi of
the (e + Fe VI) system such that 2S+1 = 5,3,1, L <= 10, J <= 8 of even and odd
parities. The completeness of the calculated dataset is verified in terms of
all possible bound levels belonging to relevant LS terms and transitions in
correspondence with the LS terms. The fine-structure averaged relativistic
values are compared with previous Opacity Project LS coupling data and other
works. The 362 forbidden transition probabilities considerably extend the
available data for the E2 and M1 transtions, and are in good agreement with
those computed by Garstang for the 3d^4 transitions.
|
0002230v1
|
2000-03-03
|
J-Band Infrared Spectroscopy of a Sample of Brown Dwarfs Using Nirspec on Keck II
|
Near-infrared spectroscopic observations of a sample of very cool, low-mass
objects are presented with higher spectral resolution than in any previous
studies. Six of the objects are L-dwarfs, ranging in spectral class from L2 to
L8/9, and the seventh is a methane or T-dwarf. These new observations were
obtained during commissioning of NIRSPEC, the first high-resolution
near-infrared cryogenic spectrograph for the Keck II 10-meter telescope on
Mauna Kea, Hawaii. Spectra with a resolving power of R=2500 from 1.135 to 1.360
microns (approximately J-band) are presented for each source. At this
resolution, a rich spectral structure is revealed, much of which is due to
blending of unresolved molecular transitions. Strong lines due to neutral
potassium (K I), and bands due to iron hydride (FeH) and steam (H2O) change
significantly throughout the L sequence. Iron hydride disappears between L5 and
L8, the steam bands deepen and the K I lines gradually become weaker but wider
due to pressure broadening. An unidentified feature occurs at 1.22 microns
which has a temperature dependence like FeH but has no counterpart in the
available FeH opacity data. Because these objects are 3-6 magnitudes brighter
in the near-infrared compared to the I-band, spectral classification is
efficient. One of the objects studied (2MASSW J1523+3014) is the coolest
L-dwarf discovered so far by the 2-Micron All-Sky Survey (2MASS), but its
spectrum is still significantly different from the methane-dominated objects
such as Gl229B or SDSS 1624+0029.
|
0003035v1
|
2000-03-06
|
X-ray Spectra of a large sample of Quasars with ASCA
|
The results from an X-ray spectral analysis of a large sample of quasars,
observed with ASCA, are presented. Data reduction leaves 62 quasars, 35
radio-loud and 27 radio-quiet, suitable for spectral analysis. Differences are
found between the radio-quiet quasars (RQQs) and the radio-loud quasars (RLQs),
in agreement with previous studies. A correlation between Gamma and optical
H-beta was also found for the radio-quiet quasars in this sample, whereby the
steepest X-ray spectra tend to be found in those objects with narrow H-beta
widths. The correlation is significant at >99% confidence, confirming the
well-known trend between Gamma and H-beta FWHM in Seyfert 1s (Brandt et al.
1997), but at higher luminosities. Other spectral complexities are observed
from this sample. A soft X-ray excess, with blackbody temperatures in the range
100 - 300 eV, is seen in many low z radio-quiet quasars. In most cases the
temperatures are probably too hot to originate directly from the disk. Iron K
line emission features are also found in the RQQs; but often from partially
ionised material. Indeed in the highest luminosity RQQs there is neither
evidence for iron line emission nor the reflection component expected from disk
models. These observations can be explained by an increase in the quasar
accretion rate with luminosity, leading to an increase in the ionisation state
of the surface layers of the disk. The occurrence of ionised or `warm'
absorbers is rare in this sample, with only 5 detections. However excess
neutral X-ray absorption is found towards several of the high z, predominantly
radio-loud, quasars. Although found to increase with quasar redshift, this
`intrinsic' absorption may be associated with radio-loud AGN.
|
0003080v1
|
2000-03-17
|
ASCA Observation of the New Transient X-ray Pulsar XTE J0111.2-7317 in the Small Magellanic Cloud
|
The new transient X-ray pulsar XTE J0111.2-7317 was observed with Advanced
Satellite for Cosmology and Astrophysics (ASCA) on 1998 November 18, a few days
after its discovery with the Proportional Counter Array onboard the Rossi X-ray
Timing Explorer. The source was detected at a flux level of 3.6x10^-10 erg
cm^-2 s^-1 in the 0.7--10.0 keV band, which corresponds to the X-ray luminosity
of 1.8x10^38 erg s^-1, if a distance of 65 kpc for this pulsar in the Small
Magellanic Cloud is assumed. Nearly sinusoidal pulsations with a period of
30.9497 +/- 0.0004 s were unambiguously detected during the ASCA observation.
The pulsed fraction is low and slightly energy dependent with average value of
\~27%. The energy spectrum shows a large soft excess below ~2 keV when fitted
to a simple power-law type model. The soft excess is eliminated if the spectrum
is fitted to an ``inversely broken power-law'' model, in which photon indices
below and above a break energy of 1.5 keV are 2.3 and 0.8, respectively. The
soft excess can also be described by a blackbody or a thermal bremsstrahlung
when the spectrum above ~2 keV is modeled by a power-law. In these models,
however, the thermal soft component requires a very large emission zone, and
hence it is difficult to explain the observed pulsations at energies below 2
keV. A bright state of the source enables us to identify a weak iron line
feature at 6.4 keV with an equivalent width of 50 +/- 14 eV. Pulse phase
resolved spectroscopy revealed a slight hardening of the spectrum and marginal
indication of an increase in the iron line strength during the pulse maximum.
|
0003251v1
|
2000-05-31
|
Accretion disk models and their X-ray reflection signatures. I. Local spectra
|
X-ray illumination of accretion disks is an invaluable diagnostic of the
structure of these disks because of the associated iron K$\alpha$ emission.
Here we point out that the resulting reflected spectra depend very sensitively
on the geometry of the X-ray source, and that this fact can be efficiently used
to test these models observationally. In particular, we discuss three different
accretion disk geometries: the ``lamppost model'', accretion disks with
magnetic flares, and the model with a full corona overlying a cold thin disk.
We show that in the case of the lamppost model, unless the X-ray luminosity of
the central source is larger than that of the cold disk by a factor of 10 or
more, a significant fraction of iron in the ionized skin of the disk is in the
hydrogen and helium-like ions. Because these ions have large fluorescence
yields, the resulting reflected spectra look strongly ionized, with Equivalent
Width (EW) of the line {\em increasing} with X-ray luminosity $L_x$ up to the
maximum of $\sim 500$ eV. This situation contrasts to the magnetic flare model,
where the large X-ray flux near flares completely ionizes the skin of the disk
and thus the resulting spectra appear to be that from a neutral material. The
line EW in this model {\em anti-correlates} with X-ray luminosity, and becomes
arbitrarily small when $L_x$ is a good fraction of the Eddington luminosity.
Finally, in the full corona case, due to the additional pressure and weight of
the corona, the gas pressure (and its density) below the corona is always large
enough to make the gas very cool and effectively neutral. No highly ionized
skin forms in such a model. If the corona is Thomson thin, then EW of the line
does not depend on the accretion disk or corona luminosities for the full
corona model.
|
0005597v1
|
2000-06-23
|
The Origin of the X-ray and Ultraviolet Emission in NGC 7469
|
We present a spectral analysis of a 30d, near-continuous observation of the
Seyfert 1 galaxy NGC 7469 with RXTE. Daily integrations show strong spectral
changes during the observation. Ourmain result is that we find the X-ray
spectral index to be correlated with the UV flux. Furthermore, the broad-band
X-ray photon flux is also correlated with the UV continuum. These correlations
point towards a model in which the X-rays originate via thermal Comptonization
of UV seed photons. Furthermore, the UV is also correlated with the
extrapolation of the X-ray power law into the soft X-ray/EUV region. Our data
analysis therefore re-opens the possibility the the UV photons and their
variability arise from reprocessing, as long as the primary source of heating
is photo-electric absorption in the reprocessor, rather than Compton
downscattering, A coherent picture of the X-ray/UV variability can therefore be
constructed whereby absorption and reprocessing of EUV/soft X-rays in a
standard accretion disk produce a variable seed photon distribution which are
in turn up-scattered into the X-ray band. We also find a significant
correlation between the 2-10 keV flux and the 6.4 keV iron K-line suggesting
that at least some portion of the line originates within ~1 lt day of the X-ray
continuum source. Neither the power law photon index nor the Compton reflection
component are correlated with the 2-10 keV flux. The latter is not correlated
with the iron K line flux either. We do find an apparent correlation between
the X-ray spectral index and the strength of the Compton reflection component.
In an Appendix we show, however, that this can be produced by a combination of
statistical and systematic errors. We conclude the apparent variations in the
Compton reflection component may be an artifact of these effects.
|
0006339v1
|
2000-09-12
|
The Composite Broad Band Spectrum of Cir X-1 at the Periastron: a Peculiar Z-source
|
We report on the spectral analysis of the peculiar source Cir X-1 observed by
the BeppoSAX satellite when the X-ray source was near the periastron. A flare
lasting $\sim 6 \times 10^3$ s is present at the beginning of the observation.
The luminosity during the persistent emission is $1 \times 10^{38}$ erg
s$^{-1}$, while during the flare is $2 \times 10^{38}$ erg s$^{-1}$. We
produced broad band (0.1-100 keV) energy spectra during the flare and the
persistent emission. At low energies the continuum is well fitted by a model
consisting of Comptonization of soft photons, with a temperature of $\sim 0.4$
keV, by electrons at a temperature of $\sim 1$ keV. Out of the flare a
power-law component, with photon index $\sim 3$, is dominant at energies higher
than 10 keV. This component contributes to $\sim 4%$ of the total luminosity.
During the flare its addition is not statistically significant. An absorption
edge at $\sim 8.4$ keV, with optical depth $\sim 1$, corresponding to the
K-edge of Fe XXIII-XXV, and an iron emission line at 6.7 keV are also present.
The iron line energy is in agreement with the ionization level of the
absorption edge. The hydrogen column deduced from the absorption edge is $\sim
10^{24}$ cm$^{-2}$, two order of magnitude larger than the absorption measured
in this source. We calculated the radius of the region originating the
comptonized seed photons, $R_W \sim 150$ km.
We propose a scenario where $R_W$ is the inner disk radius, a highly ionized
torus surrounds the accretion disk and a magnetosphere is large up to $R_W$.
The absorption edge and the emission line could originate in the photoionized
torus, while the comptonized component originates in an inner region of the
disk.
|
0009183v1
|
2000-10-26
|
A High Resolution Method for Measuring Cosmic Ray Composition beyond 10 TeV
|
The accurate determination of the elemental composition of cosmic rays at
high energies is expected to provide crucial clues on the origin of these
particles. Previous direct measurements of composition have been limited by
experiment collecting power, resulting in marginal statistics above $10^{14}$
eV, precisely the region where the ``knee'' of the cosmic-ray energy spectrum
is starting to develop. In contrast, indirect measurements using extensive air
showers can produce sufficient statistics in this region but generate elemental
measurements which have relatively large uncertainties. Here we discuss a
technique which has become possible through the use of modern ground-based
Cerenkov imaging detectors. We combine a measurement of the Cerenkov light
produced by the incoming cosmic-ray nucleus in the upper atmosphere with an
estimate of the total nucleus energy produced by the extensive air shower
initiated when the particle interacts deeper in the atmosphere. The emission
regions prior to and after the first hadronic interaction can be separated by
an imaging Cerenkov system with sufficient angular and temporal resolution.
Monte Carlo simulations indicate an expected charge resolution of $\Delta Z/Z
<5%$ for incident iron nuclei in the region of the ``knee'' of the cosmic-ray
energy spectrum. This technique also has the intriguing possibility to
unambiguously discover nuclei heavier than iron at energies above 10$^{14}$ eV.
The identification and rejection of background produced by charged particles in
ground based gamma-ray telescopes is also discussed.
|
0010554v2
|
2000-11-29
|
Variable X-ray Absorption in the Seyfert 2 Galaxy Mrk 348
|
We present RXTE monitoring observations of the Seyfert 2 galaxy Mrk 348
spanning a 6 month period. The time-averaged spectrum in the 3-20 keV band
shows many features characteristic of a Compton-thin Seyfert 2 galaxy, namely a
hard underlying power-law continuum (photon index = 1.8) with heavy soft X-ray
absorption (N_h ~ 10^23 cm^-2) plus measureable iron line emission (equivalent
width ~ 100 eV) and, at high energy, evidence for a reflection component (R <
1). During the first half of the monitoring period the X-ray continuum flux
from Mrk 348 remained relatively steady. However this was followed by a
significant brightening of the source (by roughly a factor of 4) with the
fastest change corresponding to a doubling of its X-ray flux on a timescale of
about 20 days. The flux increase was accompanied by a marked softening of X-ray
spectrum most likely attributable to a factor 3 decline in the intrinsic
line-of-sight column density. In contrast the iron line and the reflection
components showed no evidence of variability. These observations suggest a
scenario in which the central X-ray source is surrounded by a patchy
distribution of absorbing material located within about a light-week of the
nucleus of Mrk 348. The random movement of individual clouds within the
absorbing screen, across our line of sight, produces substantial temporal
variations in the measured column density on timescales of weeks to months and
gives rise to the observed X-ray spectral variability. However, as viewed from
the nucleus the global coverage and typical thickness of the cloud layer
remains relatively constant.
|
0011542v1
|
2001-01-22
|
Spectral Evolution of the Continuum and Disc Line in Dipping in GRO J1655-40
|
The discovery is reported of emission features in the X-ray spectrum of GRO
J1655-40 obtained using Rossi-XTE on 1997, Feb 26. The features have been
fitted firstly by two Gaussian lines, which in four spectra have average
energies of 5.85+/-0.08 keV and 7.32+/-0.13 keV, strongly suggestive that these
are the red- and blueshifted wings of an iron disc line from material with
velocity ~0.33 c. The blue wing is apparently less bright than expected for a
disc line subject to Doppler boosting, however, known absorption in the
spectrum of GRO J1655-40 at energies between ~7 and 8 keV can reduce the
apparent brightness of the blue wing. The spectra have also been fitted well
using the full relativistic disc line model of Laor, plus an absorption line.
This gives a restframe energy between 6.4 and 6.8 keV indicating that the line
is from highly ionized iron K_alpha. The Laor model also shows that the line
originates at radii extending from ~10 Schwarzschild radii (r_S) outwards. The
line is direct evidence for the black hole nature of the compact object. The
continuum is well described by dominant disc blackbody emission plus
Comptonized emission. During dipping, spectral evolution is well modelled by
allowing progressive covering of the disc blackbody and simple absorption of
the Comptonized emission showing that the thermal emission is more extended.
Acceptable fits are only obtained by including the disc line in the covering
term, indicating that it originates in the same inner disc region as the
thermal continuum. Dip ingress times and durations are used to provide the
radius of the disc blackbody emitter as 170-370 r_S, and the radius of the
absorber.
|
0101388v1
|
2001-05-18
|
The CHANDRA HETGS X-ray Grating Spectrum of Eta Car
|
Eta Car may be the most massive and luminous star in the Galaxy and is
suspected to be a massive, colliding wind binary system. The CHANDRA X-ray
observatory has obtained a calibrated, high-resolution X-ray spectrum of the
star uncontaminated by the nearby extended soft X-ray emisssion. Our 89 ksec
CHANDRA observation with the High Energy Transmission Grating Spectrometer
(HETGS) shows that the hot gas near the star is non-isothermal. The temperature
distribution may represent the emission on either side of the colliding wind
bow shock, effectively ``resolving'' the shock. If so, the pre-shock wind
velocities are ~ 700 and ~ 1800 km/s in our analysis, and these velocities may
be interpreted as the terminal velocities of the winds from Eta Car and from
the hidden companion star. The forbidden-to-intercombination (f/i) line ratios
for the He-like ions of S, Si and Fe are large, indicating that the line
forming region lies far from the stellar photosphere. The iron fluorescent line
at 1.93 Angstrom, first detected by ASCA, is clearly resolved from the thermal
iron line in the CHANDRA grating spectrum. The Fe fluorescent line is weaker in
our CHANDRA observation than in any of the ASCA spectra. The CHANDRA
observation also provides an uninterrupted high-time resolution lightcurve of
the stellar X-ray emission from Eta Car and suggests that there was no
significant, coherent variability during the CHANDRA observation. The Eta Car
CHANDRA grating spectrum is unlike recently published X-ray grating spectra of
single massive stars in significant ways and is generally consistent with
colliding wind emission in a massive binary.
|
0105335v2
|
2001-06-29
|
Secondary antiprotons and propagation of cosmic rays in the Galaxy and heliosphere
|
High-energy collisions of cosmic-ray nuclei with interstellar gas are
believed to be the mechanism producing the majority of cosmic ray antiprotons.
Due to the kinematics of the process they are created with a nonzero momentum;
the characteristic spectral shape with a maximum at ~2 GeV and a sharp decrease
towards lower energies makes antiprotons a unique probe of models for particle
propagation in the Galaxy and modulation in the heliosphere. On the other hand,
accurate calculation of the secondary antiproton flux provides a ``background''
for searches for exotic signals from the annihilation of supersymmetric
particles and primordial black hole evaporation. Recently new data with large
statistics on both low and high energy antiproton fluxes have become available
which allow such tests to be performed. We use our propagation code GALPROP to
calculate interstellar cosmic-ray propagation for a variety of models. We show
that there is no simple model capable of accurately describing the whole
variety of data: boron/carbon and sub-iron/iron ratios, spectra of protons,
helium, antiprotons, positrons, electrons, and diffuse gamma rays. We find that
only a model with a break in the diffusion coefficient plus convection can
reproduce measurements of cosmic-ray species, and the reproduction of primaries
(p, He) can be further improved by introducing a break in the primary injection
spectra. For our best-fit model we make predictions of proton and antiproton
fluxes near the Earth for different modulation levels and magnetic polarity
using a steady-state drift model of propagation in the heliosphere.
|
0106567v2
|
2001-07-17
|
The Unique Type Ia Supernova 2000cx in NGC 524
|
We present extensive photometric and spectroscopic observations of the Type
Ia supernova (SN Ia) 2000cx in the S0 galaxy NGC 524, which reveal it to be
peculiar. Photometrically, SN 2000cx is different from all known SNe Ia, and
its light curves cannot be fit well by the fitting techniques currently
available. There is an apparent asymmetry in the $B$-band peak, in which the
premaximum brightening is relatively fast (similar to that of the normal SN
1994D), but the postmaximum decline is relatively slow (similar to that of the
overluminous SN 1991T). The color evolution of SN 2000cx is also peculiar: the
$(B - V)_0$ color has a unique plateau phase and the $(V - R)_0$ and $(V -
I)_0$ colors are very blue.
Although the premaximum spectra of SN 2000cx are similar to those of SN
1991T-like objects (with weak Si II lines), its overall spectral evolution is
quite different. The Si II lines that emerged near maximum $B$-band brightness
stay strong in SN 2000cx until about three weeks past maximum. The change in
the excitation stages of iron-peak elements is slow. Both the iron-peak and the
intermediate-mass elements are found to be moving at very high expansion
velocities in the ejecta of SN 2000cx.
We discuss theoretical models for SN 2000cx. SN 2000cx may be an overluminous
object like SN 1991T, but with a larger yield of $^{56}$Ni and a higher kinetic
energy in the ejecta. We also briefly discuss the implications of our
observations for the luminosity vs. light-curve width relation.
|
0107318v1
|
2001-09-03
|
The BeppoSAX 0.1 - 100 keV Spectrum of the X-ray Pulsar 4U 1538-52
|
We report the results of temporal and spectral analysis performed on the
X-ray pulsar 4U 1538-52 observed by BeppoSAX. We obtained a new estimate of the
spin period of the neutron star P=528.24 \pm 0.01 s (corrected for the orbital
motion of the X-ray source): the source is still in the spin-up state, as since
1988. The pulse profile is double peaked, although significant variations of
the relative intensity of the peaks with energy are present. The broad band
(0.12-100 keV) out-of-eclipse spectrum is well described by an absorbed power
law modified by a high energy cutoff at \sim 16 keV (e-folding energy \sim 10
keV) plus an iron emission line at \sim 6.4 keV. A cyclotron line at \sim 21
keV is present. The width of the line is consistent with thermal Doppler
broadening at the temperature of the exponential cutoff. We searched for the
presence of the second harmonic, previously reported for this source. We found
no evidence of lines at \sim 42 keV, although an absorption feature at 51 keV
seems to be present (at 99% confidence level). A soft excess, modelled by a
blackbody with a temperature of \sim 0.08 keV could be present, probably
emitted by the matter at the magnetosphere. We also performed a spectral
analysis during the X-ray eclipse. The spectral evolution during the eclipse
can be well described by a progressive covering of the primary Comptonization
spectrum that is scattered into the line of sight. During the deep eclipse this
spectrum also softens, suggesting that the dust-scattered component becomes
important. An alternative, more complex model, with an emission iron line and
scattered components (as the one that has been used to fit the eclipse of
Centaurus X-3), also gives a good fit of the deep-eclipse data.
|
0109031v1
|
2001-10-08
|
The Population of Weak MgII Absorbers. II The Properties of Single-Cloud Systems
|
We present an investigation of MgII absorbers characterized as single-cloud
weak systems at z~1. We measured column densities and Doppler parameters for
MgII and FeII in 15 systems found in HIRES/Keck spectra at 6.6 km/s. Using
these quantities and CIV, Lyman alpha and Lyman limit absorption observed with
FOS/HST (resolution ~230 km/s) we applied photoionization models to each system
to constrain metallicities, densities, ionization conditions, and sizes.
We find that: (1) Single-cloud weak systems are optically thin in neutral
hydrogen and may have their origins in a population of objects distinct from
the optically thick strong MgII absorbers, which are associated with bright
galaxies. (2) Weak systems account for somewhere between 25% to 100% of the z <
1 Lyman alpha forest clouds in the range 15.8<log N(HI)<16.8 cm^-2. (3) At
least seven of 15 systems have two or more ionization phases of gas (multiphase
medium). (4) We identify a subset of weak MgII absorber that we term
``iron-rich''. These clouds are not alpha-group enhanced and are constrained to
have sizes of ~10 pc. At that size, to produce the observed redshift path
density, they would need to outnumber L* galaxies by approximately six orders
of magnitude.
We discuss these results and the implications that the iron-rich systems
require enrichment from Type Ia supernovae. Further, we address how star
clusters or supernova remnants in dwarf galaxies might give rise to absorbers
with the inferred properties. This would imply far larger numbers of such
objects than are presently known, even locally. We compare the weak systems to
the weak kinematic subsystems in strong MgII absorbers and to Galactic high
velocity clouds. (abridged)
|
0110191v1
|
2001-10-29
|
Kinetic equilibrium of iron in the atmospheres of cool dwarf stars II. Weak Fe I lines in the solar spectrum
|
NLTE line formation calculations of FeI in the solar atmosphere are extended
to include weak optical lines. Previously established atomic models are used to
discriminate between different ways of treating collisional interaction
processes. To derive a common solar FeI abundance from both strong and weak
lines, fine-tuning of the microturbulence velocity parameter and the van-der-
Waals damping constants is required. The solar FeI abundances based on all
available f-values are dominated by the large scatter already found for the
stronger lines. In particular the bulk of the data from the work of May et al.
and O'Brian et al. is not adequate for accurate abundance work. Based on
f-values measured by the Hannover and Oxford groups alone, the FeI LTE
abundances are eps(FeI,Sun)=7.57 for the empirical and eps(FeI,Sun) = 7.48 ...
7.51 for the line-blanketed solar model. The solar Fe ionization equilibrium
obtained for different atomic and atmospheric models rules out NLTE atomic
models with a low efficiency of hydrogen collisions. At variance with Paper I,
it is now in better agreement with laboratory FeII f-values for all types of
line-blanketed models. Our final model assumptions consistent with a single
unique solar Fe abundance eps(Fe,Sun) = 7.48 ... 7.51 calculated from NLTE line
formation are (a) a line-blanketed solar model atmosphere, (b) an iron model
atom with hydrogen collision rates 0.5 < S_H < 5 times the standard value to
compensate for the large photoionization cross-sections, (c) a microturbulence
velocity xi = 1.0 kms, (d) van-der-Waals damping parameters decreased by
Delta(log C6) = -0.10...-0.15 as compared to Anstee & O'Mara's calculations,
depending on S_H, (e) FeII f-values as published by Schnabel et al., and (f)
FeI f-values published by the Hannover and Oxford groups.
|
0110605v1
|
2002-02-04
|
The structure and radiation spectra of illuminated accretion discs in AGN. I. Moderate illumination
|
We present detailed computations of the vertical structure of an accretion
disc illuminated by hard X-ray radiation with the code {\sc titan-noar}
suitable for Compton thick media. The energy generated via accretion is
dissipated partially in the cold disc as well as in the X-ray source. We study
the differences between the case where the X-ray source is in the form of a
lamp post above the accretion disc and the case of a heavy corona. We consider
radiative heating via Comptonization together with heating via photo-absorption
on numerous heavy elements as carbon, oxygen, silicon, iron. The transfer in
lines is precisely calculated. A better description of the heating/cooling
through the inclusion of line transfer, a correct description of the
temperature in the deeper layers, a correct description of the entire disc
vertical structure, as well as the study of the possible coronal pressure
effect, constitute an improvement in comparison to previous works. We show that
exact calculations of hydrostatic equilibrium and determination of the disc
thickness has a crucial impact on the optical depth of the hot illuminated
zone. We assume a moderate illumination where the viscous flux equals the X-ray
radiation flux. A highly ionized skin is created in the lamp post model, with
the outgoing spectrum containing many emission lines and ionization edges in
emission or absorption in the soft X-ray domain, as well as an iron line at
$\sim 7 $ keV consisting of a blend of low ionization line from the deepest
layers and hydrogen and helium like resonance line from the upper layers, and
almost no absorption edge, contrary to the case of a slab of constant density.A
full heavy corona completely suppresses the highly ionized zone on the top of
the accretion disc and in such case the spectrum is featureless.
|
0202069v2
|
2002-02-21
|
The BeppoSAX view of bright Compton-thin Seyfert 2 galaxies
|
We present the analysis of 31 observations of 20 bright Compton thin Seyfert
2s, in the 0.1-200 keV band, performed with the BeppoSAX satellite. The sample
consists of all Seyfert 2s in the BeppoSAX public archive, with a 2-10 keV flux
higher than 5x10^(-12) erg/cm^2/s. The good statistics available and the broad
energy band permit a detailed study of the main continuum components of these
sources, i.e. the primary power-law, the reflected component, the soft emission
and the high-energy cut-off. The main results of our analysis are: (1) the
3-200 keV intrinsic power-law has a mean photon index Gamma=1.79+-0.01, with a
dispersion of sigma=0.23. (2) The high-energy exponential cut-off at E~100-300
keV is not an ubiquitous property of Seyfert galaxies: in ~30% of the objects
the continuum power-law does not drop up to energies of 300 keV or more. (3) A
reflected component is present in almost all the sources (17 out of 21). The
small variations of this component with respect to the intrinsic continuum, in
objects with multiple observations, suggests that the reflector is not the
accretion disk, but must be located much farther from the nucleus. (4) The
range of ratios between the reflected and intrinsic components suggests that
the circumnuclear medium is not homogeneous, and a significant fraction of the
solid angle is covered by a gas thicker than that along the line of sight. (5)
The iron Kalpha line is present is all but one the sources. The equivalent
width is in the typical range of Seyfert 1s (EW=100-300 eV) in sources with low
absorption (N_H < 3x10^23 cm^(-2)), and increases in more absorbed objects, as
expected according to unified models. (6) The energy resolution of BeppoSAX is
in general too low to measure the iron line width. However, in 6 cases we
measured a significant line broadening.
|
0202392v1
|
2002-03-05
|
The Reddest Quasars II. A gravitationally-lensed FeLoBAL quasar
|
We report the discovery of a z=2.65 low-ionization iron broad absorption line
quasar, FIRST J100424.9+122922, which is gravitationally-lensed by a galaxy at
z~0.95. The object was discovered as part of a program to find very red quasars
by matching the FIRST radio survey with the 2-MASS near-infrared survey.
J100424.9+122922 is the second lensed system to be found in this program,
suggesting that many gravitational lenses are probably missed from conventional
optical quasar surveys. We have made a simple lens model and a rough estimate
of the reddening in the immediate environment of the quasar which suggests that
the quasar is intrinsically very luminous and is accreting at close to the
Eddington limit of its ~10^9 M_sun black hole. The lensing galaxy has a small
amount of dust which is responsible for some excess reddening observed in the
fainter image of the quasar, but is otherwise a fairly typical massive
elliptical galaxy. We model the selection effects working against the detection
of red quasars in both lensed and unlensed samples. We show that these
selection effects are very effective at removing even lightly-reddened high
redshift quasars from magnitude-limited samples, whether they are lensed or
not. This suggests that the red quasar population in general could be very
large, and in particular the class of iron broad absorption line quasars of
which J100424.9+122922 is a member may be much larger than their rarity in
magnitude-limited samples would suggest.
|
0203065v1
|
2002-04-03
|
Characterising the complex absorber in NGC 4151
|
We present a detailed analysis of the complex absorption apparent in the 2-6
keV X-ray spectrum of the bright nearby Seyfert galaxy NGC 4151. We first
utilize the large bandpass and medium spectral resolution afforded by BeppoSAX
data to construct a 1-100 keV spectral template, which assumes the absorption
arises in both warm (i.e. partially photoionized) and cold gas present in the
line of sight to the active nucleus of the source. Application of this spectral
model to an ASCA "long-look" observation of NGC 4151 reveals a partial
correlation between the underlying continuum flux and the ionization state of
the warm absorber. Such a correlation is an intrinsic property of a warm
absorber and argues strongly in favour of this interpretation for the complex
absorbing column over alternative partial covering models. The inferred
relatively low density for the warm gas, implies an equilibration timescale for
the dominant ions of the same order or longer than the timescale of the
continuum variability. It follows that the warm component will invariably be
observed in a non-equilibrium ionization state.
We also find that (i) the reported hardening of the spectrum of NGC 4151 as
the continuum level falls may be simply due to the presence of an underlying
(hard and relatively constant) Compton-reflection component and (ii) the iron
Ka line has a relatively narrow Gaussian profile and a line flux that remains
constant over both short (days) and long (months to years) timescales - a
relativistically broadened iron Ka feature was not required in our modelling.
|
0204057v2
|
2002-06-20
|
Membership, metallicity and lithium abundances for solar-type stars in NGC 6633
|
We present spectroscopic observations of candidate F, G and K type stars in
NGC 6633, an open cluster with a similar age to the Hyades. We identify 10 new
cluster members including one short period binary system. Combining this survey
with that of Jeffries (1997), we identify a total of 30 solar-type members. We
have used the F and early G stars to spectroscopically estimate
[Fe/H]=-0.096+/-0.081 for NGC 6633 and with more precision that NGC 6633 has
(0.074+/-0.041) dex less iron than the Pleiades and (0.206+/-0.040) dex less
iron than the Hyades. Lithium abundances have been estimated for the NGC 6633
members and compared with consistently determined Li abundances in other
clusters. Several mid F stars in NGC 6633 show strong Li depletion at
approximately the same effective temperature that this phenomenon is seen in
the Hyades. At cooler temperatures the Li abundance patterns in several open
clusters with similar ages (NGC 6633, Hyades, Praesepe and Coma Berenices) are
remarkably similar, despite their differing [Fe/H]. There is however evidence
that the late G and K stars of NGC 6633 have depleted less Li than their Hyades
counterparts. This qualitatively agrees with models for pre-main sequence Li
depletion that feature only convective mixing, but these models cannot
simultaneously explain why these stars have in turn depleted Li by more than 1
dex compared with their ZAMS counterparts in the Pleiades. Two explanations are
put forward. The first is that elemental abundance ratios, particularly [O/Fe],
may have non-solar values in NGC 6633 and would have to be higher than in
either the Hyades or Pleiades. The second is that additional non-convective
mixing, driven by angular momentum loss, causes additional photospheric Li
depletion during the first few hundred Myr of main sequence evolution.
|
0206367v1
|
2002-08-08
|
BeppoSAX observations of the EXS 1737.9-2952 region II: Analysis of sources
|
(Abridged) Spectral and time variability analysis of the region of the hard
X-ray transient EXS 1737.9-2952 near the Galactic Centre (GC), using data from
the Narrow Field Instruments of BeppoSAX is reported. The main results are the
MECS spectra of the 10 identified sources. Fluxes obtained with spectral fits
are 1.7-4.8E-12 erg / cm2 s. The absorption is in the range N_H=0.5-6.7E22 /
cm2, indicating that the sources are at least at the distance of the GC, and
thus have L_x > 2-5E34 erg/s (2-10 keV). A powerlaw with photon index 1.1-1.8
generally gives a fair fit, but a strong iron line (at 6-7 keV) is evident for
5 sources and exists at lower confidence in the others. The fits indicate
differences in line position in the range 6.1-7.0 keV suggesting that the
ionisation state and/or emission mechanism may not be the same in all sources.
The time-binned data indicates that two sources are variable on a time scale of
hours at confidence > 99.99%, and one at 99.67%. These sources could be
low-mass X-ray binaries. The other sources are most probably X-ray binaries or
SNRs. A MECS spectrum from a subfield including 8 of the new sources and a
major contribution of diffuse emission yielded a fairly good fit to a power-law
spectrum with photon index 1.3 and a strong iron line at 6.8 keV. Another field
with only residual emission and no point sources yielded spectral parameters
close to the diffuse emission near GC observed by other investigators, except
for the high interstellar absorption. The PDS spectrum of the same region is
more difficult to interpret due to lack of spatial resolution and a larger FOV
that contains the bright hard X-ray source 1E1740.7-2942. The source for the
hard X-ray transient EXS 17137.9-2952 cannot be identified from the present
observations.
|
0208179v1
|
2002-08-22
|
Chemical Abundances in Twelve Red Giants of the Large Magellanic Cloud from High-Resolution Infrared Spectroscopy
|
High-resolution infrared spectra (R=50,000) have been obtained for twelve
red-giant members of the LMC with the Gemini South 8.3-meter telescope plus
Phoenix spectrometer. Quantitative chemical abundances of carbon-12, carbon-13,
nitrogen-14, and oxygen-16 were derived from molecular lines of CO, CN, and OH,
while sodium, scandium, titanium, and iron abundances were derived from neutral
atomic lines. The LMC giants have masses from about 1 to 4 solar masses and
span a metallicity range from [Fe/H]= -1.1 to -0.3. The program red giants all
show evidence of first dredge-up mixing, with low 12C/13C ratios, and low 12C
correlated with high 14N abundances. Comparisons of the oxygen-to-iron ratios
in the LMC and the Galaxy indicate that the trend of [O/Fe] versus [Fe/H] in
the LMC falls about 0.2 dex below the Galactic trend. Such an offset can be
modeled as due to an overall lower rate of supernovae per unit mass in the LMC
relative to the Galaxy, as well as a slightly lower ratio of supernovae of type
II to supernovae of type Ia.
|
0208417v1
|
2002-10-10
|
Abundances in Stars from the Red Giant Branch Tip to Near the Main Sequence Turn Off in M5
|
We present the iron abundance and abundance ratios for 18 elements with
respect to Fe in a sample of stars with a wide range in luminosity from
luminous giants to stars near the turnoff in the globular cluster M5. The
analyzed spectra, obtained with HIRES at the Keck Observatory, are of high
dispersion (R=35,000). We find that the neutron capture, the iron peak and the
alpha-element abundance ratios show no trend with Teff, and low scatter around
the mean between the top of the RGB and near the main sequence turnoff To
within the precision of the measurements (~0.1 dex), gravitationally induced
heavy element diffusion does not appear to be present among the stars near the
main sequence turnoff studied here. Our work and other recent studies suggest
that heavy element diffusion is inhibited in the surface layers of metal poor
stars. Differences in the Na abundance from star to star which extend to the
main sequence turnoff are detected in our sample in M5. The anti-correlation
between O and Na abundances, observed in other metal poor globular clusters, is
not detected in our sample, but it may be hidden among stars with only upper
limits for their O abundances. Overall the abundance ratios of M5 appear very
similar to those of M71, with the possible exception of the neutron capture
element Ba, where we argue that the apparent difference may be due to
difficulties in the analysis. As in M71, the alpha-elements Mg, Ca, Si and Ti
are overabundant relative to Fe. The results of our abundance analysis of 25
stars in M5 provide further evidence of abundance variations among specific
light elements at unexpectedly low luminosities, which cannot be explained by
our current understanding of stellar evolution.
|
0210245v1
|
2003-02-24
|
XMM-NEWTON High Resolution Spectroscopy of NGC 5548
|
We analyze a 137 ks exposure X-ray spectrum of the Seyfert 1 galaxy NGC 5548
obtained with the XMM-Newton Reflection Grating Spectrometer. Due to the long
exposure time, the spectrum is of higher statistical quality than the previous
observations of this AGN. Therefore, we detect for the first time in NGC 5548
inner-shell transitions from O III to O VI ions, and the Unresolved Transition
Array of M-shell iron. The warm absorber found from this X-ray observation
spans three orders of magnitude in ionization parameter. We detect O III, which
is as lowly ionized as the warm absorber detected in the UV band, to Fe XXIV.
For O VI the column density determined from our X-ray data is an order of
magnitude larger than the column density measured in previous UV observations.
We conclude that there is substantially more low ionized material than
previously deduced from UV observations. However, only a few percent of the
warm absorber detected in the X-rays is lowly ionized. A 99.9 % significant
increase in the derived absorbing column density with higher ionization states
is observed. The outflow velocities determined from the X-ray absorption lines
are consistent with those deduced from the UV lines, evidence, together with
the detection of O VI, that the X-ray and UV warm absorber are different
manifestations of the same phenomenon. From a simple mass conservation
argument, we indicate that our data set is consistent with an outflow with
small opening angle formed due to instabilities in the accretion disk. Possible
due to uncertainties in the radiative transport mechanism, an apparent deviant
iron to oxygen abundance is detected. No strong relativistically broadened
emission lines of O VIII, N VII and C VI were detected.
|
0302493v1
|
2003-03-03
|
XMM-Newton and Chandra Observations of the Galaxy Group NGC 5044. II. Metal Abundances and Supernova Fraction
|
Using new XMM and Chandra observations we present an analysis of the metal
abundances of the hot gas within a radius of 100 kpc of the bright nearby
galaxy group NGC 5044. Motivated by the inconsistent abundance and temperature
determinations obtained by different observers for X-ray groups, we provide a
detailed investigation of the systematic errors on the derived abundances
considering the effects of the temperature distribution, calibration, plasma
codes, bandwidth, Galactic Nh, and background rate. The iron abundance (Fe)
drops from Fe ~1 solar within R ~50 kpc to Fe ~0.4 solar near R=100 kpc. This
radial decline in Fe is highly significant: Fe=1.09 +/- 0.04 solar (stat) +/-
0.05 solar + 0.18 solar (sys) within R=48 kpc (5') compared to Fe=0.44 +/- 0.02
solar (stat) +/- 0.10 solar + 0.13 solar (sys) over R=48-96 kpc (5'-10'). The
data rule out with high confidence a very sub-solar value for Fe within R=48
kpc confirming that previous claims of very sub-solar central Fe values in NGC
5044 were primarily the result of the Fe Bias: i.e., the incorrect assumption
of spatially isothermal and single-phase gas when in fact temperature
variations exist. Within R=48 kpc we obtain Si/Fe = 0.83 +/- 0.02 (stat) +/-
0.02 + 0.07 (sys) and S/Fe = 0.54 +/- 0.02 (stat) +/- 0.01 + 0.01 (sys) in
solar units. These ratios are consistent with their values at larger radii and
imply that SNIa have contributed ~80% of the iron mass within a 100 kpc radius
of NGC 5044. This SNIa fraction is similar to the Sun and suggests an IMF
similar to that of the Milky Way. At the very center (R ~2 kpc) the XMM and
Chandra CCDs and the XMM RGS show that the Fe drops to ~50% of its value at
immediately larger radius analogously to that seen in some galaxy clusters.
(Abridged)
|
0303054v2
|
2003-04-01
|
Near-IR Properties of Galaxy Clusters: Luminosity as a Binding Mass Predictor and the State of Cluster Baryons
|
We explore the near-infrared properties of galaxies within 27 galaxy clusters
using data from the Two Micron All Sky Survey (2MASS). For a subsample of 13
clusters with available X-ray imaging data, we examine both the properties of
the galaxies and the intracluster medium. We show that the K-band luminosity is
correlated with cluster mass, providing a binding mass estimate accurate to
45%. The mass to light ratio in our ensemble increases by a factor of ~2 over
the cluster mass range (10^{14}-10^{15}M_sun). We examine the total baryon
fraction, showing that it is an increasing function of cluster mass. Using the
mass to light ratio of massive clusters, we find that Omega_M=0.19+/-0.03;
using the total baryon fraction we find that Omega_M=0.28+/-0.03, in good
agreement with recent cosmic microwave background anisotropy constraints.
Differences between these two estimates suggest that the K-band mass to light
ratio in massive clusters may be lower than that in the universe by as much as
\~30%.
We examine the stellar mass fraction, the ICM mass to stellar mass ratio and
the cluster iron mass fraction. The stellar mass fraction decreases by a factor
of 1.8 from low to high mass clusters, and the ICM to stellar mass ratio
increases from 5.9 to 10.4 over the same mass range. Together, these
measurements suggest a decrease of star formation efficiency with increasing
cluster mass and provide constraints on models of the thermodynamic history of
the intracluster medium. The cluster iron mass to total mass ratio is constant
and high, suggesting that some efficient, and uniform enrichment process may
have taken place before the bulk of stars in cluster galaxies formed.
|
0304033v2
|
2003-04-09
|
Peculiar Broad Absorption Line Quasars found in DPOSS
|
With the recent release of large (i.e., > hundred million objects),
well-calibrated photometric surveys, such as DPOSS, 2MASS, and SDSS,
spectroscopic identification of important targets is no longer a simple issue.
In order to enhance the returns from a spectroscopic survey, candidate sources
are often preferentially selected to be of interest, such as brown dwarfs or
high redshift quasars. This approach, while useful for targeted projects, risks
missing new or unusual species. We have, as a result, taken the alternative
path of spectroscopically identifying interesting sources with the sole
criterion being that they are in low density areas of the g - r and r - i
color-space defined by the DPOSS survey. In this paper, we present three
peculiar broad absorption line quasars that were discovered during this
spectroscopic survey, demonstrating the efficacy of this approach. PSS
J0052+2405 is an Iron LoBAL quasar at a redshift z = 2.4512 with very broad
absorption from many species. PSS J0141+3334 is a reddened LoBAL quasar at z =
3.005 with no obvious emission lines. PSS J1537+1227 is a Iron LoBAL at a
redshift of z = 1.212 with strong narrow Mgii and Feii emission. Follow-up high
resolution spectroscopy of these three quasars promises to improve our
understanding of BAL quasars. The sensitivity of particular parameter spaces,
in this case a two-color space, to the redshift of these three sources is
dramatic, raising questions about traditional techniques of defining quasar
populations for statistical analysis.
|
0304166v1
|
2003-04-11
|
The Contribution of Particle Impact to the Production of Fe K Emission from Accreting Black Holes
|
The iron K line is perhaps the most important spectral diagnostic available
in the study of accreting black holes. The line is thought to result from the
reprocessing of external X-rays by the surface of the accretion disk. However,
as is observed in the solar corona, illumination by energetic particles may
also produce line emission. In principle, such a process may be uncorrelated
with the observed X-rays and could explain some of the unexpected variability
behavior of the Fe line. This paper compares predictions of iron K flux
generated by impacting electrons and protons to that from photoionization.
Non-thermal power-laws of electrons are considered as well as thermal
distributions of electrons and virialized protons. The electrons are thought to
originate in a magnetically dominated accretion disk corona, while the protons
are considered in the context of a two phase (hot/cold) accretion scenario. In
each case, the Fe K flux from particle impact is found to be < 1% of that
produced by photoionization by a hard X-ray power-law (normalized to the same
energy flux as the particles). Thus, the electrons or protons must strike the
disk with 100--10,000 times more energy flux than radiation for particle impact
to be a significant producer of Fe K flux. This situation is difficult to
reconcile with the observations of hard X-ray spectra, or the proposed particle
acceleration mechanisms in the accretion disk corona. Truncated accretion flows
must be externally illuminated by hard X-rays in order to produce the Fe line,
as proton impact is very inefficient in generating line emission. In contrast
to the Sun, our conclusion is that, with the possible exception for localized
regions around magnetic footpoints, particle impact will not be an important
contributor to the X-ray emission in accreting black holes.
|
0304218v1
|
2003-04-14
|
BeppoSAX Observations of Centaurus A: the Hard Continuum and the Iron Line Feature
|
The radio galaxy Centaurus A was observed by the BeppoSAX satellite five
times from 1997 to 2000. From July 6 1999 to August 17 1999, the source was
also simultaneously pointed by COMPTEL on-board of the satellite CGRO.
Centaurus A has a complex spectrum with multiple extended components and a
strongly absorbed (N_H\sim 10^{23}$ cm${-2}) nucleus well fitted by a power law
(\Gamma \sim 1.8) which bends at high energies. When the BeppoSAX and COMPTEL
observations are combined together, an exponential cutoff with e-folding energy
\sim 1000 keV gives an adequate description of the spectral steepening. A
complex feature in emission at 6-7 keV is resolved into two Fe components, one
narrow cold line and an ionized line centred at 6.8 keV. Significant variations
have been observed in the iron feature, with the less prominent ionized line
seemingly being the only one responsible for them: its variations do not appear
to correlate with the strength of the continuum. The high energy cutoff and the
Fe feature suggest the presence of an accretion flow in the Centaurus A
nucleus. However the absence of a significant reflection, the narrowness of the
cold line as well as the lack of correlation between the continuum and 6.8 keV
line variations disfavour a standard cold/ionized thin disk (at least in the
inner regions). A more plausible configuration might be a hot thick optically
thin accretion flow surrounded by material with different opacities.
Finally, we note that high energy break observed by BeppoSAX and COMPTEL
could be also reasonably explained by Inverse Compton radiation from a jet. If
this is the case, a structured jet with outer slow layers surrounding a beamed
inner region is necessary to explain the strong Fe feature observed by
BeppoSAX.
|
0304244v1
|
2003-06-13
|
Abundances in the Neutral Interstellar Medium of I Zw 18 from FUSE Observations
|
We report on new FUSE far-UV spectroscopy of the most metal-poor blue compact
dwarf galaxy I Zw 18. The new data represent an improvement over previous FUSE
spectra by a factor of 1.7 in the signal-to-noise. Together with a larger
spectral coverage (917-1188 angstroms), this allows us to characterize
absorption lines in the interstellar medium with unprecedented accuracy. The
kinematics averaged over the large sampled region shows no clear evidence of
gas inflows or outflows. The H I absorption is interstellar with a column
density of 2.2 (+0.6,-0.5} * 10^21 cm^(-2). A conservative 3 sigma upper limit
of 5.25 * 10^(14) cm^(-2) is derived for the column density of diffuse H_2.
From a simultaneous fitting of metal absorption lines in the interstellar
medium, we infer the following abundances: [Fe/H] = -1.76 +/- 0.12, [O/H] =
-2.06 +/- 0.28, [Si/H] = -2.09 +/- 0.12, [Ar/H] = -2.27 +/- 0.13, and [N/H] =
-2.88 +/- 0.11. This is in general several times lower than in the H II
regions. The only exception is iron, whose abundance is the same. The abundance
pattern of the interstellar medium suggests ancient star-formation activity
with an age of at least a Gyr that enriched the H I phase. Around 470 SNe Ia
are required to produce the iron content. A more recent episode that started 10
to several 100 Myr ago is responsible for the additional enrichment of
alpha-elements and nitrogen in the H II regions.
|
0306290v1
|
2003-06-17
|
Kinetic equilibrium of iron in the atmospheres of cool stars III. The ionization equilibrium of selected reference stars
|
Non-LTE line formation calculations of Fe I are performed for a small number
of reference stars to investigate and quantify the efficiency of neutral
hydrogen collisions. Using the atomic model that was described in previous
publications, the final discrimination with respect to hydrogen collisions is
based on the condition that the surface gravities as determined by the Fe I/Fe
II ionization equilibria are in agreement with their astrometric counterparts
obtained from HIPPARCOS parallaxes. Depending on the choice of the hydrogen
collision scaling factor S_H, we find deviations from LTE in Fe I ranging from
0.00 (S_H = infinity) to 0.46 dex (S_H = 0 for HD140283) in the logarithmic
abundances while Fe II follows LTE.
With the exception of Procyon, for which a mild temperature correction is
needed to fulfil the ionization balance, excellent consistency is obtained for
the metal-poor reference stars if Balmer profile temperatures are combined with
S_H = 3. The correct choice of collisional damping parameters ("van-der-Waals"
constants) is found to be generally more important for these little evolved
metal-poor stars than considering departures from LTE. For the Sun the
calibrated value for S_H leads to average Fe I non-LTE corrections of 0.02 dex
and a mean abundance from Fe I lines of log epsilon(Fe) = 7.49 \pm 0.08.
We confront the deduced stellar parameters with comparable spectroscopic
analyses by other authors which also rely on the iron ionization equilibrium as
a gravity indicator. On the basis of the HIPPARCOS astrometry our results are
shown to be an order of magnitude more precise than published data sets, both
in terms of offset and star-to-star scatter.
|
0306337v1
|
2003-08-01
|
Variations in the Abundance Pattern of Extremely Metal-poor Stars and Nucleosynthesis in Population III Supernovae
|
We calculate nucleosynthesis in Population (Pop) III supernovae (SNe) and
compare the yields with various abundance patterns of extremely metal-poor
(EMP) stars. We assume that the observed EMP stars are the second generation
stars, which have the metal-abundance patterns of Pop III SNe. Previous
theoretical yields of Pop III SNe cannot explain the trends in the abundance
ratios among iron-peak elements (Mn, Co, Ni, Zn)/Fe as well as the large C/Fe
ratio observed in certain EMP stars with [Fe/H] <~ -2.5. In the present paper,
we show that if we introduce higher explosion energies and mixing-fallback in
the core-collapse SN models of M ~ 20 - 130 Msun, the above abundance features
of both typical and C-rich EMP stars can be much better explained. We suggest
that the abundance patterns of the [Fe/H] ~ -2.5 stars correspond to supernova
yields with normal explosion energies, while those of the carbon un-enhanced
([C/Fe] < 1) stars with [Fe/H] =~ -4 ~ - 3 correspond to high-energy supernova
yields. The abundance patterns of the C-rich ([C/Fe]>~ 2) and low [Fe/H] (=~ -5
\~ -3.5) stars can be explained with the yields of faint SNe that eject little
56Ni as observed in SN1997D. In the supernova-induced star formation model, we
can qualitatively explain why the EMP stars formed by the faint or energetic
supernovae have lower [Fe/H] than the EMP stars formed by normal supernovae. We
also examine how the abundance ratios among iron-peak elements depend on the
electron mole fraction Ye, and conclude that a large explosion energy is still
needed to realize the large Co/Fe and Zn/Fe ratios observed in typical EMP
stars with [Fe/H] <~ -3.5.
|
0308029v3
|
2003-11-24
|
Sulphur and zinc abundances in Galactic stars and damped Lyman-alpha systems
|
High resolution spectra of 34 halo population dwarf and subgiant stars have
been obtained with VLT/UVES and used to derive sulphur abundances from the
8694.0, 8694.6 A and 9212.9, 9237.5 A SI lines. In addition, iron abundances
have been determined from 19 FeII lines and zinc abundances from the 4722.2,
4810.5 ZnI lines. The abundances are based on a classical 1D, LTE model
atmosphere analysis, but effects of 3D hydrodynamical modelling on the [S/Fe],
[Zn/Fe] and [S/Zn] ratios are shown to be small. We find that most halo stars
with metallicities in the range -3.2 < [Fe/H] < -0.8 have a near-constant
[S/Fe] = +0.3; a least square fit to [S/Fe] vs. {Fe/H] shows a slope of only
-0.04 +/- 0.01. Among halo stars with -1.2 < [Fe/H] < -0.8 the majority have
[S/Fe] ~ +0.3, but two stars (previously shown to have low [alpha/Fe] ratios)
have [S/Fe] ~ 0. For disk stars with [Fe/H] > -1, [S/Fe] decreases with
increasing [Fe/H]. Hence, sulphur behaves like other typical alpha-capture
elements, Mg, Si and Ca. Zinc, on the other hand, traces iron over three orders
of magnitude in [Fe/H], although there is some evidence for a small systematic
Zn overabundance ([Zn/Fe] ~ +0.1) among metal-poor disk stars and for halo
stars with [Fe/H] < -2.0. Recent measurements of S and Zn in ten damped
Ly-alpha systems (DLAs) with redshifts between 1.9 and 3.4 and zinc abundances
in the range -2.1 < [Zn/H] < -0.15 show an offset relative to the [S/Zn] -
[Zn/H] relation in Galactic stars. Possible reasons for this offset are
discussed, including low and intermittent star formation rates in DLAs.
|
0311529v1
|
2004-01-21
|
First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A
|
We present the abundance analyses of three isolated A-type supergiant stars
in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES
spectrograph at the VLT. Detailed model atmosphere analyses have been used to
determine the stellar atmospheric parameters and the elemental abundances of
the stars. The mean iron group abundance was determined from these three stars
to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of
the present-day iron group abundances in Sextans A. These three stars now
represent the most metal-poor massive stars for which detailed abundance
analyses have been carried out. The mean stellar alpha element abundance was
determined from the alpha element magnesium as
[alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the
nebular alpha element abundances as determined from oxygen in the H II regions.
These results are consistent from star-to-star with no significant spatial
variations over a length of 0.8 kpc in Sextans A. This supports the nebular
abundance studies of dwarf irregular galaxies, where homogeneous oxygen
abundances are found throughout, and argues against in situ enrichment. The
alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which
is consistent with the solar ratio. This is consistent with the results from
A-supergiant analyses in other Local Group dwarf irregular galaxies but in
stark contrast with the high [alpha/Fe] results from metal-poor stars in the
Galaxy, and is most clearly seen from these three stars in Sextans A because of
their lower metallicities. The low [alpha/Fe] ratios are consistent with the
slow chemical evolution expected for dwarf galaxies from analyses of their
stellar populations.
|
0401411v1
|
2004-03-15
|
Raising the Dead: Clues to Type Ia Supernova Physics from the Remnant 0509-67.5
|
We present Chandra X-ray observations of the young supernova remnant (SNR)
0509-67.5 in the Large Magellanic Cloud (LMC), believed to be the product of a
Type Ia supernova (SN Ia). The remnant is very round in shape, with a distinct
clumpy shell-like structure. Our Chandra data reveal the remnant to be rich in
silicon, sulfur, and iron. The yields of our fits to the global spectrum
confirm that 0509-67.5 is the remnant of an SN Ia and show a clear preference
for delayed detonation explosion models for SNe Ia. We study the spectrum of
the single brightest isolated knot in the remnant and find that it is enhanced
in iron by a factor of roughly two relative to the global remnant abundances.
This feature, along with similar knots seen in Tycho's SNR, argues for the
presence of modest small-scale composition inhomogeneities in SNe Ia. The
presence of both Si and Fe, with abundance ratios that vary from knot to knot,
indicates that these came from the transition region between the Si- and
Fe-rich zones in the exploded star, possibly as a result of energy input to the
ejecta at late times due to the radioactive decay of 56Ni and 56Co. Two cases
for the continuum emission from the global spectrum were modeled: one where the
continuum is dominated by hydrogen thermal bremsstrahlung radiation; another
where the continuum arises from non-thermal synchrotron radiation. The former
case requires a relatively large value for the ambient density (~1 cm^-3).
Another estimate of the ambient density comes from using the shell structure of
the remnant in the context of dynamical models. This requires a much lower
value for the density (<0.05 cm^-3) which is more consistent with other
evidence known about 0509-67.5. We therefore conclude that the bulk of the
continuum emission from 0509-67.5 has a non-thermal origin.
|
0403343v2
|
2004-04-20
|
XMM-Newton EPIC observations of 21 low-redshift PG quasars
|
We present an X-ray spectral analysis of 21 low redshift quasars observed
with XMM-Newton EPIC. All the sources are Palomar Green quasars with redshifts
between 0.05 and 0.4 and have low Galactic absorption along the line-of-sight.
A large majority of quasars in the sample (19/21) exhibit a significant soft
excess below ~1-1.5keV, whilst two objects (PG1114+445 and IZw1) show a deficit
of soft X-ray flux due to the presence of a strong warm absorber. At least half
of the objects appear to harbor a warm absorber, as found previously in Seyfert
1 galaxies. We find significant detections of FeKalpha emission lines in at
least twelve objects, whilst there is evidence for some broadening of the line
profile, compared to the EPIC-pn resolution, in five of these quasars.
The determination of the nature of this broadening (e.g., Keplerian motion, a
blend of lines, relativistic effects) is not possible with the present data and
requires either higher S/N or higher resolution spectra. In seven objects the
line is located between 6.7-7keV, corresponding to highly ionized iron, whereas
in the other five objects the line energy is consistent with 6.4 keV, i.e.
corresponding to near neutral iron. The ionized lines tend to be found in the
quasars with the steepest X-ray spectra. We also find a correlation between the
continuum power law index and the optical Hbeta width, in both the soft and
hard X-ray bands, whereby the steepest X-ray spectra are found in objects with
narrow Hbeta widths, which confirms previous ROSAT and ASCA results. The soft
and hard band X-ray photon indices are also strongly correlated, i.e. the
steepest soft X-ray spectra correspond the steepest hard X-ray spectra.
|
0404385v1
|
2004-08-23
|
Detectability of GRB Iron Lines by Swift, Chandra and XMM
|
The rapid acquisition of positions by the upcoming Swift satellite will allow
the monitoring for X-ray lines in GRB afterglows at much earlier epochs than
was previously feasible. We calculate the possible significance levels of iron
line detections as a function of source redshift and observing time after the
trigger, for the Swift XRT, Chandra ACIS and XMM Epic detectors. For bursts
with standard luminosities, decay rates and equivalent widths of 1 keV assumed
constant starting at early source-frame epochs, Swift may be able to detect
lines up to z~1.5 with a significance of better than 3 sigma for times up to
10^4 s. The same lines would be detectable with better than 4 sigma
significance at z up to 6 by Chandra, and up to 8 by XMM, for times of up to
10^5 s. For similar bursts with a variable equivalent width peaking at 1 keV
between 0.5 and 1 days in the source frame, Swift achieves the same
significance level for z~1 at t~1 day, while Chandra reaches the previous
detection significances around t~ 1-2 days for z~ 2-4, i.e. the line is
detectable near the peak equivalent width times, and undetectable at earlier or
later times. For afterglows in the upper range of initial X-ray luminosites
afterglows, which may also be typical of pop. III bursts, similar significance
levels are obtained out to substantially higher redshifts. A distinction
between broad and narrow lines to better than 3 sigma is possible with Chandra
and XMM out to z~2 and ~6.5, respectively, while Swift can do so up to z~1, for
standard burst parameters. A distinction between different energy centroid
lines of 6.4 keV vs. 6.7 KeV (or 6.7 keV vs. Cobalt 7.2 keV) is possible up to
z~0.6, 1.2, and 2 (z~ 1, 5, 7.5), with Swift,Chandra, and XMM respectively.
|
0408414v3
|
2005-02-16
|
The FUSE Spectrum of the Planetary Nebula SwSt 1: Evidence for Inhomogeneities in the Gas and Dust
|
[Abridged] We present Far Ultraviolet Spectroscopic Explorer (FUSE)
observations of the young, compact planetary nebula (PN) SwSt 1 along the line
of sight to its central star HD 167362. We detect circumstellar absorption
lines from several species against the continuum of the central star. The
physical parameters of the nebula derived from the FUSE data differ
significantly from those found from emission lines. We derive an electron
density n_e = 8800^{+4800}_{-2400} cm^{-3} from the column density ratio of the
excited S III fine structure levels, which is at least a factor of 3 lower than
all prior estimates. The gaseous iron abundance derived from the UV lines is
quite high ([Fe/S] = -0.35+/-0.12), which implies that iron is not
significantly depleted into dust. In contrast, optical and near-infrared
emission lines indicate that Fe is more strongly depleted: [Fe/H] =
-1.64+/-0.24 and [Fe/S] = -1.15+/-0.33. We do not detect nebular H_2
absorption, to a limit N(H_2) < 7\times10^14 cm^{-2}, at least four orders of
magnitude lower than the column density estimated from infrared H_2 emission
lines. Taken together, the lack of H_2 absorption, low n_e, and high gaseous Fe
abundance derived from the FUSE spectrum provide strong evidence that dense
structures (which can shield molecules and dust from the destructive effects of
energetic stellar photons) are not present along the line of sight to the
central star. On the other hand, there is substantial evidence for dust,
molecular material, and dense gas elsewhere in SwSt 1. Therefore, we conclude
that the nebula must have an inhomogeneous structure.
|
0502321v1
|
2005-08-18
|
Spectroscopic abundance analysis of dwarfs in young open cluster IC 4665
|
We report a detailed spectroscopic abundance analysis for a sample of 18 F-K
dwarfs of the young open cluster IC 4665. Stellar parameters and element
abundances of Li, O, Mg, Si, Ca, Ti, Cr, Fe and Ni have been derived using the
spectroscopic synthesis tool SME (Spectroscopy Made Easy). Within the
measurement uncertainties the iron abundance is uniform with a standard
deviation of 0.04 dex. No correlation is found between the iron abundance and
the mass of the stellar convective zone, and between the Li abundance and the
Fe abundance. In other words, our results do not reveal any signature of
accretion and therefore do not support the scenario that stars with planets
(SWPs) acquire their on the average higher metallicity compared to field stars
via accretion of metal-rich planetary material. Instead the higher metallicity
of SWPs may simply reflect the fact that planet formation is more efficient in
high metallicity environs. However, since that many details of the planet
system formation processes remain poorly understood, further studies are needed
for a final settlement of the problem of the high metallicity of SWPs.
The standard deviation of [Fe/H] deduced from our observations, taken as an
upper limit on the metallicity dispersion amongst the IC 4665 member stars, has
been used to constrain proto-planetary disk evolution, terrestrial and giant
planets formation and evolution processes. Our results do not support the
possibility that the migration of gas giants and the circularization of
terrestrial planets' orbits are regulated by their interaction with a residual
population of planetesimals and dust particles.
|
0508387v1
|
2005-12-20
|
Elemental Abundance Survey of The Galactic Thick Disk
|
[Abridged abstract] We have performed an abundance analysis for 176 F- and G-
dwarfs of the Galactic thick disk component. Using accurate radial velocities
combined with $Hipparcos$ astrometry, kinematics (U, V, and W) and Galactic
orbital parameters were computed. We estimate the probability for a star to
belong to the thin disk, the thick disk or the halo.
Abundances of C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn, Y, Ba, Ce, Nd, and Eu have been obtained. The abundances for thick disk
stars are compared with those for thin disk members from Reddy et al. (2003).
The ratios of $\alpha$-elements (O, Mg, Si, Ca and Ti) to iron for thick disk
disk stars show a clear enhancement compared to thin disk members in the range
$-0.3 <$ [Fe/H] $ < -1.2$. There are also other elements -- Al, Sc, V, Co, and
possibly Zn -- which show enhanced ratios to iron in the thick disk relative to
the thin disk. The abundances of Na, Cr, Mn, Ni, and Cu (relative to Fe) are
very similar for thin and thick disk stars. The dispersion in abundance ratios
[X/Fe] at given [Fe/H] for thick disk stars is consistent with the expected
scatter due to measurement errors, suggesting a lack of `cosmic' scatter. The
observed compositions of the thin and thick disks seem to be consistent with
models of galaxy formation by hierarchical clustering in a $\Lambda$CDM
universe. In particular, the distinct abundance patterns observed in the thin
and thick disks, and the chemical homogeneity of the thick disk at different
galactocentric distances favor a scenario in which the majority of thick-disk
stars were formed {\it in situ}, from gas rich merging blocks.
|
0512505v1
|
2006-01-20
|
The Hamburg/ESO R-process Enhanced Star survey (HERES) III. HE 0338-3945 and the formation of the r+s stars
|
We have derived abundances of 33 elements and upper limits for 6 additional
elements for the metal-poor ([Fe/H] = -2.42) turn-off star HE 0338-3945 from
high-quality VLT-UVES spectra. The star is heavily enriched, by about a factor
of 100 relative to iron and the Sun, in the heavy s-elements (Ba, La, ..). It
is also heavily enriched in Eu, which is generally considered an r-element, and
in other similar elements. It is less enriched, by about a factor of 10, in the
lighter s-elements (Sr, Y and Zr). C is also strongly enhanced and, to a
somewhat lesser degree, N and O. These abundance estimates are subject to
severe uncertainties due to NLTE and thermal inhomogeneities which are not
taken into detailed consideration. However, an interesting result, which is
most probably robust in spite of these uncertainties, emerges: the abundances
derived for this star are very similar to those of other stars with an overall
enhancement of all elements beyond the iron peak.
We have defined criteria for this class of stars, r+s stars, and discuss nine
different scenarios to explain their origin. None of these explanations is
found to be entirely convincing. The most plausible hypotheses involve a binary
system in which the primary component goes through its giant branch and
asymptotic giant branch phases and produces CNO and s-elements which are dumped
onto the observed star. Whether the r-element Eu is produced by supernovae
before the star was formed (perhaps triggering the formation of a low-mass
binary), by a companion as it explodes as a supernova (possibly triggered by
mass transfer), or whether it is possibly produced in a high-neutron-density
version of the s-process is still unclear. Several suggestions are made on how
to clarify this situation.
|
0601476v1
|
2006-02-27
|
Chemical evolution in Sersic 159-03 observed with XMM-Newton
|
Using a new long X-ray observation of the cluster of galaxies Sersic 159-03
with XMM-Newton, we derive radial temperature and abundance profiles using
single- and multi-temperature models. The fits to the EPIC and RGS spectra
prefer multi-temperature models especially in the core. The radial profiles of
oxygen and iron measured with EPIC/RGS and the line profiles in RGS suggest
that there is a dip in the O/Fe ratio in the centre of the cluster compared to
its immediate surroundings. A possible explanation for the large scale
metallicity distribution is that SNIa and SNII products are released in the ICM
through ram-pressure stripping of in-falling galaxies. This causes a peaked
metallicity distribution. In addition, SNIa in the central cD galaxy enrich
mainly the centre of the cluster with iron. This excess of SNIa products is
consistent with the low O/Fe ratio we detect in the centre of the cluster. We
fit the abundances we obtain with yields from SNIa, SNII and Population-III
stars to derive the clusters chemical evolution. We find that the measured
abundance pattern does not require a Population-III star contribution. The
relative contribution of the number of SNIa with respect to the total number of
SNe which enrich the ICM is about 25-50%. Furthermore, we discuss the possible
presence of a non-thermal component in the EPIC spectra. A potential source of
this non-thermal emission can be inverse-Compton scattering between Cosmic
Microwave Background (CMB) photons and relativistic electrons, which are
accelerated in bow shocks associated with ram-pressure stripping of in-falling
galaxies.
|
0602582v1
|
2006-03-10
|
Metal Enrichment of the ICM: a 3-D Picture of Chemical and Dynamical Properties
|
We develop a model for the metal enrichment of the intracluster medium (ICM)
that combines a cosmological non-radiative hydrodynamical N-Body/SPH simulation
of a cluster of galaxies, and a semi-analytic model of galaxy formation. The
novel feature of our hybrid model is that the chemical properties of the
diffuse gas in the underlying simulation are dynamically and consistently
generated from stars in the galaxies. We follow the production of several
chemical elements, provided by low- and intermediate-mass stars, core collapse
and type Ia supernovae. We analyse the spatial distribution of metals in the
ICM, investigate the way in which the chemical enrichment proceeds, and use
iron emissivity as a tracer of gas motions. Fe and O radial abundance profiles
are enhanced in the inner 100 h^-1 kpc in the last Gyr because of the
convergence of enriched gas clumps to the cluster centre. Our results support a
scenario in which part of the central intracluster gas comes from gas clumps
that, in the redshift range of z~0.2 to ~0.5, have been enriched to solar
values and are at large distances from the cluster centre (from ~1 to ~6 h^-1
Mpc) moving at very high velocities (from ~1300 to ~2500 km s^-1). The
turbulent gas motions within the cluster, originated in the inhomogeneous gas
infall during the cluster assembly, are manifested in emission-weighted
velocity maps as gradients that can be as large as ~1000 km s^-1 over distances
of a few hundred kpc. Gradients of this magnitude are also seen in velocity
distributions along sightlines through the cluster centre. Doppler shifting and
broadening suffered by the Fe K 6.7 keV emission line along such sightlines
could be used to probe these gas large-scale motions when they are produced
within an area characterised by high iron line emissivity.
|
0603270v1
|
2006-04-05
|
Abundance analysis of 5 early-type stars in the young open cluster IC2391
|
It is unclear whether chemically peculiar stars of the upper main sequence
represent a class completely distinct from normal A-type stars, or whether
there exists a continuous transition from the normal to the most peculiar late
F- to early B-type stars. A systematic abundance analysis of open cluster
early-type stars would help to relate the observed differences of the chemical
abundances of the photospheres to other stellar characteristics, without being
concerned by possible different original chemical composition. Furthermore, if
a continuous transition region from the very peculiar to the so called normal
A-F stars exists, it should be possible to detect objects with mild
peculiarities. As a first step of a larger project, an abundance analysis of 5
F-A type stars in the young cluster IC2391 was performed using high resolution
spectra obtained with the UVES instrument of the ESO VLT. Our targets seem to
follow a general abundance pattern: close to solar abundance of the light
elements and iron peak elements, heavy elements are slightly overabundant with
respect to the sun, similar to what was found in previous studies of normal
field A-type stars of the galactic plane. We detected a weakly chemically
peculiar star, HD74044. Its element pattern contains characteristics of CP1 as
well as CP2 stars, enhanced abundances of iron peak elements and also higher
abundances of Sc, Y, Ba and Ce. We did not detect a magnetic field in this star
(detection limit was 2kG). We also studied the star SHJM2, proposed as a
pre-main sequence object in previous works. Using spectroscopy we found a high
surface gravity, which suggests that the star is very close to the ZAMS.
|
0604100v1
|
2006-06-20
|
Diffuse, Non-Thermal X-ray Emission from the Galactic Star Cluster Westerlund 1
|
We present the diffuse X-ray emission identified in Chandra observations of
the young, massive Galactic star cluster Westerlund 1. After removing
point-like X-ray sources down to a completeness limit of 2e31 erg/s, we
identify 3e34 erg/s (2--8 keV) of diffuse emission. The spatial distribution of
the emission can be described as a slightly-elliptical Lorentzian core with a
half-width half-maximum along the major axis of 25+/-1", similar to the
distribution of point sources in the cluster, plus a 5' halo of extended
emission. The spectrum of the diffuse emission is dominated by a hard continuum
component that can be described as a kT>3 keV thermal plasma that has a low
iron abundance (<0.3 solar), or as non-thermal emission that could be stellar
light that is inverse-Compton scattered by MeV electrons. Only 5% of the flux
is produced by a kT=0.7 keV plasma. The low luminosity of the thermal emission
and the lack of a 6.7 keV iron line suggests that <40,000 unresolved stars with
masses between 0.3 and 2 Msun are present in the cluster. Moreover, the flux in
the diffuse emission is a factor of two lower than would be expected from a
supersonically-expanding cluster wind, and there is no evidence for thermal
remnants produced by supernovae. Less than 1e-5 of the mechanical luminosity of
the cluster is dissipated as 2--8 keV X-rays, leaving a large amount of energy
that either is radiated at other wavelengths, is dissipated beyond the bounds
of our image, or escapes into the intergalactic medium.
|
0606492v1
|
2006-08-06
|
The magnetic Bp star 36 Lyncis, II. A spectroscopic analysis of its co-rotating disk
|
We report on the physical properties of the disk-like structure of B8 IIIp
star 36 Lyncis from line syntheses of phase-resolved, high resolution spectra
obtained from the IUE archives and from newly obtained ground-based H$\alpha$
spectra. This disk is highly inclined to the rotational axis and betrays its
existence every half rotation cycle as one of two opposing sectors pass in
front of the star. Although the disk absorption spectrum is at least ten times
too weak to be visible in optical iron lines during these occultations, its
properties can be readily examined in a large number of UV "iron curtain" lines
because of their higher opacities. The analysis of the variations of the UV
resonance lines brings out some interesting details about the radiative
properties of the disks: (1) they are optically thick in the C IV and Si IV
doublets, (2) the range of excitation of the UV resonance lines is larger at
the primary occultation ($\phi$ = 0.00) than at the secondary one, and (3) the
{\bf relative strengths of the absorption peaks} for the two occultations
varies substantially from line to line. We have modeled the absorptions of the
UV C IV resonance and H$\alpha$ absorptions by means of a simulated disk with
opaque and translucent components. Our simulations suggest that a gap separates
the star and the inner edge of the disk. The disk extends radially out to
$\geq$10 R$_{*}$. The disk scale height perpendicular to the plane is
$\approx$1R$_{*}$. However, the sector causing the primary occultation is about
four times thicker than the opposite sector. The C IV scattering region extends
to a larger height than the H$\alpha$ region does, probably because it results
from shock heating far from the cooler disk plane.
|
0608134v1
|
2006-09-14
|
Evidence for relativistic features in the X-ray spectrum of Mrk 335
|
We present an analysis of hard X-ray features in the spectrum of the bright
Sy 1 galaxy Mrk 335 observed by the XMM-Newton satellite. Our analysis confi
rms the presence of a broad, ionised iron Kalpha emission line in the spectrum,
first found by Gondoin et al. The broad line can be modeled successfully by
relativistic accretion disc reflection models. This interpretation is unusually
robust in the case of Mrk 335 because of the lack of any ionised (`` warm'')
absorber and the absence a clear narrow core to the line. Partial covering by
neutral gas cannot, however, be ruled out statistically as the origin of the
broad residuals. Regardless of the underlyin g continuum we report, for the
first time in this source, the detection of a narrow absorption feature at the
rest frame energy of ~5.9 keV. If the feature is identified with a resonance
absorption line of iron in a highly ionised medium, the redshift of the line
corresponds to an inflow velocity of ~0.11 -0.15 c. We present a simple model
for the inflow, accounting approximate ly for relativistic and radiation
pressure effects, and use Monte Carlo methods to compute synthetic spectra for
qualitative comparison with the data. This mode ling shows that the absorption
feature can plausibly be reproduced by infalling gas providing that the feature
is identified with Fe xxvi. We require the inflowing gas to extend over a
limited range of radii at a few tens of rg to match the observed feature. The
mass accretion rate in the flow correspond s to 60% of the Eddington limit, in
remarkable agreement with the observed rate . The narrowness of the absorption
line tends to argue against a purely gravitational origin for the redshift of
the line, but given the current data quality we stress that such an
interpretation cannot be ruled out.
|
0609414v1
|
2006-09-20
|
Type Ia Supernova Light Curves
|
The diversity of Type Ia supernova (SN Ia) photometry is explored using a
grid of 130 one-dimensional models. It is shown that the observable properties
of SNe Ia resulting from Chandrasekhar-mass explosions are chiefly determined
by their final composition and some measure of ``mixing'' in the explosion. A
grid of final compositions is explored including essentially all combinations
of 56Ni, stable ``iron'', and intermediate mass elements that result in an
unbound white dwarf. Light curves (and in some cases spectra) are calculated
for each model using two different approaches to the radiation transport
problem. Within the resulting templates are models that provide good
photometric matches to essentially the entire range of observed SNe Ia. On the
whole, the grid of models spans a wide range in B-band peak magnitudes and
decline rates, and does not obey a Phillips relation. In particular, models
with the same mass of 56Ni show large variations in their light curve decline
rates. We identify the physical parameters responsible for this dispersion, and
consider physically motivated ``cuts'' of the models that agree better with the
Phillips relation. For example, models that produce a constant total mass of
burned material of 1.1 +/- Msun do give a crude Phillips relation, albeit with
much scatter. The scatter is further reduced if one restricts that set to
models that make 0.1 to 0.3 Msun of stable iron and nickel isotopes, and then
mix the ejecta strongly between the center and 0.8 Msun. We conclude that the
supernovae that occur most frequently in nature are highly constrained by the
Phillips relation and that a large part of the currently observed scatter in
the relation is likely a consequence of the intrinsic diversity of these
objects.
|
0609562v1
|
2006-10-09
|
First stars VII. Lithium in extremely metal poor dwarfs
|
Aims. This study aims to determine the level and constancy of the Spite
plateau as definitively as possible from homogeneous high-quality VLT-UVES
spectra of 19 of the most metal-poor dwarf stars known. Methods. Our
high-resolution (R ~ 43000), high S/N spectra are analysed with OSMARCS 1D LTE
model atmospheres and turbospectrum synthetic spectra to determine effective
temperatures, surface gravities, and metallicities, as well as Li abundances
for our stars. Results. Eliminating a cool subgiant and a spectroscopic binary,
we find 8 stars to have -3.5 < [Fe/H] < -3.0 and 9 stars with -3.0 < [Fe/H] <
-2.5. Our best value for the mean level of the plateau is A(Li) =2.10 +- 0.09.
The scatter around the mean is entirely explained by our estimate of the
observational error and does not allow for any intrinsic scatter in the Li
abundances. In addition, we conclude that a systematic error of the order of
200 K in any of the current temperature scales remains possible. The iron
excitation equilibria in our stars support our adopted temperature scale, which
is based on a fit to wings of the Halpha line, and disfavour hotter scales,
which would lead to a higher Li abundance, but fail to achieve excitation
equilibrium for iron. Conclusions. We confirm the previously noted discrepancy
between the Li abundance measured in extremely metal-poor turnoff stars and the
primordial Li abundance predicted by standard Big-Bang nucleosynthesis models
adopting the baryonic density inferred from WMAP. We discuss recent work
explaining the discrepancy in terms of diffusion and find that uncertain
temperature scales remain a major question. (abridged)
|
0610245v2
|
2006-10-10
|
A longer XMM-Newton look at I Zwicky 1: Variability of the X-ray continuum, absorption, and iron Kalpha line
|
We present the second XMM-Newton observation (85 ks) of the narrow-line
Seyfert 1 galaxy (NLS1) I Zw 1 and describe its mean spectral and timing
characteristics. On average, I Zw 1 is ~35 per cent dimmer in 2005 than in the
shorter (20 ks) 2002 observation. Between the two epochs the intrinsic
absorption column density diminished, but there were also subtle changes in the
continuum shape. Considering the blurred ionised reflection model, the
long-term changes can be associated with a varying contribution of the power
law component relative to the total spectrum. Examination of normalised light
curves indicates that the high-energy variations are quite structured and that
there are delays, but only in some parts of the light curve. Interestingly, a
hard X-ray lag first appears during the most-distinct structure in the mean
light curve, a flux dip ~25 ks into the observation. The previously discovered
broad, ionised Fe Ka line shows significant variations over the course of the
2005 observation. The amplitude of the variations is 25-45 per cent and they
are unlikely due to changes in the Fe ka-producing region, but perhaps arise
from orbital motion around the black hole or obscuration in the broad iron
line-emitting region. The 2002 data are re-examined for variability of the Fe
Ka line at that epoch. There is evidence of energy and flux variations that are
associated with a hard X-ray flare that occurred during that observation.
|
0610283v2
|
2006-10-13
|
Revealing the High Energy Emission from the Obscured Seyfert Galaxy MCG -5-23-16 with Suzaku
|
We report on a 100 ks Suzaku observation of the bright, nearby (z=0.008486)
Seyfert 1.9 galaxy MCG -5-23-16. The broad-band (0.4-100 keV) X-ray spectrum
allows us to determine the nature of the high energy emission with little
ambiguity. The X-ray continuum consists of a cutoff power-law of photon index
$\Gamma=1.9$, absorbed through Compton-thin matter of column density $N_{\rm
H}=1.6\times10^{22}$ cm$^{-2}$. A soft excess is observed below 1 keV and is
likely a combination of emission from scattered continuum photons and distant
photoionized gas. The iron K line profile is complex, showing narrow neutral
iron K$\alpha$ and K$\beta$ emission, as well as a broad line which can be
modeled by a moderately inclined accretion disk. The line profile shows either
the disk is truncated at a few tens of gravitational radii, or the disk
emissivity profile is relatively flat. A strong Compton reflection component is
detected above 10 keV, which is best modeled by a combination of reflection off
distant matter and the accretion disk. The reflection component does not appear
to vary. The overall picture is that this Seyfert 1.9 galaxy is viewed at
moderate (50 degrees) inclination through Compton-thin matter at the edge of a
Compton-thick torus covering $2\pi$ steradians, consistent with unified models.
|
0610434v1
|
2006-11-15
|
The broad band spectrum and variability of NGC 4151 observed by BeppoSAX
|
We present an analysis of all SAX observations of NGC 4151. This source was
observed 5 times from 1996 to 2001 with durations ranging from a day to four
days. The intrinsic continuum, is absorbed at low energies by a complex system:
a cold patchy absorber plus a warm uniform screen photoionized by the central
continuum. We find that this dual absorber is the main driver of the observed
variability, up to a factor of eight, at 3 keV. In particular the covering
fraction of the cold absorber changes on time scales of the order of a day,
supporting its association with the Broad Line Region. The column density of
the warm gas varies on a longer time scale (months to year). Some of the small
amplitude spectral variability above 10 keV can be explained with an intrinsic
variation (\Delta\Gamma~0.2).The flux below 1 keV remains constant confirming
an extended origin. Its spectrum is reproduced by a combination of a thermal
component and a power law with the same slope as the intrinsic continuum but
with an intensity a few per cent. A Compton reflection component is
significantly detected in 1996 (averaged value of \Omega/2\pi ~0.4), with
intensity decreasing on time scale of year, and it desappears in 2000 and 2001.
The long time scale of variations argues for an association with an optically
thick torus at a distance of few light years. An iron line was detected in all
spectra. Its energy is consistent with fluorescence by cold iron. We find that
the line is variable. Its behaviour is reproduced by a variable component
proportional to the level of the reflection flux plus a constant component. The
flux of the latter is consistent with the extended line emission observed by
Chandra. We conclude that the first component is likely arising from the torus
and the second is produced in the extended Narrow Line Region.
|
0611470v1
|
2006-12-18
|
An XMM-Newton view of the X-ray flat radio-quiet quasar PG 1416-129
|
(abridged) The radio-quiet quasar PG1416-129 (z=0.129) exhibits atypical
optical and X-ray properties. Between 1990 and 2000, in response to its optical
continuum decrease, the ``classical'' broad component of Hbeta almost
completely disappeared, with a factor of 10 decrease in the line flux. In the
X-ray band, this object was observed by Ginga in 1988 to have the hardest
quasar photon index, with Gamma=1.1+/-0.1. We present an XMM/EPIC observation
of PG1416-129 performed in July 2004. We analyze the time-averaged pn spectrum
of this quasar, as well as perform time-resolved spectroscopy. We find that
during the present XMM observation, PG1416-129 still has a rather hard photon
index, both in the soft and hard energy ranges, compared to radio-quiet quasars
but compatible with the photon index value found for radio-loud quasars. This
object also shows long-term luminosity variability over 16 years by a factor of
three with a variation of photon index from ~1.2 to ~1.8. In the soft energy
band (0.2-2keV), we found a very weak soft X-ray excess compared to other RQ
quasars. The whole time averaged spectrum is fit very well either by X-ray
ionized reflection from the accretion disk surface, by a warm absorber-emitter
plus power-law, or by a smeared absorption/emission from a relativistic
outflow. While no constant narrow FeK line at 6.4keV is observed, we find the
possible presence of two non-simultaneous transient iron lines: a redshifted
narrow iron line at about 5.5keV (96.4% confidence level according to
multi-trial Monte-Carlo simulations) at the beginning of this observation and
the appearance of a line at 6.3-6.4keV (99.1% c.l.) at the end of the
observation. These variable lines could be generated by discrete hot-spots on
the accretion disk surface.
|
0612485v1
|
2006-12-21
|
Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits
|
To aid in the physical interpretation of planetary radii constrained through
observations of transiting planets, or eventually direct detections, we compute
model radii of pure hydrogen-helium, water, rock, and iron planets, along with
various mixtures. Masses ranging from 0.01 Earth masses to 10 Jupiter masses at
orbital distances of 0.02 to 10 AU are considered. For hydrogen-helium rich
planets, our models are the first to couple planetary evolution to stellar
irradiation over a wide range of orbital separations (0.02 to 10 AU) through a
non-gray radiative-convective equilibrium atmosphere model. Stellar irradiation
retards the contraction of giant planets, but its effect is not a simple
function of the irradiation level: a planet at 1 AU contracts as slowly as a
planet at 0.1 AU. For hydrogen-helium planets, we consider cores up to 90% of
the total planet mass, comparable to those of Uranus and Neptune. If "hot
Neptunes" have maintained their original masses and are not remnants of more
massive planets, radii of 0.30-0.45 times Jupiter's radius are expected. Water
planets are ~40-50% larger than rocky planets, independent of mass. Finally, we
provide tables of planetary radii at various ages and compositions, and for
ice-rock-iron planets we fit our results to analytic functions, which will
allow for quick composition estimates, given masses and radii, or mass
estimates, given only planetary radii. These results will assist in the
interpretation of observations for both the current transiting planet surveys
as well as upcoming space missions, including CoRoT and Kepler.
|
0612671v4
|
2007-01-05
|
A High-Abundance Arc in the Compact Group of Galaxies HCG 62: An AGN- or Merger-Induced Metal Outflow?
|
We present a Chandra study of the metal distribution in the X-ray bright
compact group of galaxies HCG 62. We find that the diffuse X-ray emission is
peaked at the core of the central galaxy NGC 4778, and is dominated by the
contribution of the hot gas. The diffuse emission is roughly symmetric within
\simeq0.25^{\prime}, which is straddled by double-sided X-ray cavities aligned
in the northeast-southwest direction. By mapping the emission hardness ratio
distributions and by performing the 2-dimensional spectral analysis, we
identify a remarkable high-abundance arc region at about 2^{\prime}
(33.6h_{70}^{-1} kpc) from the X-ray peak that spans over a vast region from
south to northwest, a part of which roughly coinciding with the outer edge of
the southwest X-ray cavity. The measured average abundance in this arc is
higher than that in its neighboring regions by a factor of about 2, and the
abundance ratios therein are nicely consistent with the dominance of the SN Ia
yields. We estimate that the mass of iron contained in the arc is >3\times
10^{6}h_{70}^{-2.5} solar mass, which accounts for >3% of the iron synthesized
in the galaxy. The high-abundance arc could have been formed by the AGN
activities. However, it is also possible that the arc was formed in a recent
merger as is implied by the recent optical kinematic study (Spavone et al.
2006), which implies that mergers may be as important as AGN activities in
metal redistributions in early-type galaxies and their associated groups or
clusters.
|
0701119v1
|
2007-03-16
|
Coronal Abundances in Orion Nebula Cluster Stars
|
Following the Chandra Orion Ultradeep Project (COUP) observation, we have
studied the chemical composition of the hot plasma in a sample of 146 X-ray
bright pre-main sequence stars in the Orion Nebula Cluster. We report
measurements of individual element abundances for a subsample of 86
slightly-absorbed and bright X-ray sources, using low resolution X-ray spectra
obtained from the Chandra ACIS instrument. The X-ray emission originates from a
plasma with temperatures and elemental abundances very similar to those of
active coronae in older stars. A clear pattern of abundances vs. First
Ionization Potential (FIP) is evident if solar photospheric abundances are
assumed as reference. The results are validated by extensive simulations. The
observed abundance distributions are compatible with a single pattern of
abundances for all stars, although a weak dependence on flare loop size may be
present. The abundance of calcium is the only one which appears to vary
substantially between stars, but this quantity is affected by relatively large
uncertainties. The ensemble properties of the X-ray bright COUP sources confirm
that the iron in the emitting plasma is underabundant with respect to both the
solar composition and to the average stellar photospheric values. Comparison of
the present plasma abundances with those of the stellar photospheres and those
of the gaseous component of the nebula, indicates a good agreement for all the
other elements with available measurements, and in particular for the high-FIP
elements (Ne, Ar, O, and S) and for the low-FIP element Si. We conclude that
there is evidence of a significant chemical fractionation effect only for iron,
which appears to be depleted by a factor 1.5--3 with respect to the stellar
composition.
|
0703439v1
|
2002-03-15
|
Distribution of carbon atoms in iron-carbon fcc phase: an experimental and theoretical study
|
This paper presents an experimental and theoretical study of the distribution
of carbon atoms in the octahedral interstitial sites of the face-centered cubic
(fcc) phase of the iron-carbon system. The experimental part of the work
consists of Mossbauer measurements in Fe-C alloys with up to about 12 atomic
percent C, which are interpreted in terms of two alternative models for the
distribution of C atoms in the interstitial sites. The theoretical part
combines an analysis of the chemical potential of C based on the quasichemical
approximation to the statistical mechanics of interstitial solutions, with
three-dimensional Monte Carlo simulations. The latter were performed by
assuming a gas like mixture of C atoms and vacancies (Va) in the octahedral
interstitial sites. The number of C-C, C-Va and Va-Va pairs calculated using
Monte Carlo simulations are compared with those given by the quasichemical
model. Furthermore, the relative fraction of the various Fe environments were
calculated and compared with those extracted from the Mossbauer spectra. The
simulations reproduce remarkably well the relative fractions obtained assuming
the Fe(8)C(1-y) model for Mossbauer spectra, which includes some blocking of
the nearest neighbour interstitial sites by a C atom. With the new experimental
and theoretical information obtained in the present study, a critical
discussion is reported of the extent to which such blocking effect is accounted
for in current thermodynamic models of the Fe-C fcc phase.
Abstract PACS Codes: 2.70.Uu, 76.
|
0203327v1
|
1998-09-16
|
Slow Magnetic Monopole: Interaction with Matter and New Possibility of Their Detection
|
The possibility of existence of a magnetic monopole has been surveyed by P.
Dirac even in 1931, and then from the point of view of the modern theory by
A.M. Polyakov and G.~ 'tHooft in 1974. Numerous and unsuccessful attempts of
experimental search for monopole in cosmic rays and on accelerators in high
energy particle collisions have been done. Also the searches have been carried
out in mica for monopole tracks as well as for relict monopoles, entrapped by
ferromagnetic inclusions in iron-ores, moon rock and meteorites. These
entrapped monopoles, when released, would have the lowest velocities
$\beta<10^{-6}$ and do not yield ionization at all, and are hard to detect.
Therefore it is necessary to examine thoroughly the mechanisms of slow heavy
monopole interaction with matter and their scale of energy loss.
We discuss here the interaction of a massive slow magnetic monopole with
magnetically ordered matter, with conductors, superconductors and with
condensed matter in general. Our results indicate that the energy loss of a
slow supermassive monopole reach $10^{8} eV/cm$ and more if we take into
consideration the Cherenkov radiation of magnons or phonons and conductivity of
the media. A new method of search for cosmic and relict monopoles by
magnetically ordered film is considered too. This approach resembles the
traditional method of nuclear emulsion chamber. Apparently the proposed method
is particularly attractive for detection of relict monopoles, released from
melting iron ore.
|
9809420v3
|
2004-06-08
|
Three-Neutrino Oscillations of Atmospheric Neutrinos, theta13, Neutrino Mass Hierarchy and Iron Magnetized Detectors
|
We derive predictions for the Nadir angle (theta_n) dependence of the ratio
Nmu-/Nmu+ of the rates of the mu- and mu+ multi-GeV events, and for the mu- -
mu+ event rate asymmetry, A_{mu-mu+}=[Nmu- - Nmu+]/[Nmu- + Nmu+], in
iron-magnetized calorimeter detectors (MINOS, INO) in the case of 3-neutrino
oscillations of the atmospheric nu_mu and antinu_mu, driven by one neutrino
mass squared difference, |Delta m^2_{31}| >> Delta m^2_{21}. The asymmetry
A_{mu- mu+} (the ratio Nmu-/Nmu+) is shown to be particularly sensitive to the
Earth matter effects in the atmospheric neutrino oscillations, and thus to the
values of sin^2(theta13) and sin^2(theta23), theta13 and theta23 being the
neutrino mixing angles limited by the CHOOZ and Palo Verde experiments and that
responsible for the dominant atmospheric nu_mu -> nu_tau (antinu_mu ->
antinu_tau) oscillations. It is also very sensitive to the type of neutrino
mass spectrum which can be with normal (Delta m^2_{31}>0) or with inverted
(Delta m^2_{31} < 0) hierarchy. We find that for sin^2(theta23)>0.50, sin^2(2
theta13)>0.06 and Delta m^2_{31}=(2-3) 10^{-3} eV^2, the Earth matter effects
produce a relative difference between the integrated asymmetries barA_{mu- mu+}
and barA^{2nu}_{mu- mu+}$ in the mantle (cos(theta_n)=0.30-0.84) and core
(cos(theta_n)=0.84-1.0) bins, which is bigger in absolute value than ~15%, can
reach the values of (30-35)%, and thus can be sufficiently large to be
observable. The sign of the indicated asymmetry difference is anticorrelated
with the sign of Delta m^2_{31}. An observation of the Earth matter effects in
the Nadir angle distribution of the asymmetry A_{mu- mu+} (ratio Nmu-/Nmu+)
would clearly indicate that sin^2(2 theta13)>0.06 and sin^2(theta23)>0.50, and
would lead to the determination of the sign of Delta m^2_{31}.
|
0406096v1
|
2007-05-16
|
Viscous Torque and Dissipation in the Inner Region of a Thin Accretion Disk: Implications for Measuring Black Hole Spin
|
We consider a simple Newtonian model of a steady accretion disk around a
black hole. The model is based on height-integrated hydrodynamic equations,
alpha-viscosity, and a pseudo-Newtonian potential that results in an innermost
stable circular orbit (ISCO) that closely approximates the one predicted by GR.
We find that the hydrodynamic models exhibit increasing deviations from the
standard disk model of Shakura & Sunyaev as disk thickness H/R or the value of
alpha increases. The latter is an analytical model in which the viscous torque
is assumed to vanish at the ISCO. We consider the implications of the results
for attempts to estimate black hole spin by using the standard disk model to
fit continuum spectra of black hole accretion disks. We find that the error in
the spin estimate is quite modest so long as H/R < 0.1 and alpha < 0.2. At
worst the error in the estimated value of the spin parameter is 0.1 for a
non-spinning black hole; the error is much less for a rapidly spinning hole. We
also consider the density and disk thickness contrast between the gas in the
disk and that inside the ISCO. The contrast needs to be large if black hole
spin is to be successfully estimated by fitting the relativistically-broadened
X-ray line profile of fluorescent iron emission from reflection off an
accretion disk. In our hydrodynamic models, the contrast in density and
thickness is low when H/R>0.1, sugesting that the iron line technique may be
most reliable in extemely thin disks. We caution that these results have been
obtained with a viscous hydrodynamic model and need to be confirmed with MHD
simulations of radiatively cooled thin disks.
|
0705.2244v3
|
2007-06-08
|
The Merger in Abell 576: A Line of Sight Bullet Cluster?
|
Using a combination of Chandra and XMM observations, we confirmed the
presence of a significant velocity gradient along the NE/E-W/SW direction in
the intracluster gas of the cluster Abell 576. The results are consistent with
a previous ASCA SIS analysis of this cluster. The error weighted average over
ACIS-S3, EPIC MOS 1 & 2 spectrometers for the maximum velocity difference is
>3.3E03 km/s at the 90% confidence level, similar to the velocity limits
estimated indirectly for the "bullet" cluster (1E0657-56). The probability that
the velocity gradient is generated by standard random gain fluctuations with
Chandra and XMM is <0.1%. The regions of maximum velocity gradient are in CCD
zones that have the lowest temporal gain variations. It is unlikely that the
velocity gradient is due to Hubble distance differences between projected
clusters (probability<~0.01%). We mapped the distribution of elemental
abundance ratios across the cluster and detected a strong chemical
discontinuity using the abundance ratio of silicon to iron, equivalent to a
variation from 100% SN Ia iron mass fraction in the West-Northwest regions to
32% in the Eastern region. The "center" of the cluster is located at the
chemical discontinuity boundary, which is inconsistent with the radially
symmetric chemical gradient found in some regular clusters, but consistent with
a cluster merging scenario. We predict that the velocity gradient as measured
will produce a variation of the CMB temperature towards the East of the core of
the cluster that will be detectable by current and near-future bolometers. The
measured velocity gradient opens for the possibility that this cluster is
passing through a near line-of-sight merger stage where the cores have recently
crossed.
|
0706.1073v1
|
2007-06-07
|
An X-ray Spectral Analysis of the Central Regions of NGC 4593
|
We present a detailed analysis of XMM-Newton EPIC-pn data for the Seyfert-1
galaxy NGC 4593. We discuss the X-ray spectral properties of this source as
well as its variations with time. The 0.5-10 keV spectrum shows significant
complexity beyond a simple power-law form, with clear evidence existing for a
"soft excess" as well as absorption by highly ionized plasma (a warm absorber)
within the central engine of this active galactic nucleus. We show that the
soft excess is best described as originating from thermal Comptonization by
plasma that is appreciably cooler than the primary X-ray emitting plasma; we
find that the form of the soft excess cannot be reproduced adequately by
reflection from an ionized accretion disk. The only measurable deviation from
the power-law continuum in the hard spectrum comes from the presence of cold
and ionized fluorescent iron-K emission lines at 6.4 and 6.97 keV,
respectively. While constraints on the ionized iron line are weak, the cold
line is found to be narrow at CCD-resolution with a flux that does not track
the temporal changes in the underlying continuum, implying an origin in the
outer radii of the accretion disk or the putative molecular torus of Seyfert
unification schemes. The X-ray continuum itself varies on all accessible time
scales. We detect a ~230-second time-lag between soft and hard EPIC-pn bands
that, if interpreted as scattering timescales within a Comptonizing disk
corona, can be used to constrain the physical size of the primary X-ray source
to a characteristic length scale of ~2 gravitational radii. Taken together, the
small implied coronal size and the large implied iron line emitting region
indicate a departure from the current picture of a "typical" AGN geometry.
|
0706.1083v1
|
2007-06-15
|
High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216
|
LS V +4621 is the DAO-type central star of the planetary nebula Sh 2-216. We
perform a comprehensive spectral analysis of high-resolution, high-S/N
ultraviolet observations obtained with FUSE and STIS aboard the HST as well as
the optical spectrum of LS V +4621 by means of state-of-the-art NLTE
model-atmosphere techniques in order to compare its photospheric properties to
theoretical predictions from stellar evolution theory as well as from diffusion
calculations.
From the N IV - NV, O IV - O VI, Si IV - Si V, and Fe V - Fe VII ionization
equilibria, we determined an effective temperature of 95 +/- 2 kK with high
precision. The surface gravity is log g = 6.9 +/- 0.2. An unexplained
discrepancy appears between the spectroscopic distance d = 224 +46/-58 pc and
the parallax distance d = 129 +6/-5 pc of LS V +4621. For the first time, we
have identified Mg IV and Ar VI absorption lines in the spectrum of a
hydrogen-rich central star and determined the Mg and Ar abundances as well as
the individual abundances of iron-group elements (Cr, Mn, Fe, Co, and Ni). With
the realistic treatment of metal opacities up to the iron group in the
model-atmosphere calculations, the so-called Balmer-line problem (found in
models that neglect metal-line blanketing) vanishes.
Spectral analysis by means of NLTE model atmospheres has presently arrived at
a high level of sophistication, which is now hampered largely by the lack of
reliable atomic data and accurate line-broadening tables. Strong efforts should
be made to improve upon this situation.
|
0706.2256v1
|
2007-08-09
|
Suzaku wide-band X-ray Spectroscopy of the Seyfert 2 AGN in NGC 4945
|
Suzaku observed a nearby Seyfert 2 galaxy NGC4945, which hosts one of the
brightest active galactic nuclei above 20 keV. Combining data from the X-ray
CCD camera (XIS) and the Hard X-ray Detector (HXD), the AGN intrinsic nuclear
emission and its reprocessed signals were observed simultaneously. The
intrinsic emission is highly obscured with an absorbing column of $\sim 5
\times 10^{24}$ cm$^{-2}$, and was detectable only above $\sim 10$ keV. The
spectrum below 10 keV is dominated by reflection continuum and emission lines
from neutral/ionized material. Along with a neutral iron K$\alpha$ line, a
neutral iron K$\beta$ and a neutral nickel K$\alpha$ line were detected for the
first time from this source. The neutral lines and the cold reflection
continuum are consistent with both originating in the same location. The
Compton down-scattered shoulder in the neutral Fe-K$\alpha$ line is $\sim 10%$
in flux of the narrow core, which confirms that the line originates from
reflection rather than transmission. The weakness of the Compton shoulder also
indicates that the reflector is probably seen nearly edge-on. Flux of the
intrinsic emission varied by a factor of $\sim 2$ within $\sim 20$ ks, which
requires the obscuring material to be geometrically thin. Broadband spectral
modeling showed that the solid angle of the neutral reflector is less than a
few $\times 10^{-2} \times 2\pi$. All this evidence regarding the reprocessed
signals suggests that a disk-like absorber/reflector is viewed from a near
edge-on angle.
|
0708.1201v1
|
2007-09-05
|
Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
|
We present a homogeneous photometric and spectroscopic analysis of 18 stars
along the evolutionary sequence of the metal-poor globular cluster NGC 6397
([Fe/H] = -2), from the main-sequence turnoff point to red giants below the
bump. The spectroscopic stellar parameters, in particular stellar-parameter
differences between groups of stars, are in good agreement with broad-band and
Stroemgren photometry calibrated on the infrared-flux method. The spectroscopic
abundance analysis reveals, for the first time, systematic trends of iron
abundance with evolutionary stage. Iron is found to be 31% less abundant in the
turnoff-point stars than in the red giants. An abundance difference in lithium
is seen between the turnoff-point and warm subgiant stars. The impact of
potential systematic errors on these abundance trends (stellar parameters, the
hydrostatic and LTE approximations) is quantitatively evaluated and found not
to alter our conclusions significantly. Trends for various elements (Li, Mg,
Ca, Ti and Fe) are compared with stellar-structure models including the effects
of atomic diffusion and radiative acceleration. Such models are found to
describe the observed element-specific trends well, if extra (turbulent) mixing
just below the convection zone is introduced. It is concluded that atomic
diffusion and turbulent mixing are largely responsible for the sub-primordial
stellar lithium abundances of warm halo stars. Other consequences of atomic
diffusion in old metal-poor stars are also discussed.
|
0709.0639v1
|
2007-12-20
|
Comparing Suzaku and XMM-Newton Observations of the Soft X-ray Background: Evidence for Solar Wind Charge Exchange Emission
|
We present an analysis of a pair of Suzaku spectra of the soft X-ray
background (SXRB), obtained from pointings on and off a nearby shadowing
filament in the southern Galactic hemisphere. Because of the different Galactic
column densities in the two pointing directions, the observed emission from the
Galactic halo has a different shape in the two spectra. We make use of this
difference when modeling the spectra to separate the absorbed halo emission
from the unabsorbed foreground emission from the Local Bubble (LB). The
temperatures and emission measures we obtain are significantly different from
those determined from an earlier analysis of XMM-Newton spectra from the same
pointing directions. We attribute this difference to the presence of previously
unrecognized solar wind charge exchange (SWCX) contamination in the XMM-Newton
spectra, possibly due to a localized enhancement in the solar wind moving
across the line of sight. Contemporaneous solar wind data from ACE show nothing
unusual during the course of the XMM-Newton observations. Our results therefore
suggest that simply examining contemporaneous solar wind data might be
inadequate for determining if a spectrum of the SXRB is contaminated by SWCX
emission. If our Suzaku spectra are not badly contaminated by SWCX emission,
our best-fitting LB model gives a temperature of log T = 5.98 +0.03/-0.04 and a
pressure of p/k = 13,100-16,100 cm^-3 K. These values are lower than those
obtained from other recent observations of the LB, suggesting the LB may not be
isothermal and may not be in pressure equilibrium. Our halo modeling,
meanwhile, suggests that neon may be enhanced relative to oxygen and iron,
possibly because oxygen and iron are partly in dust.
|
0712.3538v1
|
2008-01-08
|
GMASS Ultradeep Spectroscopy of Galaxies at redshift z~2. I. The stellar metallicity
|
Context: Galaxy metallicities have been measured to redshift z~2 by gas-phase
oxygen abundances of the interstellar medium using the R23 and N2 methods.
Galaxy stellar metallicities provide crucial data for chemical evolution models
but have not been assessed reliably much outside the local Universe. Aims: We
determine the iron-abundance, stellar metallicity of star-forming galaxies
(SFGs) at redshift z~2, observed as part of the Galaxy Mass Assembly ultra-deep
Spectroscopic Survey (GMASS). Methods: We compute the equivalent width of a
rest-frame mid-ultraviolet, photospheric absorption-line index, the 1978 index
found to vary monotonically with stellar metallicity by Rix and collaborators.
We normalise and combine 75 SFG spectra from the GMASS survey to produce a
spectrum corresponding to a total integration time 1652.5 hours (and a
signal-to-noise ratio ~100 for our 1.5 angstrom binning) of FORS2 spectroscopic
observations at the Very Large Telescope. Results: We measure an
iron-abundance, stellar metallicity of log (Z/Zsolar) = -0.574+/-0.159 for our
spectrum representative of a galaxy of stellar mass 9.4 x 10^9 Msolar assuming
a Chabrier IMF. We find that the R04 model SFG spectrum for log (Z/Zsolar) =
-0.699 solar metallicity provides the best description of our GMASS coadded
spectrum. For similar galaxy stellar mass, our stellar metallicity is ~0.25 dex
lower than the oxygen-abundance, gas-phase metallicity quantified by Erb and
collaborators for UV-selected star-forming galaxies at z=2. Conclusions: We
conclude that we are witnessing the establishment of a light-element
overabundance in galaxies as they are being formed at redshift z~2. Our
measurements are reminiscent of the alpha-element enhancement seen in
low-redshift, galactic bulges and early-type galaxies. (Abridged)
|
0801.1193v1
|
2008-03-10
|
X-ray absorption in distant type II QSOs
|
We present the results of the X-ray spectral analysis of an
XMM-Newton-selected type II QSO sample with z>0.5 and 0.5-10 keV flux of 0.3-33
x 10^{-14} erg/s/cm^2. The distribution of absorbing column densities in type
II QSOs is investigated and the dependence of absorption on X-ray luminosity
and redshift is studied.
We inspected 51 spectroscopically classified type II QSO candidates from the
XMM-Newton Marano field survey, the XMM-Newton-2dF wide angle survey (XWAS),
and the AXIS survey to set-up a well-defined sample with secure optical type II
identifications. Fourteen type II QSOs were classified and an X-ray spectral
analysis performed. Since most of our sources have only ~40 X-ray counts
(PN-detector), we carefully studied the fit results of the simulated X-ray
spectra as a function of fit statistic and binning method. We determined that
fitting the spectra with the Cash-statistic and a binning of minimum one count
per bin recovers the input values of the simulated X-ray spectra best. Above
100 PN counts, the free fits of the spectrum's slope and absorbing hydrogen
column density are reliable.
We find only moderate absorption (N_H=(2-10) x 10^22 cm^-2) and no obvious
trends with redshift and intrinsic X-ray luminosity. In a few cases a
Compton-thick absorber cannot be excluded. Two type II objects with no X-ray
absorption were discovered. We find no evidence for an intrinsic separation
between type II AGN and high X-ray luminosity type II QSO in terms of
absorption. The stacked X-ray spectrum of our 14 type II QSOs shows no iron
K-alpha line. In contrast, the stack of the 8 type II AGN reveals a very
prominent iron K-alpha line at an energy of ~ 6.6 keV and an EW ~ 2 keV.
|
0803.1440v1
|
2008-04-06
|
V2051 Oph after superoutburst: out-of-plane material and the superhump light source
|
We performed a detailed spectroscopic analysis of the dwarf nova V2051 Oph at
the end of its 1999 superoutburst. We studied and interpreted the simultaneous
behaviour of various emission lines.
We obtained high-resolution echelle spectroscopic data at ESO's NTT with
EMMI, covering the spectral range of 4000--7500 Angstrom. The analysis was
performed using standard IRAF tools. The indirect imaging technique of Doppler
tomography was applied, in order to map the accretion disc and distinguish
between the different emission sources.
The spectra are characterised by strong Balmer emission, together with lines
of HeI and the iron triplet FeII 42. All lines are double-peaked, but the
blue-to-red peak strength and central absorption depth vary. The primary's
velocity was found to be 84.9 km/sec. The spectrograms of the emission lines
reveal the prograde rotation of a disc-like emitting region and, for the Balmer
and HeI lines, an enhancement of the red-wing during eclipse indicates a bright
spot origin. The modulation of the double-peak separation shows a highly
asymmetric disc with non-uniform emissivity. This is confirmed by the Doppler
maps, which apart from the disc and bright spot emission also indicate an
additional region of enhanced emission in the 4th quadrant (+Vx, -Vy), which we
associate with the superhump light source. Given the behaviour of the iron
triplet and its distinct differences from the rest of the lines, we attribute
its existence to an extended gas region above the disc. Its origin can be
explained through the fluorescence mechanism.
|
0804.0898v1
|
2008-08-18
|
On the nature of unabsorbed Seyfert 2 galaxies
|
We present an analysis of six 12 um selected Seyfert 2 galaxies that have
been reported to be unabsorbed in the X-ray. By comparing the luminosities of
these galaxies in the mid-IR (12um), optical ([O III]) and hard X-ray (2-10
keV), we show that they are all under-luminous in the 2-10 keV X-ray band. Four
of the objects exhibit X-ray spectra indicative of a hard excess, consistent
with a heavily obscured X-ray component and hence a hidden nucleus. In these
objects the softer X-rays may be dominated by a strong soft scattered continuum
or contamination from the host galaxy, which is responsible for the unabsorbed
X-ray spectra observed, and accounts for the anomalously low 2-10 keV X-ray
luminosity. We confirm this assertion in NGC4501 with a Chandra observation,
which shows hard X-ray emission coincident with the nucleus, consistent with
heavy absorption, and a number of contaminating softer sources which account
for the bulk of the softer emission. We point out that such 'Compton thick'
sources need not necessarily present iron Ka emission of high EW. An example in
our sample is IRASF01475-0740, which we know must host an obscured AGN as it
hosts a HBLR seen in scattered light (Tran 2003). The X-ray spectrum is
nonetheless relatively unobscured and the iron Ka line only moderate in
strength. These observations can be reconciled if the hidden nuclear emission
is dominated by transmitted, rather than reflected X-rays, which can then be
weak compared to the soft scattered light or galactic emission. Despite these
considerations, we conclude that NGC3147 & 3660 may intrinsically lack a BLR,
confirming the recent results of Bianchi et al. (2008) for NGC3147. Neither
X-ray spectrum shows signs of hidden hard emission and both sources exhibit
X-ray variability leading us to believe we are viewing the nucleus directly.
|
0808.2385v1
|
2008-09-12
|
Absorption features in the spectra of X-ray bursting neutron stars
|
The discovery of photospheric absorption lines in XMM-Newton spectra of the
X-ray bursting neutron star in EXO0748-676 by Cottam and collaborators allows
us to constrain the neutron star mass-radius ratio from the measured
gravitational redshift. A radius of R=9-12km for a plausible mass range of
M=1.4-1.8Msun was derived by these authors. It has been claimed that the
absorption features stem from gravitationally redshifted (z=0.35) n=2-3 lines
of H- and He-like iron. We investigate this identification and search for
alternatives. We compute LTE and non-LTE neutron-star model atmospheres and
detailed synthetic spectra for a wide range of effective temperatures
(effective temperatures of 1 - 20MK) and different chemical compositions.
We are unable to confirm the identification of the absorption features in the
X-ray spectrum of EXO0748-676 as n=2-3 lines of H- and He-like iron (Fe XXVI
and Fe XXV). These are subordinate lines that are predicted by our models to be
too weak at any effective temperature. It is more likely that the strongest
feature is from the n=2-3 resonance transition in Fe XXIV with a redshift of
z=0.24. Adopting this value yields a larger neutron star radius, namely
R=12-15km for the mass range M=1.4-1.8Msun, favoring a stiff equation-of-state
and excluding mass-radius relations based on exotic matter. Combined with an
estimate of the stellar radius R>12.5km from the work of Oezel and
collaborators, the z=0.24 value provides a minimum neutron-star mass of
M>1.48Msun, instead of M>1.9Msun, when assuming z=0.35.
|
0809.2170v2
|
2008-10-02
|
Detection of blueshifted emission and absorption and a relativistic Iron line in the X-ray spectrum of ESO 323-G077
|
We report on the X-ray observation of the Seyfert 1 ESO323-G077 performed
with XMM-Newton. The spectra show a complex spectrum with conspicuous
absorption and emission features. The continuum emission can be modelled with a
power law with an index of 1.99+/-0.02 in the whole XMM-Newton energy band,
marginally consistent with typical values of Type-I objects. An absorption
component with an uncommonly high equivalent Hydrogen column,
n_H=5.82(+0.12/-0.11)x10^22 cm-2, is affecting the soft part of the spectrum.
Additionally, two warm absorption components are also present. The lower
ionised one has an ionisation parameter of Log(U)=2.14(+0.06/-0.07) and an
outflowing velocity of v=3200(+600/-200) km/s. Two absorption lines located at
~6.7 and ~7.0 keV can be modelled with the highly ionised absorber. The
ionisation parameter and outflowing velocity of the gas measured are
Log(U)=3.26(+0.19/-0.15) and v=1700(+600/-400) km/s, respectively. Four
emission lines were also detected in the soft energy band. The most likely
explanation for these emission lines is that they are associated with an
outflowing gas with a velocity of ~2000 km/s. The data suggest that the same
gas which is causing the absorption could also being responsible of these
emission features. Finally, the spectrum shows the presence of a relativistic
iron emission line likely originated in the accretion disc of a Kerr BH with an
inclination of ~25 deg. We propose a model to explain the observed X-ray
properties which invokes the presence of a two-phase outflow with cone-like
structure and a velocity of the order of 2,000-4,000 km/s. The inner layer of
the cone would be less ionised, or even neutral, than the outer layer. The
inclination angle would be lower than the opening angle of the outflowing cone.
|
0810.0513v1
|
2008-12-04
|
The evolution of the Galactic metallicity gradient from high-resolution spectroscopy of open clusters
|
Open clusters offer a unique possibility to study the time evolution of the
radial metallicity gradients of several elements in our Galaxy, because they
span large intervals in age and Galactocentric distance, and both quantities
can be more accurately derived than for field stars. We re-address the issue of
the Galactic metallicity gradient and its time evolution by comparing the
empirical gradients traced by a sample of 45 open clusters with a chemical
evolution model of the Galaxy. At variance with previous similar studies, we
have collected from the literature only abundances derived from
high--resolution spectra. The clusters have distances $7 < RGC<22$ kpc and ages
from $\sim 30$ Myr to 11 Gyr. We also consider the $\alpha$-elements Si, Ca,
Ti, and the iron-peak elements Cr and Ni. The data for iron-peak and
$\alpha$-elements indicate a steep metallicity gradient for R_GC<12$ kpc and a
plateau at larger radii. The time evolution of the metallicity distribution is
characterized by a uniform increase of the metallicity at all radii, preserving
the shape of the gradient, with marginal evidence for a flattening of the
gradient with time in the radial range 7-12 kpc. Our model is able to reproduce
the main features of the metallicity gradient and its evolution with an infall
law exponentially decreasing with radius and with a collapse time scale of the
order of 8 Gyr at the solar radius. This results in a rapid collapse in the
inner regions, i.e. $R_{\rm GC}\lesssim 12$ kpc (that we associate with an
early phase of disk formation from the collapse of the halo) and in a slow
inflow of material per unit area in the outer regions at a constant rate with
time.
|
0812.0854v1
|
2009-01-29
|
Novel Superconducting Characteristics and Unusual Normal-State Properties in Iron-based Pnictide Superconductors: 57Fe-NMR and 75As-NQR/NMR studies in REFeAsO_1-y (RE=La,Pr,Nd) and Ba0.6K0.4Fe2As2
|
We discuss the novel superconducting characteristics and unusual normal-state
properties of iron (Fe)-based pnictide superconductors REFeAsO$_{1-y}$
(RE=La,Pr,Nd) and Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$($T_{c}=$ 38 K) by means of
$^{57}$Fe-NMR and $^{75}$As-NQR/NMR. In the superconducting state of
LaFeAsO$_{0.7}$ ($T_{c}=$ 28 K), the spin component of the $^{57}$Fe-Knight
shift decreases to almost zero at low temperatures, which provide firm evidence
of the superconducting state formed by spin-singlet Cooper pairing. The nuclear
spin-lattice relaxation rates $(1/T_{1})$ in LaFeAsO$_{0.7}$ and
Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ exhibit a $T^{3}$-like dependence without a
coherence peak just below $T_{c}$, indicating that an unconventional
superconducting state is commonly realized in these Fe-based pnictide
compounds. All these events below $T_c$ are consistently argued in terms of an
extended s$_{\pm}$-wave pairing with a sign reversal of the order parameter
among Fermi surfaces. In the normal state, $1/T_1T$ decreases remarkably upon
cooling for both the Fe and As sites of LaFeAsO$_{0.7}$. In contrast, it
gradually increases upon cooling in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. Despite
the similarity between the superconducting properties of these compounds, a
crucial difference was observed in their normal-state properties depending on
whether electrons or holes are doped into the FeAs layers. These results may
provide some hint to address a possible mechanism of Fe-based pnictide
superconductors.
|
0901.4607v2
|
2009-02-06
|
Effective area calibration of the Reflection Grating Spectrometers of XMM-Newton. I. X-ray spectroscopy of the Crab nebula
|
The Crab nebula and pulsar have been widely used as a calibration source for
X-ray instruments. The in-flight effective area calibration of the Reflection
Grating Spectrometers (RGS) of XMM-Newton depend upon the availability of
reliable calibration sources. We investigate how the absolute effective area
calibration of RGS can be obtained using Crab as a standard candle. We have
analysed RGS observations of the Crab using different instrument configurations
and spatial offsets, and made use of previous determinations of the continuum
spectrum of the nebula plus pulsar. Due to the high spectral resolution of the
RGS, we resolve the main absorption edges and detect the strong 1s-2p
absorption lines of neutral oxygen. We get an excellent fit to the Crab
spectrum using this fixed continuum and the absorption spectrum determined by
RGS. We get accurate column densities for the neutral atoms of H, N, O, Ne, Mg,
and Fe, as well as a clear detection of Fe II and firm upper limits for other
ions. Our data are in good agreement with earlier optical and UV spectroscopic
measurements of some of these ions. We find solar abundances for N and O, while
Ne is overabundant by a factor of 1.7 and Fe is underabundant by a factor of
0.8. We confirm that there is less dust in the line of sight compared to the
prediction based on the absorption column. Our spectra suggest a more prominent
role of ferric iron in the dust compared to ferrous iron. Our high-resolution
observations confirm that Crab can be used as an X-ray calibration source. RGS
spectra have determined the absorption spectrum towards Crab with unprecedented
detail.
|
0902.1094v1
|
2009-04-09
|
Interplay of Rare Earth and Iron magnetism in RFeAsO with R = La, Ce, Pr, and Sm: A muon spin relaxation study and symmetry analysis
|
We report zero field muon spin relaxation (muSR) measurements on RFeAsO with
R = La, Ce, Pr, and Sm. We study the interaction of the FeAs and R (rare earth)
electronic systems in the non superconducting magnetically ordered parent
compounds of RFeAsO{1-x}Fx superconductors via a detailed comparison of the
local hyperfine fields at the muon site with available Moessbauer spectroscopy
and neutron scattering data. These studies provide microscopic evidence of long
range commensurate magnetic Fe order with the Fe moments not varying by more
than 15 % within the series RFeAsO with R = La, Ce, Pr, and Sm. At low
temperatures, long range R magnetic order is also observed. Different combined
Fe and R magnetic structures are proposed for all compounds using the muon site
in the crystal structure obtained by electronic potential calculations. Our
data point to a strong effect of R order on the iron subsystem in the case of
different symmetry of Fe and R order parameters resulting in a Fe spin
reorientation in the R ordered phase in PrFeAsO. Our symmetry analysis proves
the absence of collinear Fe--R Heisenberg interactions in RFeAsO. A strong
Fe--Ce coupling due to non--Heisenberg anisotropic exchange is found in CeFeAsO
which results in a large staggered Ce magnetization induced by the magnetically
ordered Fe sublattice far above T_N{Ce}. Finally, we argue that the magnetic
R--Fe interaction is probably not crucial for the observed enhanced
superconductivity in RFeAsO{1-x}Fx with a magnetic R ion.
|
0904.1563v3
|
2009-04-17
|
Pnictogen height as a possible switch between high-T_c nodeless and low-T_c nodal pairings in the iron based superconductors
|
We study the effect of the lattice structure on the spin-fluctuation mediated
superconductivity in the iron pnictides adopting the five-band models of
several virtual lattice structures of LaFeAsO as well as actual materials such
as NdFeAsO and LaFePO obtained from the maximally-localized Wannier orbitals.
Random phase approximation is applied to the models to solve the Eliashberg
equation. This reveals that the gap function and the strength of the
superconducting instability are determined by the cooperation or competition
among multiple spin fluctuation modes arising from several nestings among
disconnected pieces of the Fermi surface, which is affected by the lattice
structure. Specifically, the appearance of the Fermi surface $\gamma$ around
$(\pi,\pi)$ in the unfolded Brillouin zone is sensitive to the pnictogen height
$h_{\rm Pn}$ measured from the Fe plane, where $h_{\rm Pn}$ is shown to act as
a switch between high-$T_c$ nodeless and low-$T_c$ nodal pairings. We also find
that reduction of the lattice constants generally suppresses superconductivity.
We can then combine these to obtain a generic superconducting phase diagram
against the pnictogen height and lattice constant. This suggests that NdFeAsO
is expected to exhibit a fully-gapped, sign-reversing s-wave superconductivity
with a higher $T_c$ than in LaFeAsO, while a nodal pairing with a low $T_c$ is
expected for LaFePO, which is consistent with experiments.
|
0904.2612v2
|
2009-05-27
|
The detection of Broad Iron K and L line emission in the Narrow-Line Seyfert 1 Galaxy 1H0707-495 using XMM-Newton
|
Since the discovery of the first broad iron-K line in 1995 from the Seyfert
Galaxy MCG--6-30-15, broad iron-K lines have been found in several other
Seyfert galaxies, from accreting stellar mass black holes and even from
accreting neutron stars. The iron-K line is prominent in the reflection
spectrum created by the hard X-ray continuum irradiating dense accreting
matter. Relativistic distortion of the line makes it sensitive to the strong
gravity and spin of the black hole. The accompanying iron-L line emission
should be detectable when the iron abundance is high. Here we report the first
discovery of both iron-K and L emission, using XMM-Newton observations of the
Narrow-Line Seyfert 1 Galaxy 1H0707-495. The bright Fe-L emission has enabled
us, for the first time, to detect a reverberation lag of 30 s between the
direct X-ray continuum and its reflection from matter falling into the hole.
The observed reverberation timescale is comparable to the light-crossing time
of the innermost radii around a supermassive black hole. The combination of
spectral and timing data on 1H0707-495 provides strong evidence that we are
witnessing emission from matter within a gravitational radius, or a fraction of
a light-minute, from the event horizon of a rapidly-spinning, massive black
hole.
|
0905.4383v1
|
2009-06-19
|
Dynamical Mean-Field Theory within an Augmented Plane-Wave Framework: Assessing Electronic Correlations in the Iron Pnictide LaFeAsO
|
We present an approach that combines the local density approximation (LDA)
and the dynamical mean-field theory (DMFT) in the framework of the
full-potential linear augmented plane waves (FLAPW) method. Wannier-like
functions for the correlated shell are constructed by projecting local orbitals
onto a set of Bloch eigenstates located within a certain energy window. The
screened Coulomb interaction and Hund's coupling are calculated from a
first-principle constrained RPA scheme. We apply this LDA+DMFT implementation,
in conjunction with continuous-time quantum Monte-Carlo, to study the
electronic correlations in LaFeAsO. Our findings support the physical picture
of a metal with intermediate correlations. The average value of the mass
renormalization of the Fe 3d bands is about 1.6, in reasonable agreement with
the picture inferred from photoemission experiments. The discrepancies between
different LDA+DMFT calculations (all technically correct) which have been
reported in the literature are shown to have two causes: i) the specific value
of the interaction parameters used in these calculations and ii) the degree of
localization of the Wannier orbitals chosen to represent the Fe 3d states, to
which many-body terms are applied. The latter is a fundamental issue in the
application of many-body calculations, such as DMFT, in a realistic setting. We
provide strong evidence that the DMFT approximation is more accurate and more
straightforward to implement when well-localized orbitals are constructed from
a large energy window encompassing Fe-3d, As-4p and O-2p, and point out several
difficulties associated with the use of extended Wannier functions associated
with the low-energy iron bands. Some of these issues have important physical
consequences, regarding in particular the sensitivity to the Hund's coupling.
|
0906.3735v2
|
2009-07-17
|
Composition and fate of short-period super-Earths: The case of CoRoT-7b
|
The discovery of CoRoT-7b, a planet of radius 1.68 +/- 0.09 R_E, mass 4.8 +/-
0.8 M_E and orbital period of 0.854 days demonstrates that small planets can
orbit extremely close to their star. We use knowledge of hot Jupiters, mass
loss estimates and models for the interior structure and evolution of planets
to understand its composition, structure and evolution. The inferred mass and
radius of CoRoT-7b are consistent with a rocky planet that would be depleted in
iron relative to Earth. However, a one sigma increase in mass (5.6 M_E) and
decrease in size (1.59 R_E) are compatible with an Earth-like composition (33%
iron, 67% silicates). Alternatively, it is possible that CoRoT-7b contains a
significant amount of volatiles. An equally good fit to the data is found for a
vapor envelope equal to 3% (and up to 10%) by mass above an Earth-like nucleus.
Because of its intense irradiation and small size, the planet cannot possess an
envelope of H and He of more than 1/10,000 of its total mass. We show that the
mass loss is significant (~ 10^11 g/s) and independent of planetary
composition. This is because the hydrodynamical escape rate is independent of
the atmosphere's mean molecular mass, and owing to the intense irradiation,
even a bare rocky planet would possess an equilibrium vapor atmosphere thick
enough to capture stellar UV photons. This escape rate rules out the
possibility of a H-He envelope as it would escape in only ~1 Ma. A water vapor
atmosphere would escape in ~ 1 Ga, and thus it is a plausible scenario. The
origin of CoRoT-7b cannot be inferred from present observations: It may have
formed rocky; or be the remnant of a Uranus-like ice giant, or a gas giant with
a small core that was stripped of its gaseous envelope.
|
0907.3067v2
|
2009-08-17
|
Role of emission angular directionality in spin determination of accreting black holes with broad iron line
|
Spin of an accreting black hole can be determined by spectroscopy of the
emission and absorption features produced in the inner regions of an accretion
disc. We discuss the method employing the relativistic line profiles of iron in
the X-ray domain, where the emergent spectrum is blurred by general
relativistic effects. Precision of spectra fitting procedure could be
compromised by inappropriate account of the angular distribution of the disc
emission. Often a unique profile is assumed, invariable over the entire range
of radii in the disc and energy in the spectral band. We study how sensitive
the spin determination is to the assumptions about the intrinsic angular
distribution of the emitted photons. We find that the uncertainty of the
directional emission distribution translates to 20% uncertainty in
determination of the marginally stable orbit. By assuming a rotating black hole
in the centre of an accretion disc, we perform radiation transfer computations
of an X-ray irradiated disc atmosphere to determine the directionality of
outgoing X-rays in the 2-10 keV energy band. We implemented the simulation
results as a new extension to the KY software package for X-ray spectra fitting
of relativistic accretion disc models. Although the parameter space is rather
complex, leading to a rich variety of possible outcomes, we find that on
average the isotropic directionality reproduces our model data to the best
precision. Our results also suggest that an improper usage of limb darkening
can partly mimic a steeper profile of radial emissivity. We demonstrate these
results on the case of XMM-Newton observation of the Seyfert galaxy
MCG-6-30-15, for which we construct confidence levels of chi squared
statistics, and on the simulated data for the future X-ray IXO mission.
|
0908.2387v4
|
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