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1999-05-05
|
Gamma Doradus Stars: Defining a New Class of Pulsating Variables
|
In this paper we describe a new class of pulsating stars, the prototype of
which is the bright, early, F-type dwarf, Gamma Doradus. These stars typically
have between 1 and 5 periods ranging from 0.4 to 3 days with photometric
amplitudes up to 0.1 in Johnson V. The mechanism for these observed variations
is high-order, low-degree, non-radial, gravity-mode pulsation. Gamma Doradus
stars exhibit variability on a time scale that is an order of magnitude slower
than Delta Scuti stars. They may offer additional insight into stellar physics
when they are better understood (e.g., they may represent the cool portion of
an "iron opacity instability strip" currently formed by the Beta Cephei stars,
the SPB stars, and the subdwarf B stars; they may also offer insight into the
presence of g-modes in solar-like stars).
|
9905042v1
|
1999-05-20
|
The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star
|
We calculate the nucleosynthesis during stable nuclear burning on an
accreting neutron star. This is appropriate for weakly magnetic neutron stars
accreting at near-Eddington rates in low mass X-ray binaries, and for most
accreting X-ray pulsars. We show that the nuclear burning proceeds via the
rapid proton capture process (rp process), and makes nuclei far beyond the iron
group. The final mixture of nuclei consists of elements with a range of masses
between approximately A=60 and A=100. The average nuclear mass of the ashes is
set by the extent of helium burning via (alpha,p) reactions, and depends on the
local accretion rate.
Our results imply that the crust of these accreting neutron stars is made
from a complex mixture of heavy nuclei, with important implications for its
thermal, electrical and structural properties. A crustal lattice as impure as
our results suggest will have a conductivity set mostly by impurity scattering,
allowing more rapid Ohmic diffusion of magnetic fields than previously
estimated.
|
9905274v1
|
1999-05-26
|
X-ray and radio observations of RX J1826.2-1450/LS 5039
|
RX J1826.2-1450/LS 5039 has been recently proposed to be a radio emitting
high mass X-ray binary. In this paper, we present an analysis of its X-ray
timing and spectroscopic properties using different instruments on board the
RXTE satellite. The timing analysis indicates the absence of pulsed or periodic
emission on time scales of 0.02-2000 s and 2-200 d, respectively. The source
spectrum is well represented by a power-law model, plus a Gaussian component
describing a strong iron line at 6.6 keV. Significant emission is seen up to 30
keV, and no exponential cut-off at high energy is required. We also study the
radio properties of the system according to the GBI-NASA Monitoring Program. RX
J1826.2-1450/LS 5039 continues to display moderate radio variability with a
clearly non-thermal spectral index. No strong radio outbursts have been
detected after several months.
|
9905344v1
|
1999-06-09
|
A Method of Mass Measurement in Black Hole Binaries Using Timing and High Resolution X-ray Spectroscopy
|
In X-ray binaries, several percent of the compact object luminosity is
intercepted by the surface of the normal companion and re-radiated through
Compton reflection and the K-fluorescence. This reflected emission follows the
variability of the compact object with a delay approximately equal to the
orbital radius divided by the speed of light. This provides the possibility of
measuring the orbital radius and thus substantially refining the compact object
mass determination compared to using optical data alone. We demonstrate that it
may be feasible to measure the time delay between the direct and reflected
emission using cross-correlation of the light curves observed near the Kalpha
line and above the K-edge of neutral iron. In the case of Cyg X-1, the time
delay measurement is feasible with a 300--1000 ksec observation by a telescope
with a 1000 cm^2 effective area near 6.4 keV and with a ~5eV energy resolution.
With longer exposures, it may be possible to obtain mass constraints even if an
X-ray source in the binary system lacks an optical counterpart.
|
9906170v1
|
1999-07-08
|
An Ultraviolet Fe II Image of SN 1885 in M31
|
Ultraviolet imaging of the remnant of Supernova 1885 in M31 with the Hubble
Space Telescope using the F255W filter on the WFPC2 reveals a dark spot of Fe
II absorption at the remnant's known position in the bulge of M31. The diameter
of the absorbing spot is 0"55 +- 0"15, slightly smaller than, but consistent
with, the 0"70 +- 0"05 diameter measured in the higher quality WFPC2 Ca II
absorption image previously reported by us. The measured ratio of flux inside
to outside SNR 1885 in the Fe II image is 0.24 +- 0.17, consistent with the
ratio 0.33 +- 0.04 expected on the basis of a model fit to the previously
obtained near-UV FOS spectrum. The observed depth of Fe II absorption suggests
that Fe II is fully saturated, with an iron mass in the range M_Fe = 0.1-1.0
Msun. Besides Fe, ion species Mg I, Mg II, and Mn I probably make some
contribution to the absorption from the SN 1885 remnant in the F255W image.
|
9907102v1
|
1999-07-14
|
X-ray dependencies on luminosity in AGN
|
Several X-ray properties of active galactic nuclei depend, or appear to
depend, on their luminosity. It has long been suggested that alpha o-x, the
X-ray ``loudness'' decreases with luminosity. There never has been a
satisfactory explanation of this observational claim, and the statistical
soundness of the result has been disputed. The earliest systematic studies of
the X-ray variability of AGN showed that these properties also depend on
luminosity. In particular, the normalization of the power spectrum, or
alternatively the the variability amplitude, are anti-correlated with
luminosity. Most recently, tentative evidence from Ginga for an X-ray Baldwin
effect - a decrease in the iron K-alpha equivalent width with luminosity - has
been confirmed and extended by ASCA. The new data show that the reduction in
strength is accompanied by changes in profile. These results will be described
and their interpretation discussed.
|
9907192v1
|
1999-07-15
|
Stroemgren photometry of globular clusters: M55 & M22
|
We present Stroemgren CCD photometry for the two galactic globular clusters
M55 (NGC 6809) and M22 (NGC 6656). We find average Stroemgren metallicities of
-1.71 dex for M55 and -1.62 dex for M22. The determination of metal abundances
in cluster giants with the Stroemgren m1 index in comparison with spectroscopic
data from Briley et al. (1993) and Norris & Freeman (1982, 1983) shows that M55
and M22 have different distributions of cyanogen strengths. In M55, no CN
abundance variations are visible among the giant-branch stars. In striking
contrast, a large dispersion of cyanogen strengths is seen in M22. For M22 we
find patchily distributed variations in the foreground reddening of
E(B-V)=0.07, which explain the colour dispersion among the giant-branch stars.
There is no evidence for a spread in iron within M22 since the variations in m1
are dominated by the large range in CN abundances, as already found by
Anthony-Twarog et al. (1995). The difference between M55 and M22 may resemble
the difference in integral CN band strength between M31 globular clusters and
the galactic system. The colour-magnitude diagram of M55 shows the presence of
a population of 56 blue-straggler stars that are more centrally concentrated
than the red giant-branch stars.
|
9907200v1
|
1999-07-15
|
Fitting the spectrum of the X-ray background: the effects of high metallicity absorption
|
Recent work by Risaliti et al.(1999) suggests that more than half of all
Seyfert 2 galaxies in the local universe are Compton-thick (N_H > 10^24 cm^-2).
This has implications for AGN synthesis models for the X-ray background (XRB),
the flexibility of which for the inclusion of large numbers of high-z type 2
sources we examine here. We highlight the importance of Compton down-scattering
in determining the individual source spectra and the fit to the XRB spectrum,
and demonstrate how parameter space opens up considerably if a super-solar iron
abundance is assumed for the absorbing material. This is illustrated with a
model which satisfies the present constraints, but which predicts substantial
numbers of type 2 source at the faint flux levels soon to be probed for the
first time by the Chandra and XMM missions. We demonstrate also how a strong
negative K-correction facilitates the detection of sources with 10^~24 < N_H <
10^25 cm^-2 out to the highest redshifts at which they could plausibly exist.
|
9907204v1
|
1999-07-27
|
Testing Narrow-line Seyfert 1 scenarios with photoionization models
|
Narrow Line Seyfert 1 (NLSy1) galaxies have recently received a lot of
attention due to their unusual optical--X-ray properties which are not yet well
understood. Photoionization models of the circum-nuclear emission/absorption
regions allow to investigate scenarios to explain the main characteristics of
NLSy1s, i.e., (i) extremely steep X-ray spectra within the ROSAT energy range,
(ii) narrow Balmer lines and (iii) weak forbidden lines except for some
relatively strong high-ionization iron lines. Here, we focus on (iii). In
particular, we study the influence of different EUV - soft-X-ray spectral
shapes (a giant soft excess, a steep X-ray powerlaw, presence of a warm
absorber) and NLR cloud properties on the predicted optical emission-line
ratios. The calculations were carried out with Ferland's code Cloudy.
|
9907373v1
|
1999-08-06
|
Iron K-alpha line profiles driven by non-axisymmetric illumination
|
Previous calculations of Fe K-alpha line profiles are based on axisymmetric
emissivity laws. In this paper, we show line profiles driven by non-axial
symmetric illumination which results from an off-axis X-ray point source. We
find that source location and motion have significant effects on the red wing
and blue horn of the line profiles. The disk region under the source will
receive more flux, which is the most important factor to affect the line
profiles. We suggest that at least part of the variation in Fe K-alpha line
profiles is caused by the motion of X-ray sources. Future observations of Fe
K-alpha line profiles will provide more information about the distribution and
motion of the X-ray sources around black holes, and hence the underlying
physics.
|
9908079v1
|
1999-08-24
|
GRB: A Signature of Phase Transition to QGP?
|
It is suggested that the inner energetic engine of Gamma ray burst (GRB) may
be the result of the transition of normal hadron to quark-gluon plasma (QGP) in
rapidly-rotating and spin-down newborn neutron star. When such a nascent
neutron star slows down through dipole electromagnetic and quadruple
gravitational radiation, the increasing center density may reach the QCD
transition density, i.e., 5-10 nuclear density. Such kind of energy release
from the phase transition would be responsible for GRB and its possible beaming
effect. The relative dense gaseous environment of GRB location and the iron
line observed in the X-ray afterglow support this idea. Some predictions in
this model are given. keywords: Gamma-ray Bursts, Supernova, Quark-gluon plasma
|
9908262v1
|
1999-09-27
|
Blue Horizontal--Branch Stars: The "Jump" in Stromgren u, Low Gravities, and Radiative Levitation of Metals
|
We study the ``jump'' in the blue horizontal--branch (BHB) distribution first
detected by Grundahl et al. (1998) in the Galactic globular cluster (GC) M13.
On the basis of Stromgren photometry for a sample of fourteen GC's we show
that: 1) The jump is best characterized as a systematic shift, on a (u, u-y)
color-magnitude diagram, from canonical zero-age HB (ZAHB) models, in the sense
that the stars appear brighter and/or hotter than the models; 2) the jump is a
ubiquitous phenomenon, ocurring over the temperature range 11,500 < Teff <
20,000K; 3) An analogous feature is present in (log g, log Teff) diagrams --
indicating a common physical origin for the two phenomena; 4) The physical
mechanism responsible for the jump phenomenon is most likely radiative
levitation of iron and other
|
9909448v1
|
1999-09-30
|
NLTE Model Atmospheres for Central Stars of Planetary Nebulae
|
Present observational techniques provide stellar spectra with high resolution
at a high signal-to-noise ratio over the complete wavelength range -- from the
far infrared to the X-ray.
NLTE effects are particularly important for hot stars, hence the use of
reliable NLTE stellar model atmosphere fluxes is required for an adequate
spectral analysis.
State-of-the-art NLTE model atmospheres include the metal-line blanketing of
millions of lines of all elements from hydrogen up to the iron-group elements
and thus permit precise analyses of extremely hot compact stars, e.g. central
stars of planetary nebulae, PG 1159 stars, white dwarfs, and neutron stars.
Their careful spectroscopic study is of great interest in several branches of
modern astrophysics, e.g. stellar and galactic evolution, and interstellar
matter.
|
9909507v1
|
1999-10-04
|
X-ray spectral components from a broad band BeppoSAX observation of the Seyfert galaxy IC 4329A
|
From the spectral analysis of a broadband (0.1-200 keV) BeppoSAX observation
of the Seyfert 1 galaxy IC 4329A, the main results obtained are: a) the amount
of reflection, together with the intensity of the iron K line, indicate a
geometry with a solid angle substantially less than 2pi; b) the power law is
affected by a cut off with e-folding energy about 270 keV, the fourth
individual object so far where this property has been firmly detected; c) two
absorption features at about 0.7 and 1 keV are found, the first corresponding
to a blend of O vi and O vii, the other to a combination of FeL and NeK edges.
Compared to an earlier ASCA observation, when the source was 30% fainter, the
values of both the relative amount of reflection and the warm absorber
ionization degree are significantly lower: the comparison is suggestive of
sizeable delay effects in this object, due to geometrical factors in the
reflection, and to relaxation to equilibrium states in the ionization of the
absorber.
|
9910054v1
|
1999-10-14
|
The nature of the hard X-ray power-law tail in M87
|
Spatially-resolved spectroscopy of the elliptical galaxy M87 with the MECS
instrument on board BeppoSAX demonstrates that the hard X-ray power-law tail,
originally discovered by ASCA (Matsumoto et al 1996; Allen et al. 1999),
originates in the innermost 2'. Our results are consistent with it being
produced in an Accretion Dominated Flow, although a substantial jet
contribution cannot be ruled out. An origin from a Seyfert-like nucleus is
disfavored by our data. As a by-product of this result, we present an analysis
of the thermal emission coming from the center of the Virgo cluster, which
exhibits a strong positive radial temperature gradient, along with a radial
decrease of the iron abundance.
|
9910271v1
|
1999-10-20
|
The r-Process Enriched Low Metallicity Giant HD 115444
|
New high resolution, very high signal-to-noise spectra of ultra-metal-poor
(UMP) giant stars HD 115444 and HD 122563 have been gathered with the
High-Resolution Echelle Spectrometer of the McDonald Observatory 2.7m
Telescope. With these spectra, line identification and model atmosphere
analyses have been conducted, emphasizing the neutron-capture elements. Twenty
elements with Z > 30 have been identified in the spectrum of HD 115444. This
star is known to have overabundances of the neutron-capture elements, but it
has lacked a detailed analysis necessary to compare with nucleosynthesis
predictions. The new study features a line-by-line differential abundance
comparison of HD 115444 with the bright, well-studied halo giant HD 122563. For
HD 115444, the overall metallicity is [Fe/H]~ -3.0. The abundances of the light
and iron-peak elements generally show the same pattern as other UMP stars (e.g.
overdeficiencies of manganese and chromium, overabundances of cobalt), but the
differential analysis indicates several nucleosynthesis signatures that are
unique to each star.
|
9910376v1
|
1999-10-27
|
Metallicity distribution of bulge planetary nebulae and the [O/Fe] x [Fe/H] relation
|
The O/H metallicity distribution of different samples of planetary nebulae in
the bulge of the Milky Way and M31 are compared. O/H abundances are converted
into [Fe/H] metallicity by the use of theoretical [O/Fe] x [Fe/H] relationships
both for the bulge and the solar neighbourhood. It is found that these
relationships imply an offset of [Fe/H] abundances by a factor up to 0.5 dex
for bulge nebulae. Systematic errors in the O/H abundances as suggested by some
recent recombination line work, ON cycling and statistical uncertainties are
unable to explain the observed offset, suggesting that the adopted relationship
for the bulge probably overestimates the oxygen enhancement relative to iron.
|
9910496v1
|
1999-11-01
|
Effects of Kerr Spacetime on Spectral Features from X-Ray Illuminated Accretion Discs
|
We performed detailed calculations of the relativistic effects acting on both
the reflection continuum and the iron line from accretion discs around rotating
black holes. Fully relativistic transfer of both illuminating and reprocessed
photons has been considered in Kerr spacetime. We calculated overall spectra,
line profiles and integral quantities, and present their dependences on the
black hole angular momentum.
We show that the observed EW of the lines is substantially enlarged when the
black hole rotates rapidly and/or the source of illumination is near above the
hole. Therefore, such calculations provide a way to distinguish among different
models of the central source.
|
9910562v1
|
1999-11-05
|
Pulsar Kick and Asymmetric Iron Velocity Distribution in SN 1987A
|
We have investigated the relation of the direction of the momentum among the
matter, neutrino, and proto-neutron star in a collapse-driven supernova in
order to discuss the pulsar kick. In particular, we have investigated the
effects of the pulsar motion on the explosion, which are neglected in the
previous study. As a result, it is suggested that the direction of the total
momentum of the matter and neutrino is opposite to that of the momentum of the
proto-neutron star in the asymmetric explosion models. This is because the
center of the explosion deviates from the center of the progenitor due to the
pulsar motion. This picture is common among the asymmetric explosion models. So
if we assume that the pulsar motion is caused by an asymmetric supernova
explosion, the neutron star born in SN 1987A, which has not been found yet,
will be moving in the southern part of the remnant. In other words, if we can
find one neutron star in SN 1987A on the south part of the remnant, asymmetric
explosion models will be supported by the observation better than the binary
models.
|
9911077v2
|
1999-12-03
|
The Type Ib/c Supernova, Gamma-Ray Burst, Soft Gamma-Ray Repeater, Magnetar Connection
|
The polarization of core-collapse supernovae shows that many if not all of
these explosions must be strongly bi-polar. The most obvious way to produce
this axial symmetry is by the imposition of a jet as an intrinsic part of the
explosion process. These jets could arise by MHD processes in the formation of
pulsars and be especially strong in the case of magnetars. The jets will blow
iron-peak material out along the axes and other elements from the progenitor
along the equator, a very different composition structure than pictured in
simple spherical "onion skin" models. In extreme cases, these processes could
lead to the production of gamma-ray bursts powered by strong Poynting flux.
|
9912080v2
|
1999-12-07
|
Spectral analysis of four multi mode pulsating sdB stars
|
Four members of the new class of pulsating sdB stars are analysed from Keck
HIRES spectra using NLTE and LTE model atmospheres. Atmospheric parameters
(Teff, log g, log(He/H)), metal abundances and rotational velocities are
determined. Balmer line fitting is found to be consistent with the helium
ionization equilibrium for PG1605+072 but not so for PG1219+534 indicating that
systematic errors in the model atmosphere analysis of the latter have been
underestimated previously. All stars are found to be helium deficient probably
due to diffusion. The metals are also depleted with the notable exception of
iron which is solar to within error limits in all four stars, confirming
predictions from diffusion calculations of Charpinet et al. (1997). While three
of them are slow rotator's (vsini < 10km/s), PG1605+072 displays considerable
rotation (vsini = 39km/s, P<8.7h) and is predicted to evolve into an unusually
fast rotating white dwarf. This nicely confirms a prediction by Kawaler (1999)
who deduced a rotation velocity of 130km/s from the power spectrum of the
pulsations which implies a low inclination angle of the rotation axis.
|
9912133v1
|
2000-01-18
|
Implications of Abundance Gradients in Intracluster Gas
|
Analysis of spatially resolved ASCA spectra of the intracluster gas in Abell
496 confirms that metal abundances increase toward the center. We also find
spatial gradients in several abundance ratios, indicating that the fraction of
iron from SN Ia increases toward the cluster center. The dominant metal
enrichment mechanism near the cluster center must therefore be different than
in the outer parts. We show that ram pressure stripping of gas from cluster
galaxies cannot account for the central abundance enhancement. We suggest that
two successive stages of galactic winds contaminate intracluster gas:
protogalactic winds driven by SN II, followed by less energetic winds driven by
SN Ia, which have longer lived progenitors than SN II. The less energetic
secondary wind from a cD galaxy may be suppressed, due to its location at the
cluster center, leading to the observed central enhancement of SN Ia ejecta.
|
0001304v1
|
2000-01-25
|
X-Ray Afterglows of Gamma-Ray Bursts
|
The afterglow emission has become the main stream of Gamma-Ray burst research
since its discovery three years ago. With the distance-scale enigma solved, the
study of the late-time GRB emission is now the most promising approach to
disclose the origin of these explosions and their relationship with the
environment of the host galaxy in the early phase of the Universe. In this
contribution I will review X-ray observations and their implication on our
undertstanding on the GRB phenomenon. These measurements are providing a direct
probe into the nature of the progenitor and a measurement of the GRB beaming
properties, crucial to establish the total energy output. Some evidence of iron
lines connects the GRB explosion with massive progenitors, thence with
star-forming regions. Furthermore a comparison of the spectral properties with
the temporal evolution indicates that the fireball expansion should not be - on
average - highly collimated, with a jet angle $>10 \deg$.
|
0001436v1
|
2000-01-27
|
LiBeB Production and Associated Astrophysical Sites
|
The various modes of spallative LiBeB production are summarized, and
classified according to their dependence or independence on the abundance of
medium heavy elements (CNO) illustrated by that of oxygen in the interstellar
medium.
The predictions of the models are confronted to the available observational
correlations (Be, B vs O). Clearly, a primary mechanism should lead to a slope
one in the lg(Be/H) vs [O/H] plot and a secondary mechanism to a slope two. Due
to the ambiguity of the O data, another criterion, based on energetics, can
help us to select an adequate model. A purely secondary origin in the very
early Galaxy is much more energy demanding than a primary one. Indeed,
magnesium seems to be a possible surrogate of oxygen and iron since i) it is
spectroscopically more easy to cope with and ii) its nucleosynthetic yield is
independent of the mass cut and does not depend on metallicity.
|
0001474v1
|
2000-02-08
|
The hard X-ray properties of the Seyfert nucleus in NGC 1365
|
We present BeppoSAX observations of the Seyfert 1.8 galaxy NGC1365 in the
0.1--100 keV range. The source was 6 times brighter than during an ASCA
observation 3 years earlier. The 4--10 keV flux is highly variable during the
BeppoSAX observation, while the soft (0.1-4 keV) emission is constant within
the errors. Both a cold and a warm reflector and a cold absorber are required
to explain the observed spectrum. The comparison between ASCA and BeppoSAX
spectra strongly suggests that the circumnuclear material has a more complex
structure than a simple homogeneous torus, with quite different absorbing gas
columns along different lines of sight. A broad iron K_alpha line is also
present in the spectrum, with the peak energy significantly redshifted. This
can be explained by means of a relativistic disk line model. Alternatively, a
warm absorption Fe line system with N_H ~ 10^23 cm^(-2) could account for the
observed line profile.
|
0002169v1
|
2000-03-02
|
Gamma-loud quasars: a view with BeppoSAX
|
We present $Beppo$SAX observations of the $\gamma $-ray emitting quasars
0836+710, 1510-089 and 2230+114. All the objects have been detected in the PDS
up to 100 keV and have extremely flat power-law spectra above 2 keV ($\alpha
_x$=0.3--0.5). 0836+710 shows absorption higher than the galactic value and
marginal evidence for the presence of the redshifted 6.4 keV Iron line.
1510-089 shows a spectral break around 1 keV, with the low energy spectrum
steeper ($\alpha_l$=1.6) than the high energy power-law ($\alpha_h$=0.3). The
data are discussed in the light of current Inverse Compton models for the high
energy emission.
|
0003019v1
|
2000-03-13
|
Grain Survival in Supernova Remnants and Herbig-Haro Objects
|
By using the flux ratio [FeII]8617/[OI]6300, we demonstrate that most of the
interstellar dust grains survive in shocks associated with supernova remnants
and Herbig-Haro objects. The [FeII]/[OI] flux ratio is sensitive to the
gas-phase Fe/O abundance ratio, but is insensitive to the ionization state,
temperature, and density of the gas. We calculate the [FeII]/[OI] flux ratio in
shocks, and compare the results with the observational data. When only 20% of
iron is in the gas phase, the models reproduce most successfully the
observations. This finding is in conflict with the current consensus that
shocks destroy almost all the grains and 100% of metals are in the gas phase.
We comment on previous works on grain destruction, and discuss why grains are
not destroyed in shocks.
|
0003170v1
|
2000-03-27
|
Massive Stars in the Range $\rm 13-25 M_\odot$: Evolution and Nucleosynthesis. II. the Solar Metallicity Models
|
We present the evolutionary properties of a set of massive stellar models
(namely 13, 15, 20 and 25 $\rm M_\odot$) from the main sequence phase up to the
onset of the iron core collapse. All these models have initial solar chemical
composition, i.e. Y=0.285 and Z=0.02. A 179 isotope network, extending from
neutron up to $\rm ^{68}Zn$ and fully coupled to the evolutionary code has been
adopted from the Carbon burning onward. Our results are compared, whenever
possible, to similar computations available in literature.
|
0003401v1
|
2000-04-03
|
The Absolute Abundance of Iron in the Solar Corona
|
We present a measurement of the abundance of Fe relative to H in the solar
corona using a technique which differs from previous spectroscopic and solar
wind measurements. Our method combines EUV line data from the CDS spectrometer
on SOHO with thermal bremsstrahlung radio data from the VLA. The coronal Fe
abundance is derived by equating the thermal bremsstrahlung radio emission
calculated from the EUV Fe line data to that observed with the VLA, treating
the Fe/H abundance as the sole unknown. We apply this technique to a compact
cool active region and find Fe/H
= 1.56 x 10^{-4}, or about 4 times its value in the solar photosphere.
Uncertainties in the CDS radiometric calibration, the VLA intensity
measurements, the atomic parameters, and the assumptions made in the spectral
analysis yield net uncertainties of order 20%. This result implies that low
first ionization potential elements such as Fe are enhanced in the solar corona
relative to photospheric values.
|
0004007v1
|
2000-04-25
|
Non-LTE Abundances and Consequences for the Evolution of the alpha elements in the Galaxy
|
Abundances of alpha-elements such as Ca and Mg in disk and halo stars are
usually derived from equivalent widths lines measured on high resolution
spectra, and assuming Local Thermodynamic Equilibrium (LTE) . In this paper, we
present non-LTE differential abundances derived by computing the statistical
equilibrium of CaI and MgI atoms, using high resolution equivalent widths
available in the literature for 252 dwarf to subgiant stars. These non-LTE
abundances combined with recent determination of non-LTE abundances of iron,
seem to remove the dispersion of the [Ca/Fe] and [Mg/Fe] ratios in the galactic
halo and disk phases, revealing new and surprising structures. These results
have important consequences for chemical evolution models of the Galaxy. In
addition, non-LTE abundance ratios for stars belonging to the M92 cluster
apparently have the same behavior. More high resolution observations, mainly of
globular clusters, are urgently needed to confirm our results.
|
0004337v1
|
2000-05-04
|
Oscillating structure of γ-bursts and their possible origin
|
As it is well-known that the hydrodinamic collapse of the massive star iron
core should lead to the production of a hot neutron star. The assumption is
made that the thermonuclear burning of the envelope matter, accreting onto the
hot neutron star, can proceed in the oscillatoric regime (analogously to that
happens during heat explosion of the carbon-oxigene cores of stars with smaller
masses). Local density oscillations in the vicinity of the neutron star surface
can generate shock waves, in which the stratification of the electron-positron
plasma from the rest of the matter can happen due to the light preasure. In the
case of the spherically symmetric collapse of the compact star it can lead to
the production of the expanding relativistic fireball shells with
characteristic oscillation time of ~ 10^{-2} s, observed in the cosmological
\gamma-bursts (GRB), can occur. It is pointed out that nonrotating massive
Wolf-Rayet's (WR) stars could be the source for the GRB, whose collapses,
according to a number of observations, can happen without any noticeable
ejection of the envelope.
|
0005078v1
|
2000-05-04
|
Composition of UHE Cosmic Ray Primaries
|
Project GRAND presents results on the atomic composition of primary cosmic
rays. This is accomplished by determining the average height of primary
particles that cause extensive air showers detected by Project GRAND. Particles
with a larger cross sectional area, such as iron nuclei, are likely to start an
extensive air shower higher in the atmosphere whereas protons, with a smaller
cross section, would pass through more air before interacting and thus start
showers at lower heights. Such heights can be determined by extrapolating
identified muon tracks backward (upward) to determine their height of origin
(Gress et al., 1997). Since muons are from the top, hadronic part of the
shower, they are a good estimator for the beginning of the shower. The data for
this study were taken during the previous year with 20 million shower events.
|
0005089v1
|
2000-05-08
|
X-ray Continuum Slope and X-ray Spectral Features in NLS1 Galaxies
|
The idea that some of the unusual features in the X-ray spectra of
Narrow-Line Seyfert 1 galaxies (NLS1s) are due to the steep X-ray continuum is
tested by comparing photoionization model calculations with various observed
properties of Seyfert 1 galaxies. A meaningful comparison must involve the
careful use of the right X-ray ionization parameter, designated here U(oxygen).
When this is done, it is found that the strength of the continuum absorption
features is insensitive to the exact slope of the 0.1-50 keV continuum. It is
also shown that the complex of iron L-shell lines near 1 keV can produce strong
absorption and emission features, depending on the gas distribution and line
widths. While this may explain some unusual X-ray features in AGN, the
predicted intensity of the features do not distinguish NLS1 from broader line
sources. Finally, acceleration of highly ionized gas, by X-ray radiation
pressure, is also not sensitive to the exact slope of the X-ray continuum.
|
0005142v1
|
2000-05-08
|
The Fe II problem in NLS1s
|
For more than twenty years, strong Fe II emission lines have been observed in
Active Galactic Nuclei and in particular in Narrow Line Seyfert 1 galaxies. A
quick overview of the observations and of the models proposed to interpret the
Fe II spectrum is given. The influence of atomic data and of physical
parameters are discussed, and it is shown that the strengths of the Fe II lines
cannot be explained in the framework of photoionization models. A non-radiative
heating, for instance due to shocks, with an overabundance of iron, can help to
solve the problem. A comparison with other objects emitting intense Fe II lines
favors also the presence of strong outflows and shocks. We suggest some issues
in the context of AGN evolution.
|
0005153v1
|
2000-05-08
|
X-ray reprocessing in Narrow-Line Seyfert 1 Galaxies: Ton S180 and Ark 564
|
We present the results of spectral analysis of the ASCA data for the
Narrow-Line Seyfert 1 galaxy (NLS1) Ton S180 and simultaneous ASCA and RXTE
data modelling for the NLS1 Ark 564. We model both the primary and reflected
continuum as well as the iron K alpha line, the energy of which depends on the
ionization state of the reprocessor. We show that the reprocessing matter is
mildly ionized, and we find the soft to hard luminosity ratio to be about 2.5.
The accretion rate approximately corresponds to the Eddington limit value.
|
0005156v1
|
2000-05-08
|
Modelling of the X-ray broad absorption features in Narrow-Line Seyfert 1s
|
We investigate the origin of the broad absorption features detected near
1-1.4 keV in several Narrow Line Seyfert 1 galaxies, by modelling the absorbing
medium with various physical parameters, using the ionization code PEGAS. The
observed properties of the X-ray absorption features can be reproduced by
taking into account the peculiar soft X-ray excess which is well fitted by a
blackbody plus an underlying power law. We equally stress that the emission
coming from the absorbing medium (related to the covering factor) has a strong
influence on the resulting X-ray spectrum, in particular on the apparent
position and depth of the absorption features. A non-solar iron abundance may
be required to explain the observed deep absorption. We also investigate the
influence of an additional collisional ionization process (hybrid case) on the
predicted absorption features.
|
0005169v1
|
2000-05-08
|
Studies of the high luminosity quasar, PDS 456
|
X-ray and multi-wavelength observations of the most luminous known local
(z<0.3) AGN, the recently discovered radio-quiet quasar PDS 456, are presented.
The spectral energy distribution shows that PDS 456 has a bolometric luminosity
of 1e47 erg/s, peaking in the UV. The X-ray spectrum obtained by ASCA and RXTE
shows considerable complexity. The most striking feature observed is a deep,
highly-ionised, iron K edge (8.7 keV, rest-frame), originating via reprocessing
from highly ionised material, possibly the inner accretion disk. PDS 456 was
found to be remarkably variable for its luminosity; in one flare the X-ray flux
doubled in just about 15 ksec. If confirmed this would be an unprecedented
event in a high-luminosity source, with a light-crossing time corresponding to
about 2RS. The implications are that either flaring occurs within the very
central regions, or else that PDS 456 is a super-Eddington or relativistically
beamed system.
|
0005175v1
|
2000-06-02
|
Equilibrium configurations of relativistic White Dwarfs
|
The Feynman-Metropolis-Teller treatment for compressed atoms is here
reconsidered in the framework of the relativistic generalised Fermi-Thomas
model, obtained by Ruffini et al. Physical properties of a zero temperature
plasma is thus investigated and the resulting equation of state, which keeps
into account quantum, relativistic and electromagnetic effects, is applied to
the study of equilibrium configurations of relativistic White Dwarfs.
It is shown that numerical evaluation of such configuration leads, for the
same central density $\rho_c$, to smaller values of radius R and of mass M than
in the classical works of Chandrasekhar and Salpeter, the deviations being most
marked at the lowest densities (up to 30% from the
Chandrasekhar model and 10% from the Salpeter one for $\rho_c \sim
10^6g/cm^3$, corresponding to $M \sim 0.2 M_{\odot}$).
At high densities we considered the occurrence of inverse beta decays, whose
effect is to introduce gravitational instability of the configurations. We
consequently find the maximum mass of White Dwarfs, which, for an Oxygen and an
Iron WD, is respectively $1.365 M_{\odot}$ and $1.063 M_{\odot}$.
|
0006036v1
|
2000-06-03
|
Analysis of Four A-F Supergiants in M31 from Keck HIRES Spectroscopy
|
The first stellar abundances in M31 are presented, based on Keck I HIRES
spectroscopy and model atmospheres analyses of three A-F supergiants, 41-2368,
41-3712, and A-207. We also present the preliminary analysis of a fourth star,
41-3654. We find that the stellar oxygen abundances are in good agreement with
those determined from nebular studies, even though the stars do {\it not} show
a clear radial gradient in oxygen. The uncertainties in the stellar abundances
are smaller than the range in the nebular results, making these stars ideal
objects for further studies of the distribution of oxygen in M31. We show that
the stars can be used to study the abundance distributions of other elements as
well, including iron-group and heavier elements.
The A-F supergiants also provide direct information on the metallicity and
reddening of nearby Cepheid stars. We have examined the metallicity and
reddening assumptions used for Cepheids within 1' of our targets and noted the
differences from values used in the literature.
|
0006046v1
|
2000-06-20
|
The Effect of Diffusion on Pulsations of Stars on the Upper Main Sequence. delta Scuti and Metallic A Stars
|
Recent dramatic improvements in the modeling of abundance evolution due to
diffusion in A stars have been achieved with the help of monochromatic opacity
tables from the OPAL group. An important result in the context of stellar
pulsations is the substantial helium abundance shown to be left over in the
driving region of delta Scuti-type pulsations in chemically peculiar Am stars.
An accurate opacity profile in the entire stellar envelope including the full
effect of heavy elements is also now available for the first time.
Pulsations are shown to be excluded for young Am stars but occur naturally
when these stars evolve off the ZAMS. The predicted variable metallic A stars
all lie towards the red edge of the instability strip, in qualitative agreement
with the observed variable delta Delphini and mild Am stars.
Results show little direct excitation from iron-peak elements in A-type
stars. The main abundance effect is due to the settling of helium, along with a
marginal effect due to the enhancement of hydrogen.
|
0006272v1
|
2000-06-23
|
The Metallicity Distribution Function of Red Giants in the LMC
|
We report new metallicity determinations for 39 red giants in a 220 square
arcminute region, 1.8 degrees southwest of the bar of the Large Magellanic
Cloud. These abundance measurements are based on spectroscopy of the Ca II
infrared triplet. The metallicity distribution function (MDF) shows a strong
peak at [Fe/H] = -0.57 +/- 0.04. Half the red giants in our field fall within
the range -0.83 < [Fe/H] < -0.41. We find a striking contrast in the shape of
the MDF below [Fe/H] < -1 between our inner disk field and the distant outer
field studied by Olszewski (1993). Our field-star MDF seems similar to that of
the intermediate-age (1-3 Gyr) star clusters. We have also obtained abundance
estimates using Stromgren photometry for ~1000 red giants in the same field.
The Stromgren measurements, which are sensitive to a combination of cyanogen
and iron lines, correlate well with the Ca II measurements, but a
metallicity-dependent offset is found (abridged).
|
0006327v1
|
2000-07-05
|
Face-on galaxies NGC 524 and NGC 6340: chemically decoupled nuclei and inclined circumnuclear disks
|
Central regions of the early-type disk galaxies NGC 524 and NGC 6340 have
been investigated with the Multi-Pupil Field Spectrograph at the 6m telescope
of the Special Astrophysical Observatory of the Russian Academy of Sciences. I
confirm the existence of chemically distinct stellar nuclei in these galaxies
which have been claimed earlier. The metallicity differences which are found
between the nuclei and the bulges, 0.5 - 1.0 kpc from the centers, reach 0.5 -
0.6 dex. Both nuclei are magnesium overabundant, but the bulges have different
magnesium-to-iron ratios: it is solar in NGC 6340 and the same as the nuclear
one in NGC 524. The kinematical and morphological analyses reveal the existence
of inclined central disks in these galaxies. In NGC 524 the central disk
consists of stars, dust, and ionized gas; its extension may be as large as up
to R=3 kpc, and it is inclined by more than 20 deg to the global galactic
plane. In NGC 6340 only a gaseous polar disk with the radius less than 500 pc
is detected.
|
0007051v1
|
2000-07-06
|
Galaxy Mergers: A Search for Chemical Signatures
|
We have gathered high resolution echelle spectra for more than two dozen
high-velocity metal-poor field stars, including BD+80 245, a star previously
known to have extremely low [alpha/Fe] abundances, as well as G4-36, a new
low-alpha star with unusually large [Ni/ Fe]. In this kinematically selected
sample, other chemically anomalous stars have also been uncovered. In addition
to deriving the alpha-element abundances, we have also analysed iron-group and
s-process elements. Not only does chemical substructure exist in the halo, but
the chemical anomalies are not all the same within all elemental groups.
|
0007065v1
|
2000-07-17
|
Compton-Thick X-ray Absorption in the Seyfert Galaxies Tololo 0109-383 and ESO 138-G1
|
We present analyses of the ASCA X-ray spectra of two Seyfert galaxies, Tololo
0109-383 and ESO 138-G1. In both cases, spectral fitting reveals two
statistically acceptable continuum models: Compton reflection and partial
covering. Both spectra have strong iron K-alpha lines, with equivalent widths
greater than 1.5 keV. These large equivalent widths are suggestive of heavier
obscuration than that directly indicated by the partial-covering models
(approximately 2 x 10^23 cm^-2), with the actual column densities being
`Compton-thick' (i.e. N_H > 1.5 x 10^24 cm^-2). We use the hard X-ray/[O III]
flux correlation for Seyferts and data from the literature to provide
additional support for this hypothesis. Since Tololo 0109-383 is known to have
optical type 1 characteristics such as broad Balmer line components and Fe II
emission, this result marks it as a notable object.
|
0007237v2
|
2000-07-21
|
Probing Dense Matter in the cores of AGN: Observations with RXTE and ASCA
|
Preliminary results from an X-ray spectral study of Seyfert 1 galaxies with
ASCA and RXTE are presented. From an analysis of X-ray reprocessing features of
Compton reflection and Fe K-alpha fluorescence, it is found that iron line
strength is not necessarily a good predictor of the amount of reflection. The
variability properties of Fe K-alpha and reflection do not necessarily scale
together and substantial decoupling of the behavior of the reprocessed flux
with respect to continuum variability is common. Such trends suggest the
presence of multiple and/or complex regions of dense matter in AGN cores and
that standard accretion disk models drastically oversimplify reality.
|
0007327v1
|
2000-07-27
|
Presupernova collapse models with improved weak-interaction rates
|
Improved values for stellar weak interaction rates have been recently
calculated based upon a large shell model diagonalization. Using these new
rates (for both beta decay and electron capture), we have examined the
presupernova evolution of massive stars in the range 15-40 Msun. Comparing our
new models with a standard set of presupernova models by Woosley and Weaver, we
find significantly larger values for the electron-to-baryon ratio Ye at the
onset of collapse and iron core masses reduced by approximately 0.1 Msun. The
inclusion of beta-decay accounts for roughly half of the revisions, while the
other half is a consequence of the improved nuclear physics. These changes will
have important consequences for nucleosynthesis and the supernova explosion
mechanism.
|
0007412v1
|
2000-08-04
|
On the peculiar X-ray properties of the bright nearby radio-quiet quasar PDS456
|
BeppoSAX and ASCA observations of the nearby (z=0.184), high-luminosity,
radio-quiet quasar PDS456 are presented. The X-ray spectrum is characterized by
a prominent ionized edge at 8-9 keV (originally discovered by RXTE, Reeves et
al. 2000) and by a soft excess below 1.5 keV. The lack of any significant iron
K alpha emission line suggests for the edge an origin from line-of-sight
material rather than from reflection from a highly ionized accretion disc. The
hard X-ray continuum is indeed well modelled by transmission through a
highly-ionized medium with a large column density (N_H warm = 4.5 x 10^24
cm^-2) plus an additional cold absorber with a lower column density (N_H cold =
2.7 x 10^22 cm^-2).
|
0008090v1
|
2000-08-22
|
Interstellar X-ray Absorption Spectroscopy of Oxygen, Neon, and Iron with the Chandra LETGS Spectrum of X0614+091
|
We find resolved interstellar O K, Ne K, and Fe L absorption spectra in the
Chandra Low Energy Transmission Grating Spectrometer spectrum of the low mass
X-ray binary X0614+091. We measure the column densities in O and Ne, and find
direct spectroscopic constraints on the chemical state of the interstellar O.
These measurements probably probe a low-density line of sight through the
Galaxy and we discuss the results in the context of our knowledge of the
properties of interstellar matter in regions between the spiral arms.
|
0008349v1
|
2000-08-23
|
Strangeness in Neutron Stars
|
It is generally agreed on that the tremendous densities reached in the
centers of neutron stars provide a high-pressure environment in which numerous
novel particles processes are likely to compete with each other. These
processes range from the generation of hyperons to quark deconfinement to the
formation of kaon condensates and H-matter. There are theoretical suggestions
of even more exotic processes inside neutron stars, such as the formation of
absolutely stable strange quark matter, a configuration of matter even more
stable than the most stable atomic nucleus, iron. In the latter event, neutron
stars would be largely composed of pure quark matter, eventually enveloped in a
thin nuclear crust. No matter which physical processes are actually realized
inside neutron stars, each one leads to fingerprints, some more pronounced than
others though, in the observable stellar quantities. This feature combined with
the unprecedented progress in observational astronomy, which allows us to see
vistas with remarkable clarity that previously were only imagined, renders
neutron stars to nearly ideal probes for a wide range of physical studies,
including the role of strangeness in dense matter.
|
0008376v1
|
2000-09-27
|
Unusual Properties of X-Ray Emission near the Galactic Center
|
The X-ray spectrum in a $1^{\circ}\times1^{\circ}$ region of the Galactic
center observed with the ASCA satellite is examined in detail, following the
first report by Koyama et al. (1996, AAA 65.155.208). The observed spectrum
contains prominent emission lines from helium-like and hydrogen-like ions of
various elements, and is essentially the same all over the region. If the
observed spectrum is thermal emission from hot plasmas, it requires
multi-temperature plasma components, each at a different degree of ionization
and with a different amount of absorption. The absence of adiabatic cooling and
of systematic changes in the degree of ionization over the region is against
the Galactic center origin of hot plasmas. A significant broadening of the
helium-like and hydrogen-like iron K-lines is confirmed. The line width
corresponds to a rms velocity of $\sim3300$ km~s$^{-1}$, which far exceeds the
sound velocity in a plasma of $kT\sim14$ keV measured with the Ginga satellite.
These facts cast doubt on a thermal origin of the observed X-ray emission.
|
0009445v1
|
2000-09-29
|
The beryllium abundance in the very metal-poor halo star G 64-12 from VLT/UVES observations
|
We report on a new spectroscopic analysis of the very metal deficient star G
64-12 ([Fe/H]=-3.3), aimed at determining, for the first time, its Be content.
The spectra were observed during the Science Verification of UVES, the ESO VLT
Ultraviolet and Visible Echelle Spectrograph. The high resolution (~48,000) and
high S/N (~130 per pixel) achieved at the wavelengths of the BeII resonance
doublet allowed an accurate determination of its abundance: log N(Be/H) =
-13.10 +/- 0.15 dex. The Be abundance is significantly higher than expected
from previous measurements of Be in stars of similar metallicity (3D and NLTE
corrections acting to make a slightly higher value than an LTE analysis). When
compared to iron, the high [Be/Fe] ratio thus found may suggest a flattening in
the beryllium evolutionary trend at the lowest metallicity end or the presence
of dispersion at early epochs of galactic evolution.
|
0009482v1
|
2000-10-06
|
ASCA Observation of an X-Ray-Luminous Active Nucleus in Markarian 231
|
We have obtained a moderately long (100 kilosecond) ASCA observation of the
Seyfert 1 galaxy Markarian 231, the most luminous of the local ultraluminous
infrared galaxy (ULIRG) population. In the best-fitting model we do not see the
X-ray source directly; the spectrum consists of a scattered power-law component
and a reflection component, both of which have been absorbed by a column N_H
\approx 3 X 10^(22)/cm^2. About 3/4 of the observed hard X-rays arise from the
scattered component, reducing the equivalent width of the iron K alpha line.
The implied ratio of 1-10 keV X-ray luminosity to bolometric luminosity,
L_x/L_bol \sim 2%, is typical of Sy 1 galaxies and radio-quiet QSOs of
comparable bolometric luminosities, and indicates that the bolometric
luminosity is dominated by the AGN. Our estimate of the X-ray luminosity also
moves Mrk 231 in line with the correlations found for AGN with extremely strong
Fe II emission. A second source separated by about 2 arcminutes is also clearly
detected, and contributes about 25% of the total flux.
|
0010125v1
|
2000-10-13
|
Iron K-alpha Emission from a Decaying Magnetar Model of Gamma-Ray Bursts
|
The recent report of X-ray Fe features in the afterglow of the gamma-ray
burst GRB 991216 may provide important clues for identifying the nature of its
progenitor and constraining the burst mechanism. We argue that the strong line
emission can be attributed to the interaction of a continuing (but decaying)
post-burst relativistic outflow from the central engine with the progenitor
stellar envelope at distances less than a light-hour. Only a small mass of Fe
is then required, which could have been readily produced by the star itself.
|
0010258v1
|
2000-11-09
|
Population synthesis of neutron stars, strange (quark) stars and black holes
|
We compute and present the distribution in mass of single and binary neutron
stars, strange stars, and black holes. The calculations were performed using a
stellar population synthesis code. We follow all phases of single and binary
evolution, starting from a ZAMS binary and ending in the creation of one
compact object (neutron star, black hole, strange star) and a white dwarf, or
two compact objects (single or binary). We assume that neutron stars are formed
in the collapse of iron/nickel cores in the mass range M0 < M < M1, quark stars
in the range M1 < M < M2, and black holes for core masses M > M2 and find that
the population of quark stars can easily be as large as the population of black
holes, even if there is only a small mass window for their formation.
|
0011183v1
|
2000-11-14
|
The 2000 outburst of the recurrent nova CI Aquilae: optical spectroscopy
|
We present low- and medium resolution spectra of the recurrent nova CI
Aquilae taken at 14 epochs in May and June, 2000. The overall appearance is
similar to other U Sco-type recurrent novae (U Sco, V394 CrA). Medium
resolution (R=7000-10000) hydrogen and iron profiles suggest an early expansion
velocity of 2000-2500 km/s. The H\alpha evolution is followed from Dt = -0.6 d
to +53 d, starting from a nearly Gaussian shape to a double peaked profile
through strong P-Cyg profiles. The interstellar component of the sodium D line
and two diffuse interstellar bands put constraints on the interstellar
reddening which is estimated to be E(B-V)=0.85\pm0.3. The available visual and
CCD-V observations are used to determine t0,t2 and t3. The resulting parameters
are: t0=2451669.5\pm0.1, t2=30\pm1 d, t3=36\pm1 d. The recent lightcurve is
found to be generally similar to that observed in 1917 with departures as large
as 1-2 mag in certain phases. This behaviour is also typical for the U Sco
subclass.
|
0011270v1
|
2000-11-28
|
Chandra detection of reflected X-ray emission from the type 2 QSO in IRAS 09104+4109
|
We present X-ray imaging spectroscopy of the extremely luminous infrared
galaxy IRAS 09104+4109 (z=0.442) obtained with the Chandra X-ray Observatory.
With the arcsec resolution of Chandra, an unresolved source at the nucleus is
separated from the surrounding cluster emission. A strong iron K line at 6.4
keV on a very hard continuum is detected from the nuclear source, rendering
IRAS 09104+4109 the most distant reflection-dominated X-ray source known.
Combined with the BeppoSAX detection of the excess hard X-ray emission, it
provides further strong support to the presence of a hidden X-ray source of
quasar luminosity in this infrared galaxy. Also seen is a faint linear
structure to the North, which coincides with the main radio jet. An X-ray
deficit in the corresponding region suggests an interaction between the cluster
medium and the jet driven by the active nucleus.
|
0011518v1
|
2000-12-01
|
The Seyfert-Starburst Connection in X-rays. I. The Data
|
We analyze X-ray spectra and images of a sample of Seyfert 2 galaxies that
unambiguously contain starbursts, based on their optical and UV
characteristics. Although all sample members contain active galactic nuclei
(AGNs), supermassive black holes or other related processes at the galactic
centers alone cannot account for the total X-ray emission in all instances.
Eleven of the twelve observed galaxies are significantly resolved with the
ROSAT HRI, while six of the eight sources observed with the lower-resolution
PSPC also appear extended on larger scales. The X-ray emission is extended on
physical scales of 10 kpc and greater, which we attribute to starburst-driven
outflows and supernova-heating of the interstellar medium. Spectrally, a
physically-motivated composite model of the X-ray emission that includes a
heavily absorbed (N_H > 10^{23} cm^{-2}) nuclear component (the AGN), power-law
like scattered AGN flux, and a thermal starburst describes this sample well.
Half the sample exhibit iron K alpha lines, which are typical of AGNs.
|
0012035v1
|
2000-12-05
|
Chandra--ASCA--RXTE observations of the micro-quasar GRS 1915+105
|
A Chandra AO1 30ks HETGS observation of the X-ray transient micro-quasar GRS
1915+105 reveals absorption edges and faint line emission over the HETG energy
range. We find from a preliminary analysis evidence for prominent neutral K
edges associated with iron, silicon, magnesium, and tentatively sulphur. The
column densities assuming solar abundances are consistent with ~ few x $10^{22}
cm^{-2}$ in excess of the Galactic value, and may point to surrounding cold
material associated with GRS 1915+105. Neutral Fe K$\alpha$ emission, and
ionized absorption from Fe XXV and Fe XXVI are resolved. We limit our
discussion to the Chandra results.
|
0012111v1
|
2000-12-17
|
Stellar spectroscopy far beyond the Local Group
|
Multi-object spectroscopic observations of blue supergiants in NGC 3621, a
spiral galaxy at a distance of 6.7 Mpc, carried out with the ESO VLT and FORS
are presented. We demonstrate the feasibility of quantitative stellar
spectroscopy at distances approaching a ten-fold increase over previous
investigations by determining chemical composition, stellar parameters,
reddening, extinction and wind properties of one of our targets, a supergiant
of spectral type A1 Ia located in the outskirts of NGC 3621. The metallicity
(determined from iron group elements) is reduced by a factor of two relative to
the sun in qualitative agreement with results from previous abundance studies
based on H II region oxygen emission lines. Reddening and extinction are E(B-V)
= 0.12 and Av = 0.37, respectively, mostly caused by the galactic foreground.
Comparing stellar wind momentum and absolute V magnitude with galactic and M31
counterparts we confirm the potential of the wind momentum-luminosity
relationship as an alternative tool to estimate extragalactic distances.
|
0012372v1
|
2001-02-15
|
Ionized Iron Lines in X-ray Reflection Spectra
|
We present results from new calculations of the X-ray reflection spectrum
from ionized accretion discs. These computations improve on our previous models
by including the condition of hydrostatic balance in the vertical direction,
following the work of Nayakshin, Kazanas & Kallman. We find that an ionized Fe
K$\alpha$ line is prominent in the reflection spectra for a wide variety of
physical conditions. The results hold for both gas and radiation pressure
dominated discs and when the metal abundances have been varied.
|
0102268v2
|
2001-03-11
|
Contribution of nuclei accelerated by gamma-ray pulsars to cosmic rays in the Galaxy
|
We consider the galactic population of gamma-ray pulsars as possible sources
of cosmic rays at and just above the ``knee'' in the observed cosmic ray
spectrum at $10^{15}$--$10^{16}$ eV. We suggest that iron nuclei may be
accelerated in the outer gaps of pulsars, and then suffer partial
photo-disintegration in the non-thermal radiation fields of the outer gaps. As
a result, protons, neutrons, and surviving heavier nuclei are injected into the
expanding supernova remnant. We compute the spectra of nuclei escaping from
supernova remnants into the interstellar medium, taking into account the
observed population of radio pulsars.
Our calculations, which include a realistic model for acceleration and
propagation of nuclei in pulsar magnetospheres and supernova remnants, predict
that heavy nuclei accelerated directly by gamma-ray pulsars could contribute
about 20% of the observed cosmic rays in the knee region. Such a contribution
of heavy nuclei to the cosmic ray spectrum at the knee can significantly
increase the average value of $<\ln A>$ with increasing energy as is suggested
by recent observations.
|
0103160v1
|
2001-03-14
|
The Early Formation, Evolution and Age of the Neutron-Capture Elements in the Early Galaxy
|
Abundance observations indicate the presence of rapid-neutron capture (i.e.,
r-process) elements in old Galactic halo and globular cluster stars. These
observations demonstrate that the earliest generations of stars in the Galaxy,
responsible for neutron-capture synthesis and the progenitors of the halo
stars, were rapidly evolving. Abundance comparisons among several halo stars
show that the heaviest neutron-capture elements (including Ba and heavier) are
consistent with a scaled solar system r-process abundance distribution, while
the lighter such elements do not conform to the solar pattern. These
comparisons suggest two r-process sites or at least two different sets of
astrophysical conditions. The large star-to-star scatter observed in the
neutron-capture/iron ratios at low metallicities -- which disappears with
increasing [Fe/H] -- suggests an early, chemically unmixed and inhomogeneous
Galaxy. The stellar abundances indicate a change from the r-process to the slow
neutron capture (i.e., s-) process at higher metallicities in the Galaxy. The
detection of thorium in halo and globular cluster stars offers a promising,
independent age-dating technique that can put lower limits on the age of the
Galaxy.
|
0103222v1
|
2001-03-26
|
A hard X-ray constraint on the presence of an AGN in the ultra-luminous infrared galaxy Arp220
|
We present X-ray results on the ultraluminous infrared galaxy Arp220 obtained
with BeppoSAX. The X-ray emission up to 10 keV is detected. No significant
signal is detected with the PDS detector in the higher energy band. The 2-10
keV emission has a hard spectrum (photon-index of 1.7), similar to M82, and a
luminosity of 1e41 erg/s. A population of X-ray binaries may be a major source
of this X-ray emission. The upper limit of an iron K line equivalent width at
6.4 keV is ~600 eV. This observation imposes so far the tightest constraint on
an active nucleus if present in Arp220. We find that a column density of X-ray
absorption must exceed 10**25 cm-2 for an obscured active nucleus to be
significant in the energetics, and the covering factor of the absorption should
be almost unity. The underluminous soft X-ray starburst emission may need a
good explanation, if the bolometric luminosity is primarily powered by a
starburst.
|
0103417v1
|
2001-03-26
|
Precipitating Condensation Clouds in Substellar Atmospheres
|
We present a method to calculate vertical profiles of particle size
distributions in condensation clouds of giant planets and brown dwarfs. The
method assumes a balance between turbulent diffusion and sedimentation in
horizontally uniform cloud decks. Calculations for the Jovian ammonia cloud are
compared with results from previous methods. An adjustable parameter describing
the efficiency of sedimentation allows the new model to span the range of
predictions made by previous models. Calculations for the Jovian ammonia cloud
are consistent with observations. Example calculations are provided for water,
silicate, and iron clouds on brown dwarfs and on a cool extrasolar giant
planet. We find that precipitating cloud decks naturally account for the
characteristic trends seen in the spectra and colors of L- and T-type ultracool
dwarfs.
|
0103423v2
|
2001-04-10
|
Oxygen Abundances in Metal-Poor Stars (-2.2 < [Fe/H] < -1.2) from Infrared OH lines
|
Infrared OH lines at 1.55 - 1.56 um in the H-band were obtained with the
Phoenix high-resolution spectrograph at the 2.1m telescope of the Kitt Peak
National Observatory for a sample of 14 metal-poor stars. Detailed analyses of
the sample stars have been carried out, deriving stellar parameters based on
two methods: (a) spectroscopic parameters; (b) IRFM effective temperatures,
trigonometric gravities and metallicities from Fe II lines. The Fe I lines
present in the H-band region observed showed to be well fitted by the stellar
parameters within $\Delta$[Fe/H] < 0.15 dex. The oxygen abundances were derived
from fits of spectrum synthesis calculations to the infrared OH lines. CO lines
in the H- and K-bands were obtained for a subsample in order to determine their
carbon abundances. Adopting the spectroscopic parameters a mean oxygen-to-iron
ratio of [O/Fe] ~ +0.52 is obtained, whereas using the IRFM temperatures,
Hipparcos gravities and [FeII/H], [O/Fe] ~ +0.25 is found. A mean of the two
methods gives a final value of [O/Fe] ~ +0.4 for the metallicity range -2.2 <
[Fe/H] < -1.2 of the sample metal-poor stars.
|
0104184v1
|
2001-04-11
|
Evolution of Lithium-Beryllium-Boron and Oxygen in the early Galaxy
|
Oxygen is a much better evolutionary index than iron to describe the history
of Lithium-Beryllium-Boron (LiBeB) since it is the main producer of these light
elements at least in the early Galaxy. The O-Fe relation is crucial to the
determination of the exact physical process responsible for the LiBeB
production. At low metallicity, if [O/Fe] vs [Fe/H] is flat, then the
production mode is independent of the interstellar metallicity, BeB is
proportional to oxygen, i.e. is of primary nature. If not, the production mode
is function of the progressive enrichment in O of the interstellar medium, BeB
varies rather as the square of O, i.e. is of secondary nature. In the first
case, fast nuclei enriched into He, C and O injected by supernovae and
accelerated in surrounding superbubbles would explain the primary trend.
In the second case, the main spallative agent would be the standard galactic
cosmic rays. Calculated nucleosynthetic yields of massive stars, estimates of
the energy cost of production of beryllium nuclei, and above all recent
observations reported in this meeting seem to favor the primary mechanism, at
least in the early Galaxy.
|
0104194v1
|
2001-04-12
|
NLTE Synthetic Spectral Fits to the Type Ia Supernova 1994D in NGC 4526
|
We have fit the normal, well observed, Type Ia Supernova (SN Ia) SN 1994D
with non-LTE spectra of the deflagration model W7. We find that well before
maximum luminosity W7 fits the optical spectra of SN 1994D. After maximum
brightness the quality of the fits weakens as the spectrum forms in a core rich
in iron peak elements. We show the basic structure of W7 is likely to be
representative of the typical SN Ia. We have shown that like W7, the typical SN
Ia has a layer of unburned C+O composition at v > 15000 \kmps, followed by
layers of C-burned and O-burned material with a density structure similar to
W7. We present UVOIR (UBVRIJKH) synthetic photometry and colors and compare
with observation. We have computed the distance to the host galaxy, NGC 4526,
obtaining a distance modulus of \mu = 30.8 \pm 0.3. We discuss further
application of this direct measurement of SNe Ia distances. We also discuss
some simple modifications to W7 that could improve the quality of the fits to
the observations.
|
0104225v1
|
2001-04-17
|
A Search for Fine Structure of the Knee in EAS Size Spectra
|
28 size spectra of extensive air showers from 7 different experiments are
analysed consistently. They are fitted by adjusting either 4 or 5 parameters:
knee position, power law exponents above and below the knee, overall intensity
and, in addition, a parameter describing the smoothness of the bend. The
residuals are then normalized to the same knee position and averaged. When 5
parameters are employed no systematic deviation from a single smooth knee is
apparent at the 1% level up to a factor of 4 above the knee. At larger shower
sizes a moderately significant deviation can be seen whose shape and position
are compatible with a second knee caused by iron group nuclei.
|
0104282v1
|
2001-06-04
|
Ionisation Structure in Accretion Shocks with a Composite Cooling Function
|
We have investigated the ionisation structure of the post-shock regions of
magnetic cataclysmic variables using an analytic density and temperature
structure model in which effects due to bremsstrahlung and cyclotron cooling
are considered. We find that in the majority of the shock-heated region where
H- and He-like lines of the heavy elements are emitted, the
collisional-ionisation and corona-condition approximations are justified. We
have calculated the line emissivity and ionisation profiles for Iron as a
function of height within the post-shock flow. For low-mass white dwarfs, line
emission takes place near the shock. For high-mass white dwarfs, most of the
line emission takes place in regions well below the shock and hence it is less
sensitive to the shock temperature. Thus, the line ratios are useful to
determine the white-dwarf masses for the low-mass white dwarfs, but the method
is less reliable when the white dwarfs are massive. Line spectra can, however,
be used to map the hydrodynamic structure of the post-shock accretion flow.
|
0106052v1
|
2001-06-04
|
L-dwarf variability: Magnetic star spots or non-uniform clouds?
|
The recent discovery of photometric variations in L dwarfs has opened a
discussion on the cause of the variations. We argue against the existence of
magnetic spots in these atmospheres and favor the idea that non-uniform
condensate coverage (i.e. clouds) is responsible for the variations. The
magnetic Reynolds number (Rm) in the atmosphere of L dwarfs, which describes
how well the gas couples with the magnetic field, is too small (<<1) to support
the formation of magnetic spots. In constrast silicate and iron clouds form in
the photospheres of L dwarfs. Inhomogeneities in such cloud decks can plausibly
produce the observed photometric variations. Further evidence in support of
clouds is the tendency for variable L dwarfs to be bluer in J-Ks than the
average L dwarf of a given spectral type. This color effect is expected if
clear holes appear in an otherwise uniform cloud layer.
|
0106062v1
|
2001-06-06
|
The nebular spectra of the hypernova SN 1998bw and evidence for asymmetry
|
The nebular spectra of the energetic Type Ic supernova SN1998bw (hypernova)
are studied. The transition to the nebular phase occurred at an epoch of about
100 days after outburst, which is assumed to coincide with GRB980425. Early in
the nebular epoch the spectra show the characteristics of a typical SN Ic
spectrum, with strong lines of [OI], CaII and MgI], and lines of [FeII].
However, the [FeII] lines are unusually strong for a SN Ic. Also, lines of
different elements have different widths, indicating different expansion
velocities. In particular, iron appears to expand more rapidly than oxygen.
Furthermore, the [OI] nebular lines decline more slowly than the [FeII] ones,
signalling deposition of gamma-rays in a slowly-moving O-dominated region.
These facts suggest that the explosion was aspherical. The absence of [FeIII]
nebular lines can be understood if the ejecta are significantly clumped. A
schematic picture of what this very unusual stellar explosion may have looked
like is presented.
|
0106095v1
|
2001-06-12
|
The Stellar Parameters and Evolutionary State of the Primary in the d'-Symbiotic System StH\alpha190
|
We report on a high-resolution, spectroscopic stellar parameter and abundance
analysis of a d' symbiotic star: the yellow component of StH\alpha190. This
star has recently been discovered, and confirmed here, to be a rapidly rotating
(vsini=100 km/s) subgiant, or giant, that exhibits radial-velocity variations
of probably at least 40 km/s, indicating the presence of a companion (a white
dwarf star). It is found that the cool stellar component has Teff=5300K and log
g=3.0. The iron and calcium abundances are close to solar, however, barium is
overabundant, relative to Fe and Ca, by about +0.5 dex. The barium enhancement
reflects mass-transfer of s-process enriched material when the current white
dwarf was an asymptotic giant branch (AGB) star. The past and future evolution
of this binary system depends critically on its current orbital period, which
is not yet known. Concerted and frequent radial-velocity measurements are
needed to provide crucial physical constraints to this d' symbiotic system.
|
0106215v1
|
2001-06-13
|
ASCA observations of massive medium-distant clusters of galaxies. II
|
We have selected seven medium-distant clusters of galaxies (z ~ 0.1 - 0.3)
for multi-wavelength observations with the goal of investigating their
dynamical state. Following Paper I (Pierre et al. 1999) which reported the ASCA
results about two of them, we present here the analysis of the ASCA
observations of the other five clusters; RXJ1023.8-2715 (A3444),
RXJ1031.6-2607, RXJ1050.5-0236 (A1111), RXJ1203.2-2131(A1451), and
RXJ1314.5-2517. Except for RXJ1031.6, whose X-ray emission turned out to be
dominated by an AGN, the ASCA spectra are well fitted by a one-temperature thin
thermal plasma model. We compare the temperature-luminosity relation of our
clusters with that of nearby ones (z<0.1). Two clusters, RXJ1050.5 and
RXJ1023.8, show larger luminosities than the bulk of clusters at similar
temperatures, which suggests the presence of a cooling flow. The temperature
vs. iron-abundance relationship of our sample is consistent with that of nearby
clusters.
|
0106248v1
|
2001-06-19
|
Bremsstrahlung of Flavor-Degenerate Pairs by Neutrinos in the Nuclear Field
|
Neutrino Bremsstrahlung of flavor-degenerate pairs in the field of a nucleus
is of potential importance for neutrino astrophysics and is representative of a
class of processes connecting leptonic electroweak sectors to real or virtual
photons. We focus on first generation flavor production by both electron and
muon neutrinos and present Standard Model cross sections and distributions for
lead and iron nuclei. The results (of order 10-41 cm2 for electron neutrino -
lead collisions at 100 MeV) have been fitted to empirical formulae that can be
used to estimate backgrounds to neutrino detection experiments and flux
normalizations. A compact form of the matrix element obtained by analytic
reduction is used to explain the distributions. The V-A limits of the cross
sections are shown to agree with published work from the pre-neutral current
era. Event signatures and the possible roles of these processes in stellar and
laboratory neutrino physics are discussed. Cross sections are compared with
those for neutrino-electron scattering for neutrino spectra corresponding to
typical supernovae temperatures.
|
0106317v2
|
2001-06-19
|
The K-alpha complex of He-like iron with dielectronic satellites
|
It is shown that the dielectronic satellites (DES) dominate X-ray spectral
formation in the 6.7 keV K-alpha complex of Fe XXV at temperatures below that
of maximum abundance in collisional ionization equilibrium T_m. Owing to their
extreme temperature sensitivity the DES are excellent spectral diagnostics for
T < T_m in photoionized, colllisional, or hybrid plasmas; whereas the
forbidden, intercombination, and resonance lines of Fe XXV are not. A
diagnostic line ratio GD(T) is defined including the DES and the lines, with
parameters from new relativistic atomic calculations. The DES absorption
resonance strengths may be obtained from differential oscillator strengths to
possibly yield the Fe XXIV/Fe XXV column densities. The DES contribution to
highly ionized Fe should be of interest for models of redward broadening of
K-alpha features, ionized accretion discs, accretion flows, and K-alpha
temporal-temperature variability in AGN.
|
0106330v1
|
2001-07-05
|
Spectral Indices of Stars at Super-solar Regime
|
We derived Lick narrow-band indices for 139 candidate super metal-rich stars
of different luminosity class previously studied in Malagnini et al. (2000).
Indices include Iron Fe50, Fe52, Fe53, and Magnesium Mgb and Mg2 features. By
comparing observations with Kurucz' synthetic model atmospheres, no evidence is
found for non-standard Mg vs. Fe relative abundance (i.e. [Mg/Fe]~ 0, on the
average, for our sample). A comparison with the Worthey et al.(1994) and
Buzzoni et al.(1992, 1994) fitting function predictions for [Fe/H] > 0 is
performed and briefly discussed.
|
0107105v1
|
2001-07-06
|
In-flight performances of SilEye-2 Experiment and cosmic ray abundances inside space station Mir
|
Cosmic ray measurements performed with the instrument SilEye-2 on Mir space
station are presented. SilEye-2 is a silicon detector telescope for the study
of the causes of Light Flashes perception by astronauts. As a stand-alone
device, it monitors in the short and long term the radiation composition inside
Mir. The cosmic ray detector consists of an array of 6 active silicon strip
detectors which allow nuclear identification of cosmic rays up to Iron. The
device was operational for more than 1000 hours in the years 1998-2000,
measuring also several Solar Particle Events. In this work we present the
in-flight performance of the instrument and nuclear abundance data from Boron
to Silicon above ~150 MeV/n inside Mir.
|
0107128v1
|
2001-07-13
|
BeppoSAX observations of the Narrow-Line Seyfert 1 galaxy RX J1702.5+3247
|
We report optical, radio and X-ray observations of the Narrow-Line Seyfert 1
galaxy RX J1702.5+3247. The soft (0.1-2 keV) X-ray flux, measured by BeppoSAX,
is characterized by strong variability on short time scales (< 500 sec). The
most extreme amplitude variations require a radiative efficiency exceeding the
maximum for a Kerr black hole, implying the presence of relativistic effects. A
comparison with archival ROSAT data reveals long term temporal and spectral
variability. The 0.1-10 keV spectrum is equally well fitted either by an
ionized reflection disk model, or by a broken power law plus a Gaussian line,
consistent with a hydrogen-like iron line at 6.97 keV from a highly ionized
accretion disk.
|
0107240v1
|
2001-07-18
|
A Possible Emission Feature in an X-ray Afterglow of GRB970828 as a Radiative Recombination Edge
|
A gamma-ray burst of 28 August 1997 was localized by the All-Sky Monitor on
the Rossi XTE satellite and its coordinates were promptly disseminated. An ASCA
followup started 1.17 days after the burst as a Target of Opportunity
Observation and detected an X-ray afterglow. The spectral data displayed a hump
around ~5 keV and an absorption column of 7.1 x 10^21 cm^{-2}. This hump
structure is likely a recombination edge of iron in the vicinity of the source,
taking account of the redshift z = 0.9578 found for the likely host galaxy of
the associated radio flare. Radiative Recombination edge and Continuum model
can interpret the spectrum from highly ionized plasma in a non equilibrium
ionization state. The absorption could be also due to the medium presumably in
the vicinity of the GRB.
|
0107331v1
|
2001-08-28
|
Neutrinos from Early-Phase, Pulsar-Driven Supernovae
|
Neutron stars, just after their formation, are surrounded by expanding,
dense, and very hot envelopes which radiate thermal photons. Iron nuclei can be
accelerated in the wind zones of such energetic pulsars to very high energies.
These nuclei photo-disintegrate and their products lose energy efficiently in
collisions with thermal photons and with the matter of the envelope, mainly via
pion production. When the temperature of the radiation inside the envelope of
the supernova drops below $\sim 3\times 10^6$ K, these pions decay before
losing energy and produce high energy neutrinos. We estimate the flux of muon
neutrinos emitted during such an early phase of the pulsar - supernova envelope
interaction. We find that a 1 km$^2$ neutrino detector should be able to detect
neutrinos above 1 TeV within about one year after the explosion from a
supernova in our Galaxy. This result holds if these pulsars are able to
efficiently accelerate nuclei to energies $\sim 10^{20}$ eV, as postulated
recently by some authors for models of Galactic acceleration of the extremely
high energy cosmic rays (EHE CRs).
|
0108447v2
|
2001-09-07
|
X-ray Spectra and Light Curves of AR Lac: Temperature Structure, Abundances, and Variability
|
We observed AR Lac, an eclipsing RS CVn binary star, with the Chandra High
Energy Grating Spectrometer for a total of 100 ks divided into six intervals
covering both quadratures and eclipses. We repeated observations at each phase.
At least two flares were seen, in which the flux increased by factors of two
and four. The flares occurred near eclipse phases and compromised detection of
eclipse modulation. The quadrature fluxes were the most stable, but they also
show non-repeating trends. Quadrature line profiles are broadened relative to
eclipse profiles, presumably due to orbital velocity Doppler shifted emission
from each binary component. The spectrum appears to be iron poor and neon rich,
similar to HR 1099, but not as extreme as II Pegasi. We will examine line
strengths, widths, and positions vs. phase, and present preliminary
differential emission measure models and abundance determinations using the
APED emissivity database.
This work is supported by NASA contract NAS8-38249 (HETG) and SAO SV1-61010
(CXC) to MIT.
|
0109120v1
|
2001-09-10
|
A multivariate study of mass composition for simulated showers at the Auger South Observatory
|
The output parameters from the ground array of the Auger South observatory,
were simulated for the typical instrumental and environmental conditions at its
Malarg\"ue site using the code sample-sim. Extensive air showers started by
photons, protons and iron nuclei at the top of the atmosphere were used as
triggers. The study utilized the air shower simulation code Aires with both
QGSJet and Sibyll hadronic interaction models. A total of 1850 showers were
used to produce more than 35,000 different ground events. We report here on the
results of a multivariate analysis approach, including principal component
analysis and neural networks, to the development of new primary composition
diagnostics.
|
0109140v1
|
2001-09-18
|
Inclinations and black hole masses of Seyfert 1 galaxies
|
A tight correlation of black hole mass and central velocity dispersion has
been found recently for both active and quiescent galaxies. By applying this
correlation, we develop a simple method to derive the inclination angles for a
sample of 11 Seyfert 1 galaxies that have both measured central velocity
dispersions and black hole masses estimated by reverberation mapping. These
angles, with a mean value of 36 degree that agrees well with the result
obtained by fitting the iron K$\alpha$ lines of Seyfert 1s observed with ASCA,
provide further support to the orientation-dependent unification scheme of AGN.
A positive correlation of the inclinations with observed FWHMs of H$\beta$ line
and a possible anti-correlation with the nuclear radio-loudness have been
found. We conclude that more accurate knowledge on inclinations and broad line
region dynamics is needed to improve the black hole mass determination of AGN
with the reverberation mapping technique.
|
0109283v1
|
2001-09-19
|
Soft X-Ray Absorption by Fe$^{0+}$ to Fe$^{15+}$ in Active Galactic Nuclei
|
A full set of calculations is presented for inner-shell n = 2 to 3
photoexcitation of the 16 iron charge states: Fe I through Fe XVI. The blend of
the numerous absorption lines arising from these excitations (mainly 2p - 3d)
forms an unresolved transition array (UTA), which has been recently identified
as a prominent feature between 16 - 17 \AA in the soft X-ray spectra of active
galactic nuclei (AGN). Despite the blending within charge-states, the ample
separation between the individual-ion features enables precise diagnostics of
the ionization range in the absorbing medium. Column density and turbulent
velocity diagnostics are also possible, albeit to a lesser accuracy. An
abbreviated set of atomic parameters useful for modeling the Fe 2p - 3d UTA is
given. It is shown that the effects of accompanying photoexcitation to higher
levels ($n \ge$ 4), as well as the associated photoionization edges, may also
be relevant to AGN spectra.
|
0109314v1
|
2001-09-21
|
The Chandra LETGS high resolution X-ray spectrum of the isolated neutron star RX J1856.5-3754
|
We present the Chandra LETGS X-ray spectrum of the nearby (~60 pc) neutron
star RX J1856.5-3754. Detailed spectral analysis of the combined X-ray and
optical data rules out the nonmagnetic neutron star atmosphere models with
hydrogen, helium, iron and solar compositions. We also conclude that strongly
magnetized atmosphere models are unable to represent the data. The data can be
explained with a two-component blackbody model. The harder component with
temperature of kT_bb~63 eV and a radius R_bb~2.2 km of the emitting region well
fits the X-ray data and can be interpreted as radiation from a hot region on
the star's surface.
|
0109374v1
|
2001-09-28
|
Molybdenum Evidence for Inherited Planetary Scale Isotope Heterogeneity of the Protosolar Nebula
|
Isotope anomalies provide important information about early solar system
evolution. Here we report molybdenum isotope abundances determined in samples
of various meteorite classes. There is no fractionation of molybdenum isotopes
in our sample set within 0.1 permil and no contribution from the extinct
radionuclide 97Tc at mass 97 (97Tc/92Mo<3E-6). Instead, we observe clear
anomalies in bulk iron meteorites, mesosiderites, pallasites, and chondrites
characterized by a coupled excess in p- and r- or a mirror deficit in s-process
nuclides (Mo-HL). This large scale isotope heterogeneity of the solar system
observed for molybdenum must have been inherited from the interstellar
environment where the sun was born, illustrating the concept of ``cosmic
chemical memory''. The presence of molybdenum anomalies is used to discuss the
filiation between planetesimals.
|
0109549v1
|
2001-10-08
|
Abundances and Kinematics of Field Stars II: Kinematics and Abundance Relationships
|
As an investigation of the origin of ``$\alpha$-poor'' halo stars, we analyze
kinematic and abundance data for 73 intermediate metallicity stars ($-1 >$
[Fe/H] $\geq -2$) selected from Paper I of this series. We find evidence for a
connection between the kinematics and the enhancement of certain
element-to-iron ([X/Fe]) ratios in these stars. Statistically significant
correlations were found between [X/Fe] and galactic rest-frame velocities
(\vrf{}) for Na, Mg, Al, Si, Ca and Ni, with marginally significant
correlations existing for Ti and Y as well. We also find that the [X/Fe] ratios
for these elements all correlate with a similar level of significance with
[Na/Fe]. Finally, we compare the abundances of these halo stars against those
of stars in nearby dSph galaxies. We find significant differences between the
abundance ratios in the dSph stars and halo stars of similar metallicity. From
this result, it is unlikely that the halo stars in the solar neighborhood,
including even the ``$\alpha$-poor'' stars, were once members of disrupted dSph
galaxies similar to those studied to date.
|
0110164v1
|
2001-11-01
|
Extremely Weak Reflection Features in the X-ray Spectrum of XTE J1118+480: Possible Evidence for X-ray-Emitting Jets?
|
We have simultaneously fit Chandra and RXTE spectra of the Galactic black
hole XTE J1118+480 with three models for X-ray reflection. We explored a range
of accretion disc ionizations (log(xi)=1-4; xi=L_X/nR^{2}) and iron abundances
(0.10-1.00). Our fits with the constant density ionized disc models of Ross &
Fabian indicate that less than 0.5 per cent (90 per cent confidence
upper-limit) of the observed flux is reflected. Fits with the "pexrav" of model
Magdziarz & Zdziarski indicate that the two-dimensional solid angle (Omega/2pi)
subtended by the disc relative to a central source of incident hard X-rays is
0.01 +0.06 -0.01. A combination of the high inclination (i=81 degrees),
Comptonization, and bulk velocities may each contribute to the low reflection
fractions we have measured. The results are also consistent with extended jets
being the source of the hard X-ray flux, as the disc would then represent a
small solid angle as seen from the emission region.
|
0111027v2
|
2001-11-02
|
Evidence for Accretion: High-Resolution X-ray Spectroscopy of the Classical T Tauri Star TW Hydrae
|
We present high resolution X-ray spectra of the X-ray bright classical T
Tauri star, TW Hydrae, covering the wavelength range of 1.5-25 AA. The
differential emission measure derived from fluxes of temperature-sensitive
emission lines shows a plasma with a sharply peaked temperature distribution,
peaking at log T = 6.5. Abundance anomalies are apparent, with iron very
deficient relative to oxygen, while neon is enhanced relative to oxygen.
Density-sensitive line ratios of Ne IX and O VII indicate densities near log
n_e = 13. A flare with rapid (~1 ks) rise time was detected during our 48 ksec
observation; however, based on analysis of the emission-line spectrum during
quiescent and flaring states, the derived plasma parameters do not appear
strongly time-dependent. The inferred plasma temperature distribution and
densities are consistent with a model in which the bulk of the X-ray emission
from TW Hya is generated via mass accretion from its circumstellar disk.
Assuming accretion powers the X-ray emission, our results for log n_e suggest
an accretion rate of ~10^{-8} M_sun yr^{-1}.
|
0111049v1
|
2001-11-02
|
Disclosing the true nature of the Sy 2 galaxy NGC 3281: one more Compton-thick source
|
We present the BeppoSAX broad-band X-ray spectrum of the Seyfert 2 galaxy NGC
3281. The source high-energy spectrum is characterized by the nuclear
transmitted component, with an absorbing column density of about 2 10^24 cm^-2,
while the MECS spectrum is reflection-dominated, with a prominent (EW about
0.5-1.2 keV) iron K_alpha emission line. The source is detected at only the 5
sigma significance level in the LECS band, because of the strong obscuration
which hampers at low energies the direct view of the active nucleus harbored in
NGC 3281. BeppoSAX results are consistent with the scenario where NGC 3281 is
inclined more than 60 degrees with respect to the line-of-sight. Combining the
N_H value obtained from the present X-ray analysis with the A_V measurement, a
N_H/A_V about 50 times the Galactic value is derived.
|
0111056v1
|
2001-11-15
|
Millions of Single Cloud Weak MgII Systems
|
We report on a population of absorption systems selected by the presence of
very weak Mg II doublets. A sub-population of these systems are iron enriched
and have near solar metallicities. This would indicated advanced stages (i.e.
few Gyr) of in situ star formation within the absorbing structures. From
photoionization modeling, we infer low ionization fractions of f(HI/H)~0.01,
and gas densities of ~0.1 cm^-3. Since the maximum HI column densities are
\~10^17 cm^-2, the inferred cloud sizes are ~10 pc. From their redshift number
densities, this implies that their co-moving spatial density outnumbers normal
bright galaxies by a factor of a few million.
|
0111315v1
|
2001-11-21
|
Chandra Grating Observations of Seyfert 1 Galaxies
|
We present new results from a Chandra HETG observation of NGC 5548 and give a
comparison of absorption and emission features found in other Seyfert 1
galaxies using Chandra grating observations. Deep soft X-ray edges are seen in
Mkn 509 and NGC 3783, consistent with ASCA data. In NGC 5548 and NGC 4501 the
edges are weak but consistent with the low column densities. We show that the
detection of a narrow, probably non-disk, component of the iron line is very
common. However, just as importantly, it is not detected in some cases. We show
the effect of removing this narrow component from the ASCA Fe-K line profile in
NGC 4151, revealing the underlying true shape of the relativistic Fe-K line
component.
|
0111428v1
|
2001-12-17
|
Spectral properties of the Narrow-Line Seyfert 1 Quasar PG1211+143
|
We propose a theoretical model to explain the spectrum of the quasar
PG1211+143 emitted in the Optical/X-ray bands. In particular, we suggest that
the inner accretion disk may develop a warm, optically thick skin, which
produces a profound emission feature observed in the soft X-ray band. This is
well modelled with the Comptonized black body emission.
The same warm, mildly ionized medium may also be responsible for the hard
X-ray reflection and the presence of the iron $K_{\alpha}$ line. However, in
our model it still remains an open question, whether the seed photons for
Comptonization come from the cold accretion disc or from the hotter plasma.
High resolution spectroscopy available through the Chandra and XMM data may
provide now an independent test of the physical conditions in the Comptonizing
and reflecting warm skin.
|
0112387v1
|
2002-01-24
|
Inner-shell 1s - 2p Soft X-ray Absorption Lines
|
The HULLAC atomic code is used to compute wavelengths and oscillator
strengths for the 1s - np inner-shell absorption lines in Li-like to F-like
ions of neon, magnesium, aluminum, silicon, sulfur, argon, calcium, and iron.
Many of these lines are expected to be observed in Chandra and XMM-Newton
high-resolution X-ray spectra of active galaxies. The new atomic data are
incorporated in the ION code for spectral modeling of photoionized plasmas. The
calculated spectra are subsequently compared with the spectrum of NGC 3783 and
show very good agreement. The usefulness of these lines as diagnostics for the
ionization state, column densities, and velocities in line-of-sight
photoionized gas is called attention to.
|
0201416v1
|
2002-01-28
|
Theoretical Uncertainties in the Subgiant--Mass Age Relation and the Absolute Age of Omega Cen
|
The theoretical uncertainties in the calibration of the relationship between
the subgiant mass and age in metal-poor stars are investigated using a Monte
Carlo approach. Assuming that the mass and iron abundance of a subgiant star
are known exactly, uncertainties in the input physics used to construct stellar
evolution models and isochrones lead to a Gaussian 1-sigma uncertainty of
+/-2.9% in the derived ages. The theoretical error budget is dominated by the
uncertainties in the calculated opacities.
Observations of detached double lined eclipsing binary OGLEGC-17 in the
globular cluster Omega Cen have found that the primary is on the subgiant
branch with a mass of M = 0.809+/-0.012 M_sun and [Fe/H]= -2.29+/-0.15 (Kaluzny
et al. 2001). Combining the theoretical uncertainties with the observational
errors leads to an age for OGLEGC-17 of 11.10+/-0.67 Gyr. The one-sided, 95%
lower limit to the age of OGLEGC-17 is 10.06 Gyr, while the one-sided, 95%
upper limit is 12.27 Gyr.
|
0201443v1
|
2002-01-31
|
XMM-Newton EPIC observations of Her X-1
|
We present spin-resolved X-ray data of the neutron star binary Her X-1. We
find evidence that the Iron line at 6.4 keV originates from the same location
as the blackbody X-ray component. The line width and energy varies over both
the spin period and the 35 day precession period. We also find that the
correlation between the soft and hard X-ray light curves varies over the 35 day
period.
|
0201526v1
|
2002-02-04
|
The s-process in rotating AGB stars
|
We discuss the occurrence of the s-process during the radiative interpulse
phase of rotating AGB stars. Due to differential rotation, protons are mixed
into C12-rich layers after thermal pulses, in the course of the so called third
dredge up episode. We follow the time evolution of key isotope abundances in
the relevant layers with a post-processing code which includes time dependant
mixing and nucleosynthesis. In rotating AGB models, the mixing persists during
the entire interpulse phase due to the steep gradient of angular velocity at
the envelope-core interface. As the layers containing protons and C12, which
are formed this way, become hotter, a C13-pocket is formed in a natural way.
However, in this situation also N14 is formed and spread over the entire
C13-pocket. We include the neutron consuming N14(n,p) reaction in our network
and determine to what extent it reduces the production of trans-iron elements.
We propose that rotation may be responsible for the spread of efficiencies of
the C13 neutron source as required by observations.
|
0202067v1
|
2002-02-04
|
Discovery of complex narrow X-ray absorption features from the low-mass X-ray binary GX13+1 with XMM-Newton
|
We report the detection of a complex of narrow X-ray absorption features from
the low-mass X-ray binary GX13+1 during 3 XMM-Newton observations in 2000 March
and April. The features are consistent with being due to resonant scattering of
the K_alpha and K_beta lines of He- and H-like iron (Fe xxv and Fe xxvi) and
H-like calcium (Ca xx) K_alpha. Only the Fe xxvi K_alpha line has been
previously observed from GX13+1. Due to the closeness in energy the Fe xxv and
Fe xxvi K_beta features may also be ascribed to Nixxvii and Nixxviii K_alpha,
respectively. We also find evidence for the presence of a deep (tau~0.2) Fe xxv
absorption edge at 8.83 keV. The equivalent widths of the lines do not show any
obvious variation on a timescale of a few days suggesting that the absorbing
material is a stable feature of the system and present during a range of
orbital phases.
|
0202077v1
|
2002-02-19
|
Atomic Processes in Planetary Nebulae
|
A hot central star illuminating the surrounding ionized H II region usually
produces very rich atomic spectra resulting from basic atomic processes:
photoionization, electron-ion recombination, bound-bound radiative transitions,
and collisional excitation of ions. Precise diagnostics of nebular spectra
depend on accurate atomic parameters for these processes. Latest developments
in theoretical computations are described, especially under two international
collaborations known as the Opacity Project (OP) and the Iron Project (IP),
that have yielded accurate and large-scale data for photoionization cross
sections, transition probabilities, and collision strengths for electron impact
excitation of most astrophysically abundant ions. As an extension of the two
projects, a self-consistent and unified theoretical treatment of
photoionization and electron-ion recombination has been developed where both
the radiative and the dielectronic recombination processes are considered in an
unified manner. Results from the Ohio State atomic-astrophysics group, and from
the OP and IP collaborations, are presented. A description of the electronic
web-interactive database, TIPTOPBASE, with the OP and the IP data, and a
compilation of recommended data for effective collision strengths, is given.
|
0202343v1
|
2002-02-27
|
Long Term Spectral Variability of Seyfert Galaxies from RXTE Color-Flux Diagrams
|
We present results from RXTE data obtained during a systematic monitoring
program of four Seyfert galaxies (NGC5548, NGC5506, MCG-6-30-15 and NGC4051).
We studied the variability of three hardness ratios derived from the light
curves in four energy bands. All the objects show similar spectral variations
in all ratios. In order to interpret the results we computed the hardness
ratios corresponding to a simple spectral model of a power law plus iron line
plus reflection component, considering two possibilities: a)variations with
constant line equivalent width and reflection parameter R and b) variations
with constant line and reflection flux. The overall, mean observed trends can
be explained by spectral slope variations and a constant flux Fe line and
reflection component, although the existence of a line component which is
variable on short time scales cannot be excluded. Finally, we find that the
data are not consistent with an increase of R with flux for individual sources,
indicating that, as a single source varies, softer spectra do not correspond to
larger R values.
|
0202498v1
|
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