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2006-02-04
|
Comprehensive Analysis of RXTE Data from Cyg X-1: Spectral Index-Quasi-Periodic Oscillation Frequency-Luminosity Correlations
|
We present timing and spectral analysis of ~ 2.2 Ms of RXTE archival data
from Cyg X-1. Using a generic Comptonization model we reveal that the spectrum
of Cyg X-1 consists of three components: a thermal seed photon spectrum, a
Comptonized part of the seed photon spectrum and the iron line. We find a
strong correlation between the 0.1-20 Hz frequencies of quasiperiodic
oscillations (QPOs) and the spectral index. Presence of two spectral phases
(states) are clearly seen in the data when the spectral indices saturate at low
and high values of QPO frequencies. This saturation effect was discovered
earlier in a number of black hole candidate (BHC) sources and now we strongly
confirm this phenomenon in Cyg X-1. In the soft state this index-QPO frequency
correlation shows a saturation of the photon index Gamma ~ 2.1 at high values
of the low frequency \nu_{L}. The saturation level of Gamma~ 2.1 is the lowest
value found yet in BHCs. The bolometric luminosity does not show clear
correlation with the index. We also show that Fe K_{\alpha} emission line
strength (equivalent width, EW) correlates with the QPO frequency. The EW
increases from 200 eV in the low/hard state to 1.5 keV in the high/soft state.
The observational correlations revealed compel us to propose a scenario for the
spectral transition and iron line formation which occur in BHC sources. We also
present the spectral state (power-law index) evolution for eight years of Cyg
X-1 observations by RXTE.
|
0602091v1
|
2006-02-27
|
Primordial Stellar Feedback and the Origin of Hyper Metal-Poor Stars
|
The apparent absence of stars in the Milky Way halo with -5 ~< [Fe/H] ~< -4
suggests that the gas out of which the halo stars were born experienced a
period of low or delayed star formation after the local universe was lit up by
the first, metal-free generation of stars (Pop III). Negative feedback owed to
the Pop III stars could initially have prevented the pre-Galactic halo from
cooling, which thereby delayed the collapse and inhibited further star
formation. During this period, however, the nucleosynthesis products of the
first supernovae (SNe) had time to mix with the halo gas. As a result, the
initially primordial gas was already weakly enriched in heavy elements, in
particular iron, at the time of formation of the Galactic halo. The very high,
observed C/Fe ratios in the two recently discovered hyper metal-poor stars
([Fe/H]<-5) HE 0107-5240 and HE 1327-2326 as well as the diversity of C/Fe
ratios in the population of extremely metal-poor stars ([Fe/H]<-3) are then
naturally explained by a combination of pre-enrichment by Pop III stars and
local enrichment by subsequent generations of massive, rotating stars, for
which the most massive ones end their lives as black hole-forming SNe, only
ejecting their outer (carbon-rich) layers. The possible existence of
populations of mega metal-poor/iron-free stars ([Fe/H]<-6) is also discussed.
|
0602597v1
|
2006-04-03
|
Analysis of 26 Barium Stars I. Abundances
|
We present a detailed analysis of 26 barium stars, including dwarf barium
stars, providing their atmospheric parameters (Teff, log g, [Fe/H], vt) and
elemental abundances. We aim at deriving gravities and luminosity classes of
the sample stars, in particular to confirm the existence of dwarf barium stars.
Accurate abundances of chemical elements were derived. Abundance ratios between
nucleosynthetic processes, by using Eu and Ba as representatives of the r- and
s-processes are presented. High-resolution spectra with the FEROS spectrograph
at the ESO-1.5m Telescope, and photometric data with Fotrap at the Zeiss
telescope at the LNA were obtained. The atmospheric parameters were derived in
an iterative way, with temperatures obtained from colour-temperature
calibrations. The abundances were derived using spectrum synthesis for Li, Na,
Al, alpha-, iron peak, s- and r-elements atomic lines, and C and N molecular
lines. Atmospheric parameters in the range 4300 < Teff < 6500, -1.2 < [Fe/H] <
0.0 and 1.4 < log g < 4.6 were derived, confirming that our sample contains
giants, subgiants and dwarfs. The abundance results obtained for Li, Al, Na,
alpha- and iron peak elements for the sample stars show that they are
compatible with the values found in the literature for normal disk stars in the
same range of metallicities. Enhancements of C, N and heavy elements relative
to Fe, that characterise barium stars, were derived and showed that [X/Ba] vs.
[Ba/H] and [X/Ba] vs. [Fe/H] present different behaviour as compared to [X/Eu]
vs. [Eu/H] and [X/Eu] vs. [Fe/H], reflecting the different nucleosynthetic
sites for the s- and r-processes.
|
0604036v1
|
2006-04-13
|
Extinction and metal column density of HI regions up to redshift z~2
|
We used the photometric database of the Sloan Digital Sky Survey (SDSS) to
estimate the reddening of 13 SDSS quasars selected on the basis of the presence
of zinc absorption lines in an intervening Damped Ly alpha (DLA) system. In 5
of these quasars the reddening is detected at ~2 sigma confidence level in two
independent color indices of the SDSS ugriz photometric system. A detailed
analysis of the data supports an origin of the reddening in the intervening
absorbers. We used these rare measurements of extinction in DLA systems to
probe the relation between extinction and metal column density in the interval
of absorption redshift 0.7 </~ z </~ 2.0. We find that the mean extinction in
the V band per atom of iron in the dust is remarkably similar to that found in
interstellar clouds of the Milky Way. This result lends support to previous
estimates of the dust obscuration effect in DLA systems based on a Milky Way
extinction/metal column density relation. We propose a simple mechanism, based
on dust grain destruction/accretion properties, which may explain the
approximate constancy of the extinction per atom of iron in the dust.
|
0604296v1
|
2006-05-01
|
Broad band X-ray spectrum of KS 1947+300 with BeppoSAX
|
We present results obtained from three BeppoSAX observations of the
accretion-powered transient X-ray pulsar KS 1947+300 carried out during the
declining phase of its 2000 November -- 2001 June outburst. A detailed spectral
study of KS 1947+300 across a wide X-ray band (0.1--100.0 keV) is attempted for
the first time here. Timing analysis of the data clearly shows a 18.7 s
pulsation in the X-ray light curves in the above energy band. The pulse profile
of KS 1947+300 is characterized by a broad peak with sharp rise followed by a
narrow dip. The dip in the pulse profile shows a very strong energy dependence.
Broad-band pulse-phase-averaged spectroscopy obtained with three of the
BeppoSAX instruments shows that the energy spectrum in the 0.1--100 keV energy
band has three components, a Comptonized component, a ~0.6 keV blackbody
component, and a narrow and weak iron emission line at 6.7 keV with a low
column density of material in the line of sight. We place an upper limit on the
equivalent width of the iron K_\alpha line at 6.4 keV of ~13 eV (for a width of
100 eV). Assuming a spherical blackbody emitting region and the distance of the
source to be 10 kpc, the radius of the emitting region is found to be in the
range of 14--22 km, which rules out the inner accretion disk as the soft X-ray
emitting region.
|
0605024v1
|
2006-06-06
|
Detailed XMM-Newton Observation of the Cluster of Galaxies Abell 1060
|
We present results from the XMM-Newton observation of the non-cooling flow
cluster A1060. Large effective area of XMM-Newton enables us to investigate the
nature of this cluster in unprecedented detail. From the observed surface
brightness distribution, we have found that the gravitational mass distribution
is well described by the NFW profile but with a central density slope of ~1.5.
We have undoubtedly detected a radial temperature decrease of as large as ~30%
from the center to the outer region (r ~13'), which seems much larger than that
expected from the temperature profile averaged over nearby clusters. We have
established that the temperature of the region ~7' southeast of the center is
higher than the azimuthally averaged temperature of the same radius by ~20%.
Since the pressure of this region already reaches equilibrium with the
environment, the temperature structure can be interpreted as having been
produced between 4*10^7 yr (the sound-crossing time) and 3*10^8 yr (the thermal
conduction time) ago. We have found that the high-metallicity blob located at
\~1.'5 northeast of NGC 3311 is more extended and its iron mass of 1.9*10^7
M_solar is larger by an order of magnitude than estimated from our Chandra
observation. The amount of iron can still be considered as being injected
solely from the elliptical galaxy NGC3311.
|
0606115v1
|
2006-07-13
|
RHESSI Observations of the Solar Flare Iron-line Feature at 6.7 keV
|
Analysis of RHESSI 3--10 keV spectra for 27 solar flares is reported. This
energy range includes thermal free--free and free--bound continuum and two line
features, at 6.7keV and 8keV, principally due to highly ionized iron (Fe). We
used the continuum and the flux in the so-called Fe-line feature at 6.7keV to
derive the electron temperature T_e, the emission measure, and the Fe-line
equivalent width as functions of time in each flare. The Fe/H abundance ratio
in each flare is derived from the Fe-line equivalent width as a function of
T_e. To minimize instrumental problems with high count rates and effects
associated with multi-temperature and nonthermal spectral components, spectra
are presented mostly during the flare decay phase, when the emission measure
and temperature were smoothly varying. We found flare Fe/H abundance ratios
that are consistent with the coronal abundance of Fe (i.e. 4 times the
photospheric abundance) to within 20% for at least 17 of the 27 flares; for 7
flares, the Fe/H abundance ratio is possibly higher by up to a factor of 2. We
find evidence that the Fe XXV ion fractions are less than the theoretically
predicted values by up to 60% at T_e=25 MK appear to be displaced from the most
recent theoretical values by between 1 and 3 MK.
|
0607309v1
|
2006-07-17
|
Chemical Evolution of the Galactic Bulge as Derived from High-Resolution Infrared Spectroscopy of K and M Red Giants
|
We present chemical abundances in K and M red-giant members of the Galactic
bulge derived from high-resolution infrared spectra obtained with the Phoenix
spectrograph on Gemini-South. The elements studied are carbon, nitrogen,
oxygen, sodium, titanium, and iron. The evolution of C and N abundances in the
studied red-giants show that their oxygen abundances represent the original
values with which the stars were born. Oxygen is a superior element for probing
the timescale of bulge chemical enrichment via [O/Fe] versus [Fe/H]. The
[O/Fe]-[Fe/H] relation in the bulge does not follow the disk relation, with
[O/Fe] values falling above those of the disk. Titanium also behaves similarly
to oxygen with respect to iron. Based on these elevated values of [O/Fe] and
[Ti/Fe] extending to large Fe abundances, it is suggested that the bulge
underwent a more rapid chemical enrichment than the halo. In addition, there
are declines in both [O/Fe] and [Ti/Fe] in those bulge targets with the largest
Fe abundances, signifying another source affecting chemical evolution: perhaps
Supernovae of Type Ia. Sodium abundances increase dramatically in the bulge
with increasing metallicity, possibly reflecting the metallicity dependant
yields from supernovae of Type II, although Na contamination from H-burning in
intermediate mass stars cannot be ruled out.
|
0607393v1
|
2006-08-24
|
Fall-back crust around a quark-nova compact remnant I: The degenerate shell case with applications to SGRs, AXPs and XDINs
|
We explore the formation and evolution of debris ejected around quark stars
in the Quark Nova scenario, and the application to Soft Gamma-ray Repeaters
(SGRs) and Anomolous X-ray Pulsars (AXPs). If an isolated neutron star explodes
as a Quark Nova, an Iron-rich shell of degenerate matter forms out of the
fall-back (crust) material. Our model can account for many of the observed
features of SGRs and AXPs such as: (i) the two types of bursts (giant and
regular); (ii) the spin-up and spin-down episodes during and following the
bursts with associated persistant increases in $\dot{P}$; (iii) the energetics
of the boxing day burst, SGR1806$+$20; (iv) the presence of an Iron line as
observed in SGR1900$+$14; (v) the correlation between the far-Infrared and the
X-ray fluxes during the bursting episode and the quiescent phase; (vi) the hard
X-ray component observed in SGRs during the giant bursts, and (vii) the
discrepancy between the ages of SGRs/AXPs and their supernova remnants. We also
find a natural evolutionary relationship between SGRs and AXPs in our model
which predicts that only the youngest SGRs/AXPs are most likely to exhibit
strong bursting. Many features of X-ray Dim Isolated Neutron stars (XDINs) are
also accounted for in our model such as, (i) the two-component blackbody
spectra; (ii) the absorption lines around 300 eV; and (iii) the excess optical
emission.
|
0608536v4
|
2006-09-18
|
The Sun is a plasma diffuser that sorts atoms by mass
|
The Sun is a magnetic plasma diffuser that selectively moves light elements
like H and He and the lighter isotopes of each element to its surface. The Sun
formed on the collapsed core of a supernova. It consists mostly of iron,
oxygen, nickel, silicon and sulfur made near the SN core, like the rocky
planets and ordinary meteorites. H ions, generated by emission and decay of
neutrons at the core, are accelerated upward by deep magnetic fields, thus
acting as a carrier gas that maintains mass separation in the Sun. Neutron
emission from the central neutron star triggers a series of reactions that
generate solar luminosity, solar neutrinos, solar mass-fractionation, and an
outpouring of the neutron decay product, H, in the solar wind. Mass
fractionation appears to have operated in the parent star as well, and likely
occurs in other stars.
|
0609509v3
|
2006-09-19
|
Suzaku observations of the hard X-ray variability of MCG-6-30-15: the effects of strong gravity around a Kerr black hole
|
Suzaku has, for the first time, enabled the hard X-ray variability of the
Seyfert 1 galaxy MCG-6-30-15 to be measured. The variability in the 14-45 keV
band, which is dominated by a strong reflection hump, is quenched relative to
that at a few keV. This directly demonstrates that the whole reflection
spectrum is much less variable than the power-law continuum. The broadband
spectral variability can be decomposed into two components - a highly variable
power-law and constant reflection - as previously inferred from other
observations in the 2-10 keV band. The strong reflection and high iron
abundance give rise to a strong broad iron line, which requires the inner disc
radius to be at about 2 gravitational radii. Our results are consistent with
the predictions of the light bending model which invokes the very strong
gravitational effects expected very close to a rapidly spinning black hole.
|
0609521v1
|
2006-09-26
|
Models for the X-ray spectra and variability of luminous accreting black holes
|
The X-ray spectra of luminous Seyfert 1 galaxies often appear to be
reflection dominated. In a number of Narrow Line Seyfert 1 (NLS1) galaxies and
galactic black holes in the very high state, the variability of the continuum
and of the iron line are decoupled, the reflected component being often much
less variable than the continuum. These properties have been interpreted as
effects of gravitational light bending. In this framework, we present detailed
Monte-Carlo simulations of the reflection continuum in the Kerr metric. These
calculations confirm that the spectra and variability behaviour of these
sources can be reproduced by the light bending model. As an alternative to the
light bending model, we show that similar observational properties are expected
from radiation pressure dominated discs subject to violent clumping
instabilities and, as a result, have a highly inhomogeneous two-phase
structure. In this model, most of the observed spectral and variability
features originate from the complex geometrical structure of the inner regions
of near-Eddington accretion flows and are therefore a signature of accretion
physics rather than general relativity.
|
0609703v1
|
2006-09-29
|
3D Collapse of Rotating Stellar Iron Cores in General Relativity including Deleptonization and a Nuclear Equation of State
|
We present results from the first 2D and 3D simulations of the collapse of
rotating stellar iron cores in general relativity employing a
finite-temperature equation of state and an approximate treatment of
deleptonization during collapse. We compare fully nonlinear and conformally
flat spacetime evolution methods and find that the conformally flat treatment
is sufficiently accurate for the core-collapse supernova problem. We focus on
the gravitational wave (GW) emission from rotating collapse, core bounce, and
early postbounce phases. Our results indicate that the GW signature of these
phases is much more generic than previously estimated. In addition, we track
the growth of a nonaxisymmetric instability of dominant m = 1 character in one
of our models that leads to prolonged narrow-band GW emission at ~930 Hz over
several tens of milliseconds.
|
0609819v2
|
2006-10-02
|
Suzaku Observation of Diffuse X-ray Emission from the Carina Nebula
|
We studied extended X-ray emission from the Carina Nebula taken with the
Suzaku CCD camera XIS on 2005 Aug. 29. The X-ray morphology, plasma temperature
and absorption to the plasma are consistent with the earlier Einstein results.
The Suzaku spectra newly revealed emission lines from various species including
oxygen, but not from nitrogen. This result restricts the N/O ratio
significantly low, compared with evolved massive stellar winds, suggesting that
the diffuse emission is originated in an old supernova remnant or a super shell
produced by multiple supernova remnants. The X-ray spectra from the north and
south of Eta Carinae showed distinct differences between 0.3-2 keV. The south
spectrum shows strong L-shell lines of iron ions and K-shell lines of silicon
ions, while the north spectrum shows them weak in intensity. This means that
silicon and iron abundances are a factor of 2-4 higher in the south region than
in the north region. The abundance variation may be produced by an SNR ejecta,
or relate to the dust formation around the star forming core.
|
0610024v1
|
2006-11-30
|
Mass loss and yield uncertainty in low-mass asymptotic giant branch stars
|
We investigate the uncertainty in surface abundances and yields of asymptotic
giant branch (AGB) stars. We apply three different mass loss laws to a 1.5
solar mass star of metallicity Z=0.008 at the beginning of the thermally
pulsing asymptotic giant branch (TP-AGB) phase. Efficient third dredge-up is
found even at very low envelope mass, contrary to previous simulations with
other evolution codes. We find that the yield of carbon is uncertain by about
15% and for most other light elements the yield is uncertain at the level of
20-80%. For iron group elements the uncertainty varies from around 30% for the
more abundant species to over a factor of two for the less abundant radioactive
species, like iron-60. The post-AGB surface abundances for this mass and
metallicity are much more uncertain due to the dilution of dredged-up material
in differing envelope masses in the later stages of the models. Our results are
compared to known planetary nebula (PN) and post-AGB abundances. We find that
the models are mostly consistent with observations but we are unable to
reproduce observations of some of the isotopes.
|
0612005v1
|
2006-12-12
|
Abundances and chemical stratification in the atmosphere of the HgMn star HD 175640
|
We present the results of a study of the photospheric abundances of the HgMn
star HD 175640, conducted using archival ESO-UVES spectra. A large number of
unblended (titanium, chromium, manganese and iron) lines were studied to search
for the presence of chemical stratification in the atmosphere of this star. The
selected lines are located in the visible region of the spectrum, longward of
the Balmer jump, in orders with S/N $\geq$ 300. We derived the abundance of
each element by calculating independently the abundance associated each line.
We then characterized the depth of formation of each line, and examined the
dependence of abundance on optical depth. Titanium, chromium, manganese and
iron show no variation of their abundance with optical depth. These four
elements do not appear to be strongly stratified in the atmosphere of HD
175640. This indicates that if stratification occurs, it must be in atmospheric
layers which are not diagnosed by the spectral lines studied, or that it is too
weak to detect using these data. We also report evidence that HD 175640 is an
SB1, and furthermore report anomalous shifts of some strong Fe {\sc II} lines,
the origin of which is unclear.
|
0612323v1
|
2007-01-17
|
Detailed chemical composition of Galactic Cepheids. A determination of the Galactic abundance gradient in the 8-12 kpc region
|
The recent introduction of high resolution/large spectral range spectrographs
provided the opportunity to investigate in detail the chemical composition of
classical Cepheids. This paper is focussed on new abundance determinations for
iron and 6 light metals (O, Na, Mg, Al, Si, Ca) in 30 Galactic Cepheids. We
also give a new estimate of the Galactic radial abundance gradient.
The stellar effective temperatures have been determined using the method of
line depth ratios, while the surface gravity and the microturbulent velocity
v$_{t}$ by imposing the ionization balance between Fe I and Fe II and the help
of the curves of growth. Abundances were calculated with classical LTE
atmosphere models.
Abundances are obtained with RMS accuracies of the order of 0.05-0.10 dex for
Fe, and 0.05-0.20 dex for the other elements. Cepheids in our sample have
solar-like abundances and current measurements agree quite-well with previous
determinations. We computed ``single zone'' Galactic radial abundance gradients
for the 8-12 kpc region and found a slope for iron of -0.061 dex kpc$^{-1}$.
|
0701499v2
|
2001-07-13
|
Iron under Earth's core conditions: Liquid-state thermodynamics and high-pressure melting curve
|
{\em Ab initio} techniques based on density functional theory in the
projector-augmented-wave implementation are used to calculate the free energy
and a range of other thermodynamic properties of liquid iron at high pressures
and temperatures relevant to the Earth's core. The {\em ab initio} free energy
is obtained by using thermodynamic integration to calculate the change of free
energy on going from a simple reference system to the {\em ab initio} system,
with thermal averages computed by {\em ab initio} molecular dynamics
simulation. The reference system consists of the inverse-power pair-potential
model used in previous work. The liquid-state free energy is combined with the
free energy of hexagonal close packed Fe calculated earlier using identical
{\em ab initio} techniques to obtain the melting curve and volume and entropy
of melting. Comparisons of the calculated melting properties with experimental
measurement and with other recent {\em ab initio} predictions are presented.
Experiment-theory comparisons are also presented for the pressures at which the
solid and liquid Hugoniot curves cross the melting line, and the sound speed
and Gr\"{u}neisen parameter along the Hugoniot. Additional comparisons are made
with a commonly used equation of state for high-pressure/high-temperature Fe
based on experimental data.
|
0107307v1
|
2002-11-14
|
Ferromagnetism of a graphite nodule from the Canyon Diablo meteorite
|
There have recently been various reports of weak ferromagnetism in graphite
(1,2) and synthetic carbon materials (3) such as rhombohedral C60 (4), as well
as a theoretical prediction of a ferromagnetic instability in graphene sheets
(5). With very small ferromagnetic signals, it is difficult to be certain that
the origin is intrinsic, rather than due to minute concentrations of iron-rich
impurities. Here we take a different experimental approach to study
ferromagnetism in graphitic materials, by making use of meteoritic graphite,
which is strongly ferromagnetic at room temperature. We examined ten samples of
extraterrestrial graphite from a nodule in the Canyon Diablo meteorite.
Graphite is the major phase in every sample but there are minor amounts of
magnetite, kamacite, akaganeite, and other phases. By analysing the phase
composition of a series of samples, we find that these iron-rich minerals can
only account for about two-thirds of the observed magnetization. The remainder
is somehow associated with graphite, corresponding to an average magnetization
of 23 Am2kg-1, or 0.05 Bohr magnetons per carbon atom. The magnetic ordering
temperature is near 570 K. We suggest that the ferromagnetism is a magnetic
proximity effect induced at the interface with magnetite or kamacite
inclusions.
|
0211275v1
|
2003-05-29
|
Iron valence in double-perovskite (Ba,Sr,Ca)2FeMoO6: Isovalent substitution effect
|
In the Fe-Mo based B-site ordered double-perovskite, A2FeMoO6.0, with iron in
the mixed-valence II/III state, the valence value of Fe is not precisely fixed
at 2.5 but may be fine-tuned by means of applying chemical pressure at the
A-cation site. This is shown through a systematic 57Fe Mossbauer spectroscopy
study using a series of A2FeMoO6.0 [A = (Ba,Sr) or (Sr,Ca)] samples with high
degree of Fe/Mo order, the same stoichiometric oxygen content and also almost
the same grain size. The isomer shift values and other hyperfine parameters
obtained from the Mossbauer spectra confirm that Fe remains in the
mixed-valence state within the whole range of A constituents. However, upon
increasing the average cation size at the A site the precise valence of Fe is
found to decrease such that within the A = (Ba,Sr) regime the valence of Fe is
closer to II, while within the A = (Sr,Ca) regime it is closer to the actual
mixed-valence II/III state. As the valence of Fe approaches II, the difference
in charges between Fe and Mo increases, and parallel with this the degree of
Fe/Mo order increases. Additionally, for the less-ordered samples an increased
tendency of clustering of the anti-site Fe atoms is deduced from the Mossbauer
data.
|
0305679v1
|
2005-10-21
|
Carbon diffusion in alpha-iron: Evidence for quantum mechanical tunneling
|
Recent experimental data on the diffusion coefficient of carbon in alpha-iron
below liquid nitrogen temperature (LNT) question the classical approach to the
observed temperature dependence. As the temperature is lowered below LNT, the
diffusion constant tends to a nearly temperature-independent value rather than
continuing its activated trend. The low temperature branch is apparently
characteristic of a quantum mechanical process dominated by tunneling in ground
state. Concommitantly, we apply an occurrence probability approach to
describing the overall temperature dependence as a single continuous rate.
Within the adiabatic approximation the electronic eigenvalue depending
parametrically on the nuclear coordinates is taken to be the potential energy
to control the motion of the nuclei. The resulting rate involves all
horizontal-tunneling energy-conserving elastic transitions at the quantized
energy levels of the migrating atom. A small though not negligible slope of in
the temperature dependence as the temperature is raised below 100 K is dealt
with by complementing for the rate of a parallel one-phonon inelastic tunneling
process in excess to the basic elastic-tunneling rate. Our combined approach
agrees well with the experimental data. In particular, the frequency of the
coupled vibration is obtained virtually identical to the carbon vibrational
frequency from inelastic neutron-scattering data. The migrational barrier is
also found to be within the limits expected for alpha-iron.
|
0510589v1
|
2006-12-11
|
Chemical ordering and composition fluctuations at the (001) surface of the Fe-Ni Invar alloy
|
We report on a study of (001) oriented fcc Fe-Ni alloy surfaces which
combines first-principles calculations and low-temperature STM experiments.
Density functional theory calculations show that Fe-Ni alloy surfaces are
buckled with the Fe atoms slightly shifted outwards and the Ni atoms inwards.
This is consistent with the observation that the atoms in the surface layer can
be chemically distinguished in the STM image: brighter spots (corrugation
maxima with increased apparent height) indicate iron atoms, darker ones nickel
atoms. This chemical contrast reveals a c2x2 chemical order (50% Fe) with
frequent Fe-rich defects on Invar alloy surface. The calculations also indicate
that subsurface composition fluctuations may additionally modulate the apparent
height of the surface atoms. The STM images show that this effect is pronounced
compared to the surfaces of other disordered alloys, which suggests that some
chemical order and corresponding concentration fluctuations exist also in the
subsurface layers of Invar alloy. In addition, detailed electronic structure
calculations allow us to identify the nature of a distinct peak below the Fermi
level observed in the tunneling spectra. This peak corresponds to a surface
resonance band which is particularly pronounced in iron-rich surface regions
and provides a second type of chemical contrast with less spatial resolution
but one that is essentially independent of the subsurface composition.
|
0612262v1
|
2007-03-21
|
Optical spectra, crystal-field parameters, and magnetic susceptibility of the new multiferroic NdFe3(BO3)4
|
We report high-resolution optical absorption spectra for NdFe3(BO3)4 trigonal
single crystal which is known to exhibit a giant magnetoelectric effect below
the temperature of magnetic ordering TN = 33 K. The analysis of the
temperature-dependent polarized spectra reveals the energies and, in some
cases, symmetries and exchange splittings of Nd3+ 84 Kramers doublets. We
perform crystal-field calculations starting from the exchange-charge model,
obtain a set of six real crystal-field parameters, and calculate wave functions
and magnetic g-factors. In particular, the values g(perpendicular) = 2.385,
g(parallel) = 1.376 were found for the Nd3+ ground-state doublet. We obtain
Bloc=7.88 T and |JFN|= 0.48 K for the values of the local effective magnetic
field at liquid helium temperatures at the Nd3+ site and the Nd - Fe exchange
integral, respectively, using the experimentally measured Nd3+ ground-state
splitting of 8.8 cm-1. To check reliability of our set of crystal field
parameters we model the magnetic susceptibility data from literature. A dimer
containing two nearest-neighbor iron ions in the spiral chain is considered to
partly account for quasi-one-dimensional properties of iron borates, and then
the mean-field approximation is used. The results of calculations with the
exchange parameters for Fe3+ ions Jnn = -6.25 K (intra-chain interactions) and
Jnnn = -1.92 K (inter-chain interactions) obtained from fitting agree well with
the experimental data.
|
0703549v1
|
2007-04-03
|
Diffuse X-ray Emission from the Carina Nebula Observed with Suzaku
|
A number of giant HII regions are associated with soft diffuse X-ray
emission. Among these, the Carina nebula possesses the brightest soft diffuse
emission. The required plasma temperature and thermal energy can be produced by
collisions or termination of fast winds from main-sequence or embedded young O
stars, but the extended emission is often observed from regions apart from
massive stellar clusters. The origin of the X-ray emission is unknown.
The XIS CCD camera onboard Suzaku has the best spectral resolution for
extended soft sources so far, and is therefore capable of measuring key
emission lines in the soft band. Suzaku observed the core and the eastern side
of the Carina nebula (Car-D1) in 2005 Aug and 2006 June, respectively. Spectra
of the south part of the core and Car-D1 similarly showed strong L-shell lines
of iron ions and K-shell lines of silicon ions, while in the north of the core
these lines were much weaker. Fitting the spectra with an absorbed thin-thermal
plasma model showed kT~0.2, 0.6 keV and NH~1-2e21 cm-2 with a factor of 2-3
abundance variation in oxygen, magnesium, silicon and iron. The plasma might
originate from an old supernova, or a super shell of multiple supernovae.
|
0704.0346v1
|
2007-04-12
|
Possible X-ray diagnostic for jet/disk dominance in Type 1 AGN
|
Using Rossi X-ray Timing Explorer Seyfert 1 and 1.2 data spanning 9 years, we
study correlations between X-ray spectral features. The sample consists of 350
time-resolved spectra from 12 Seyfert 1 and 1.2 galaxies. Each spectrum is
fitted to a model with an intrinsic powerlaw X-ray spectrum produced close to
the central black hole that is reprocessed and absorbed by material around the
black hole. To test the robustness of our results, we performed Monte Carlo
simulations of the spectral sample. We find a complex relationship between the
iron line equivalent width (EW) and the underlying power law index (Gamma). The
data reveal a correlation between Gamma and EW which turns over at Gamma <~ 2,
but finds a weak anti-correlation for steeper photon indices. We propose that
this relationship is driven by dilution of a disk spectrum (which includes the
narrow iron line) by a beamed jet component and, hence, could be used as a
diagnostic of jet-dominance. In addition, our sample shows a strong correlation
between the reflection fraction (R) and Gamma, but we find that it is likely
the result of modeling degeneracies. We also see the X-ray Baldwin effect (an
anti-correlation between the 2-10 keV X-ray luminosity and EW) for the sample
as a whole, but not for the individual galaxies and galaxy types.
|
0704.1587v1
|
2007-05-12
|
Iron line profiles and self-shadowing from relativistic thick accretion discs
|
We present Fe Kalpha line profiles from and images of relativistic discs with
finite thickness around a rotating black hole using a novel code. The line is
thought to be produced by iron fluorescence of a relatively cold X-ray
illuminated material in the innermost parts of the accretion disc and provides
an excellent diagnostic of accretion flows in the vicinity of black holes.
Previous studies have concentrated on the case of a thin, Keplerian accretion
disc. This disc must become thicker and sub-Keplerian with increasing accretion
rates. These can affect the line profiles and in turn can influence the
estimation of the accretion disc and black hole parameters from the observed
line profiles. We here embark on, for the first time, a fully relativistic
computation which offers key insights into the effects of geometrical thickness
and the sub-Keplerian orbital velocity on the line profiles. We include all
relativistic effects such as frame-dragging, Doppler boost, time dilation,
gravitational redshift and light bending. We find that the separation and the
relative height between the blue and red peaks of the line profile diminish as
the thickness of the disc increases. This code is also well-suited to produce
accretion disc images. We calculate the redshift and flux images of the
accretion disc and find that the observed image of the disc strongly depends on
the inclination angle. The self-shadowing effect appears remarkable for a high
inclination angle, and leads to the black hole shadow being completely hidden
by the disc itself.
|
0705.1796v2
|
2007-06-08
|
Evolution of interstellar dust and stardust in the solar neighbourhood
|
The abundance evolution of interstellar dust species originating from stellar
sources and from condensation in molecular clouds in the local interstellar
medium of the Milky Way is studied and the input of dust material to the Solar
System is determined. A one-zone chemical evolution model of the Milky Way for
the elemental composition of the disk combined with an evolution model for its
interstellar dust component similar to that of Dwek (1998) is developed. The
dust model considers dust-mass return from AGB stars as calculated from
synthetic AGB models combined with models for dust condensation in stellar
outflows. Supernova dust formation is included in a simple parameterized form
which is gauged by observed abundances of presolar dust grains with supernova
origin. For dust growth in the ISM a simple method is developed for coupling
this with disk and dust evolution models. The time evolution of the abundance
of the following dust species is followed in the model: silicate, carbon,
silicon carbide, and iron dust from AGB stars and from SNe as well as silicate,
carbon, and iron dust grown in molecular clouds. It is shown that the
interstellar dust population is dominated by dust accreted in molecular clouds;
most of the dust material entering the Solar System at its formation does not
show isotopic abundance anomalies of the refractory elements, i.e.,
inconspicuous isotopic abundances do not point to a Solar System origin of dust
grains. The observed abundance ratios of presolar dust grains formed in SN
ejecta and in AGB star outflows requires that for the ejecta from SNe the
fraction of refractory elements condensed into dust is 0.15 for carbon dust and
is quite small ($\sim10^{-4}$) for other dust species.
|
0706.1155v1
|
2007-08-06
|
On the relativistic iron line and soft excess in the Seyfert 1 galaxy Markarian 335
|
We report on a 133 ks XMM-Newton observation of the Seyfert 1 galaxy
Markarian 335. The 0.4-12 keV spectrum contains an underlying power law
continuum, a soft excess below 2 keV, and a double-peaked iron emission feature
in the 6-7 keV range. We investigate the possibility that the double-peaked
emission might represent the characteristic signature of the accretion disc.
Detailed investigations show that a moderately broad, accretion disc line is
most likely present, but that the peaks may be owing to narrower components
from more distant material. The peaks at 6.4 and 7 keV can be identified,
respectively, with the molecular torus in active galactic nucleus unification
schemes, and very highly ionized, optically thin gas filling the torus. The
X-ray variability spectra on both long (~100 ks) and short (~1 ks) timescales
disfavour the recent suggestion that the soft excess is an artifact of
variable, moderately ionized absorption.
|
0708.0751v1
|
2007-08-28
|
The Symbiotic System SS73 17 Seen with Suzaku
|
We observed with Suzaku the symbiotic star SS73 17, motivated by the
discovery by the INTEGRAL satellite and the Swift BAT survey that it emits hard
X-rays. Our observations showed a highly-absorbed X-ray spectrum with NH >
10^23 cm-2, equivalent to A_V > 26, although the source has B magnitude 11.3
and is also bright in UV. The source also shows strong, narrow iron lines
including fluorescent Fe K as well as Fe xxv and Fe xxvi. The X-ray spectrum
can be fit with a thermal model including an absorption component that
partially covers the source. Most of the equivalent width of the iron
fluorescent line in this model can be explained as a combination of
reprocessing in a dense absorber plus reflection off a white dwarf surface, but
it is likely that the continuum is partially seen in reflection as well. Unlike
other symbiotic systems that show hard X-ray emission (CH Cyg, RT Cru, T CrB,
GX1+4), SS73 17 is not known to have shown nova-like optical variability, X-ray
flashes, or pulsations, and has always shown faint soft X-ray emission. As a
result, although it is likely a white dwarf, the nature of the compact object
in SS73 17 is still uncertain. SS73 17 is probably an extreme example of the
recently discovered and relatively small class of hard X-ray emitting symbiotic
systems.
|
0708.3833v1
|
2007-10-03
|
Abundance Patterns in Stars in the Bulge and Galactic Center
|
We discuss oxygen and iron abundance patterns in K and M red-giant members of
the Galactic bulge and in the young and massive M-type stars inhabiting the
very center of the Milky Way. The abundance results from the different bulge
studies in the literature, both in the optical and the infrared, indicate that
the [O/Fe]-[Fe/H] relation in the bulge does not follow the disk relation, with
[O/Fe] values falling above those of the disk. Based on these elevated values
of [O/Fe] extending to large Fe abundances, it is suggested that the bulge
underwent a rapid chemical enrichment with perhaps a top-heavy initial mass
function. The Galactic Center stars reveal a nearly uniform and slightly
elevated (relative to solar) iron abundance for a studied sample which is
composed of 10 red giants and supergiants. Perhaps of more significance is the
fact that the young Galactic Center M-type stars show abundance patterns that
are reminiscent of those observed for the bulge population and contain enhanced
abundance ratios of alpha-elements relative to either the Sun or Milky Way disk
at near-solar metallicities.
|
0710.0866v1
|
2007-10-24
|
About Compiled Catalogue of Spectroscopically Determined $α$-Elements Abundances for Stars with Accurate Parallaxes
|
We present a new version of the compiled catalogue of nearby stars for which
was published the spectoscopically determined effective temperatures, surface
gravities, and abundances of iron, magnesium, calcium, silicon, and titanium.
Distances, velocity components, galactic orbital elements, and ages was
calculated for all stars. The atmospheric parameters and iron abundances were
found from 4700 values in 136 publications, while relative abundances of
alpha-elements were found from 2800 values in 81 publications for ~2000 dwarfs
and giants using a three-step iteration averaging procedure, with weights
assigned to each source of data as well as to each individual determination and
taking into account systematic deviations of each scale relative to the reduced
mean scale. The estimated assumed completeness for data sources containing more
than five stars, up to late April 2007, exceeds 90%. For the vast majority of
stars in the catalogue, the spatial-velocity components were derived from
modern high-precision astrometric observations, and their Galactic orbit
elements were computed using a three-component model of the Galaxy, consisting
of a disk, a bulge, and a massive extended halo. Ages was determined for dwarfs
and subgiants using Yale isochrones~2004. For this purpose the original codes
was developed, based on interpolation with the 3D-spline functions of
theoretical isochrones, and with subsequent interpolation in metallicity and
abundances of \alpha-elements.
|
0710.4531v1
|
2007-10-25
|
Spectral analysis of LMC X-2 with XMM/Newton: unveiling the emission process in the extragalactic Z-source
|
We present the results of the analysis of an archival observation of LMC X-2
performed with XMM/Newton. The spectra taken by high-precision instruments have
never been analyzed before. We find an X-ray position for the source that is
inconsistent with the one obtained by ROSAT, but in agreement with the Einstein
position and that of the optical counterpart. The correlated spectral and
timing behaviour of the source suggests that the source is probably in the
normal branch of its X-ray color-color diagram. The spectrum of the source can
be fitted with a blackbody with a temperature 1.5 keV plus a disk blackbody at
0.8 keV. Photoelectric absorption from neutral matter has an equivalent
hydrogen column of 4 x 10^{20} cm^{-2}. An emission line, which we identify as
the O VIII Lyman alpha line, is detected, while no feature due to iron is
detected in the spectrum. We argue that the emission of this source can be
straightforwardly interpreted as a sum of the emission from a boundary layer
between the NS and the disc and a blackbody component coming from the disc
itself. Other canonical models that are used to fit Z-sources do not give a
satisfactory fit to the data. The detection of the O VIII emission line (and
the lack of detection of lines in the iron region) can be due to the fact that
the source lies in the Large Magellanic Cloud.
|
0710.4934v1
|
2007-10-30
|
On variability and spectral distortion of the fluorescent iron lines from black-hole accretion discs
|
We investigate properties of iron fluorescent line arising as a result of
illumination of a black hole accretion disc by an X-ray source located above
the disc surface. We study in details the light-bending model of variability of
the line, extending previous work on the subject.
We indicate bending of photon trajectories to the equatorial plane, which is
a distinct property of the Kerr metric, as the most feasible effect underlying
reduced variability of the line observed in several objects. A model involving
an X-ray source with a varying radial distance, located within a few central
gravitational radii around a rapidly rotating black hole, close to the disc
surface, may explain both the elongated red wing of the line profile and the
complex variability pattern observed in MCG--6-30-15 by XMM-Newton.
We point out also that illumination by radiation which returns to the disc
(following the previous reflection) contributes significantly to formation of
the line profile in some cases. As a result of this effect, the line profile
always has a pronounced blue peak (which is not observed in the deep minimum
state in MCG--6-30-15), unless the reflecting material is absent within the
innermost 2--3 gravitational radii.
|
0710.5578v2
|
2007-12-01
|
Linear arrays of non homogeneous Cu sites in the CuO2 plane, a new scenario for pairing mechanisms in a currugated-iron-like plane
|
Experimental results obtained by using x-ray absorption spectroscopy show
that the configurations of Cu sites in the CuO2 plane of Bi 2:2:1:2 high Tc
superconductors are not homogeneous. Different Cu sites are characterized by
short 0.23 nm and long 0.245 nm Cu-O(apical) distances. The linear arrays of
different Cu sites forming domains with a corrugated-iron like shape is
proposed to be a key characteristic of superconducting domains in the CuO2
plane. The wavelength of the modulation is close to the superconducting
coherence length. The ordering of the distorted Cu sites is suggested to be
evidence for ordering of polarons driven by the pseudo Jahn Teller electron
lattice interaction. The Cu L3 XAS experiments on Bi 2:2:1:2 system indicate
that the electronic states added by doping, 4% have the a1 symmetry (i.e. with
Cu 3d(3z2-r2), the combination of O (planar) 2px,y orbital with a1 symmetry
L(a1), and O(apical) 2pz orbital character) and 15% have the b1 symmetry
(3d(x2-y2) and the combination of O (planar) 2p(x,y) orbital with b1 symmetry
L(b1)). This new scenario supports the pairing mechanisms for high Tc
superconductivity in the presence of two components: 1) the more delocalized
component with b1 symmetry and 2) the more localized component, with partially
a1 symmetry associated with different parts of the Fermi surface.
|
0712.0065v1
|
2007-12-18
|
Neutrino oscillation signatures of oxygen-neon-magnesium supernovae
|
We discuss the flavor conversion of neutrinos from core collapse supernovae
that have oxygen-neon-magnesium (ONeMg) cores. Using the numerically calculated
evolution of the star up to 650 ms post bounce, we find that, for the normal
mass hierarchy, the electron neutrino flux in a detector shows signatures of
two typical features of an ONeMg-core supernova: a sharp step in the density
profile at the base of the He shell and a faster shock wave propagation
compared to iron core supernovae. Before the shock hits the density step (t ~
150 ms), the survival probability of electron neutrinos is about 0.68, in
contrast to values of 0.32 or less for an iron core supernova. The passage of
the shock through the step and its subsequent propagation cause a decrease of
the survival probability and a decrease of the amplitude of oscillations in the
Earth, reflecting the transition to a more adiabatic propagation inside the
star. These changes affect the lower energy neutrinos first; they are faster
and more sizable for larger theta_13. They are unique of ONeMg-core supernovae,
and give the possibility to test the speed of the shock wave. The time
modulation of the Earth effect and its negative sign at the neutronization peak
are the most robust signatures in a detector.
|
0712.3000v3
|
2008-01-31
|
Cosmic ray velocity and electric charge measurements with the AMS/RICH detector: prototype results
|
The Alpha Magnetic Spectrometer (AMS) to be installed on the International
Space Station (ISS) will measure charged cosmic ray spectra of elements up to
iron, in the rigidity range from 1 GV to 1 TV, for at least three years. AMS is
a large angular spectrometer composed of different subdetectors, including a
proximity focusing Ring Imaging CHerenkov (RICH) detector. This will be
equipped with a mixed radiator made of aerogel and sodium fluoride (NaF), a
lateral conical mirror and a detection plane made of 680 photomultipliers
coupled to light guides. The RICH detector allows measurements of particle's
electric charge up to iron, and particle's velocity. Two possible methods for
reconstructing the Cherenkov angle and the electric charge with the RICH will
be discussed.
A RICH prototype consisting of a detection matrix with 96 photomultipliers, a
segment of a conical mirror and samples of the radiator materials was built and
its performance was evaluated using ion beam data. Results from the last test
beam performed with ion fragments resulting from the collision of a 158
GeV/c/nucleon primary beam of indium ions (CERN SPS) on a lead target are
reported. The large amount of collected data allowed to test and characterize
different aerogel samples and the NaF radiator. In addition, the reflectivity
of the mirror was evaluated. The data analysis confirms the design goals.
|
0801.4952v1
|
2008-01-31
|
Cosmological black hole spin evolution by mergers and accretion
|
Using recent results from numerical relativity simulations of black hole
mergers, we revisit previous studies of cosmological black hole spin evolution.
We show that mergers are very unlikely to yield large spins, unless alignment
of the spins of the merging holes with the orbital angular momentum is very
efficient. We analyze the spin evolution in three specific scenarios: (1) spin
evolves only through mergers, (2) spin evolves through mergers and prolonged
accretion episodes, (3) spin evolves through mergers and short-lived (chaotic)
accretion episodes. We study how different diagnostics can distinguish between
these evolutionary scenarios, assessing the discriminating power of
gravitational-wave measurements and X-ray spectroscopy. Gravitational radiation
can produce three different types of spin measurements, yielding respectively
the spins of the two black holes in a binary inspiral prior to merger, the spin
of the merger remnant (as encoded in the ringdown waves), and the spin of
``single'' black holes during the extreme mass-ratio inspiral (EMRI) of compact
objects. The latter spin population is also accessible to iron-line
measurements. We compute and compare the spin distributions relevant for these
different observations. If iron-line measurements and gravitational-wave
observations of EMRIs only yield dimensionless spins j=J/M^2>0.9, then
prolonged accretion should be responsible for spin-up, and chaotic accretion
scenarios would be very unlikely. If only a fraction of the whole population of
low-redshift black holes spins rapidly, spin-alignment during binary mergers
(rather than prolonged accretion) could be responsible for spin-ups.
|
0802.0025v2
|
2008-02-07
|
On the Dynamical Origin of the ICM Metallicity Evolution
|
We present a study on the origin of the metallicity evolution of the
intra-cluster medium (ICM) by applying a semi-analytic model of galaxy
formation to N-body/SPH (smoothed particle hydrodynamic) non-radiative
numerical simulations of clusters of galaxies. The semi-analytic model includes
gas cooling, star formation, supernovae feedback and metal enrichment, and is
linked to the diffuse gas of the underlying simulations so that the chemical
properties of gas particles are dynamically and consistently generated from
stars in the galaxies. This hybrid model let us have information on the spatial
distribution of metals in the ICM. The results obtained for a set of clusters
with virial masses of ~1.5*10^15 h^{-1} M_sun contribute to the theoretical
interpretation of recent observational X-ray data, which indicate a decrease of
the average iron content of the intra-cluster gas with increasing redshift. We
find that this evolution arises mainly as a result of a progressive increase of
the iron abundance within ~0.15 R_vir. The clusters have been considerably
enriched by z~1 with very low contribution from recent star formation. Low
entropy gas that has been enriched at high redshift sinks to the cluster centre
contributing to the evolution of the metallicity profiles.
|
0802.0975v1
|
2008-02-15
|
Evolution of the metal content of the intra-cluster medium with hydrodynamical simulations
|
We present a comparison between simulation results and X-ray observational
data on the evolution of the metallicity of the intra-cluster medium (ICM). The
simulations of galaxy clusters were performed with the Tree-SPH Gadget2 code
that includes a detailed model of chemical evolution, by assuming three
different shapes for the stellar initial mass function (IMF), namely the
Salpeter (1955), Kroupa (2001) and Arimoto-Yoshii (1987) IMF. Our simulations
predict significant radial gradients of the Iron abundance, which extend over
the whole cluster virialized region. At larger radii, we do not detect any
flattening of the metallicity profiles. As for the evolution of the ICM metal
(Iron) abundance out to z=1, we find that it is determined by the combined
action of (i) the sinking of already enriched gas, (ii) the ongoing metal
production in galaxies and (iii) the locking of ICM metals in newborn stars. As
a result, rather than suppressing the metallicity evolution, stopping star
formation at z=1 has the effect of producing an even too fast evolution of the
emission-weighted ICM metallicity with too high values at low redshift.
Finally, we compare simulations with the observed rate of type-Ia supernovae
per unit B-band luminosity (SnU_B). We find that our simulated clusters do not
reproduce the decreasing trend of SnU_B at low redshift, unless star formation
is truncated at z=1.
|
0802.2224v1
|
2008-02-25
|
NLTE Model of NGC 6543's Central Star and its Relation with the Surrounding Planetary Nebula
|
We analyze the chemical composition of the central star of the planetary
nebula NGC 6543 based upon a detailed NLTE model of its stellar wind. The
logarithmic abundances by number are H=12.00, He=11.00, C=9.03, N=8.36, O=9.02,
Si=8.19, P=5.53, S=7.57 and Fe=7.24. Compared with the solar abundances, most
of the elements have solar composition with respect to hydrogen except C which
is overabundant by 0.28 dex and Fe which is depleted by $\sim 0.2$ dex.
Contrary to most previous work, we find that the star is not H-poor and has a
normal He composition. These abundances are compared with those found in the
diffuse X-ray plasma and the nebular gas. Compared to the plasma emitting in
diffuse X-rays, the stellar wind is much less depleted in iron. Since the iron
depletions in the nebular gas and X-ray plasma are similar, we conclude that
the plasma emitting diffuse X-rays is derived from the nebular gas rather than
the stellar wind. Excellent agreement is obtained between the abundances in the
stellar wind and the nebular recombination line abundances for He, C, and O
relative to H. On the other hand, the derived stellar N abundance is smaller
than the nebular N abundance derived from recombination lines and agrees with
the abundance found from collisionally-excited lines. The mean temperature
variation determined by five different methods indicates that the difference in
the nebular abundances between the recombination lines and collisionally
excited lines can be explained as due to the temperature variations in a
chemically homogeneous medium.
|
0802.3692v1
|
2008-03-04
|
The Chemical Evolution of Dwarf Spheroidal Galaxies: Dissecting the Inner Regions and their Stellar Populations
|
Using 3-dimensional hydrodynamical simulations of isolated dwarf spheroidal
galaxies (dSphs), we undertake an analysis of the chemical properties of their
inner regions, identifying the respective roles played by Type Ia (SNe Ia) and
Type II (SNe II) supernovae. The effect of inhomogeneous pollution from SNe Ia
is shown to be prominent within two core radii, with the stars forming therein
amounting to ~20% of the total. These stars are relatively iron-rich and
alpha-element-depleted compared to the stars forming in the rest of the galaxy.
At odds with the projected stellar velocity dispersion radial profile, the
actual 3-dimensional one shows a depression in the central region, where the
most metal-rich (ie. [Fe/H]-rich) stars are partly segregated. This naturally
results in two different stellar populations, with an anti-correlation between
[Fe/H] and velocity dispersion, in the same sense as that observed in the
Sculptor and Fornax dSphs. Because the most iron-rich stars in our model are
also the most alpha-depleted, a natural prediction and test of our model is
that the same radial segregation effects should exist between [alpha/Fe] and
velocity dispersion.
|
0803.0385v1
|
2008-03-20
|
Tracing the [FeII]/[NeII] ratio and its relationship with other ISM indicators within star forming dwarf galaxies: a Spitzer IRS archival study
|
Archival Spitzer observations of 41 starburst galaxies that span a wide range
in metallicity reveal for the first time a correlation between the
[FeII]/[NeII] 26.0/12.8 micron ratio and the electron gas density as traced by
the 18.7/33.4 micron [SIII] ratio, with the [FeII]/[NeII] ratio decreasing with
increasing gas density. The correlations of the [FeII]/[NeII] ratio, the PAH
peak to continuum strength & metallicity found in an earlier paper were
confirmed for a larger sample. We also find a strong correlation between the
gas density and the PAH peak to continuum strength. Using shock and
photoionization models, we see the driver of the observed [FeII]/[NeII] ratios
is metallicity. The majority of [FeII] emission in low metallicity galaxies may
be shock-derived, whilst at high metallicity, the [FeII] emission may be
instead dominated by contributions from HII and in particular from dense PDR
regions. However, the observed [FeII]/[NeII] ratios may instead be following a
metallicity-abundance relationship, with iron being less depleted onto grains
in low metallicity galaxies - a result that would have profound implications
for the use of iron emission lines as unambiguous tracers of shocks.
|
0803.3000v1
|
2008-03-31
|
A Generic Two-band Model for Unconventional Superconductivity and Spin-Density-Wave Order in Electron and Hole Doped Iron-Based Superconductors
|
Based on experimental data on the newly synthesized iron-based
superconductors and the relevant band structure calculations, we propose a
minimal two-band BCS-type Hamiltonian with the interband Hubbard interaction
included. We illustrate that this two-band model is able to capture the
essential features of unconventional superconductivity and spin density wave
(SDW) ordering in this family of materials. It is found that bound
electron-hole pairs can be condensed to reveal the SDW ordering for zero and
very small doping, while the superconducting ordering emerges at small finite
doping, whose pairing symmetry is qualitatively analyzed to be of nodal d-wave.
The derived analytical formulas not only give out a nearly symmetric phase
diagram for electron and hole doping, but also is likely able to account for
existing main experimental results. Moreover, we also derive two important
relations for a general two-band model and elaborate how to apply them to
determine the band width ratio and the effective interband coupling strength
from experimental data.
|
0803.4346v3
|
2008-04-04
|
Magnetic Order versus superconductivity in the Iron-based layered La(O1-xFx)FeAs systems
|
In high-transition temperature (high-Tc) copper oxides, it is generally
believed that antiferromagnetism plays a fundamental role in the
superconducting mechanism because superconductivity occurs when mobile
electrons or holes are doped into the antiferromagnetic parent compounds. The
recent discovery of superconductivity in the rare-earth (R) iron-based oxide
systems [RO1-xFxFeAs] has generated enormous interest because these materials
are the first noncopper oxide superconductors with Tc exceeding 50 K. The
parent (nonsuperconducting) LaOFeAs material is metallic but shows anomalies
near 150 K in both resistivity and dc magnetic susceptibility. While optical
conductivity and theoretical calculations suggest that LaOFeAs exhibits a
spin-density-wave (SDW) instability that is suppressed with doping electrons to
form superconductivity, there has been no direct evidence of the SDW order.
Here we use neutron scattering to demonstrate that LaOFeAs undergoes an abrupt
structural distortion below ~150 K, changing the symmetry from tetragonal
(space group P4/nmm) to monoclinic (space group P112/n) at low temperatures,
and then followed with the development of long range SDW-type antiferromagnetic
order at ~134 K with a small moment but simple magnetic structure. Doping the
system with flourine suppresses both the magnetic order and structural
distortion in favor of superconductivity. Therefore, much like high-Tc copper
oxides, the superconducting regime in these Fe-based materials occurs in close
proximity to a long-range ordered antiferromagnetic ground state. Since the
discovery of long
|
0804.0795v1
|
2008-04-14
|
The candidate filament close to the 3C295 galaxy cluster: optical and X-ray spectroscopy
|
We present a detailed analysis of the overdensity of X-ray sources colse to
the 3C 295 galaxy cluster (z=0.46) to assess whether it is associated with a
filament of the large-scale structure of the Universe. We obtained optical
spectra of the optical counterparts of eleven sources associated with the
filament, finding that one is at z=0.474. This is a type 1 AGN at 1.5 arcmin
from the cluster center. We found three more sources with a redshift in the
range 0.37 - 0.53. We extracted the stacked X-ray spectrum of 47 X-ray sources
belonging to the putative filament. We found a significant narrow (at the
resolution of the Chandra ACIS-I) line at E ~ 4.4 keV, the energy of the iron
Kalpha line at the redshift of the cluster. The detection of this line is
confirmed at a confidence level of better than 3sigma and its energy is
constrained to be in the range 6.2--6.47 (at a 90% confidence level), excluding
an identification with the 6.7 helium-like iron line from the hot cluster ICM
at better than 4 sigma. We conclude that the detection of the redshifted line
is a strong indication that at least several of the excess sources lie at z ~
0.46 and that AGNs are efficient tracers of the ``filament'' connected with the
central cluster of galaxies.
|
0804.2143v1
|
2008-04-23
|
Specific heat of the iron-based high-$T_c$ superconductor SmO$_{1-x}$F$_x$FeAs
|
The specific heat $C(T)$ of new iron-based high-$T_c$ superconductor
SmO$_{1-x}$F$_x$FeAs ($0 \leq x \leq 0.2$) was systematically studied. For
undoped $x$ = 0 sample, a specific heat jump was observed at 130 K. This is
attributed to the structural or spin-density-wave (SDW) transition, which also
manifests on resistivity as a rapid drop. However, this jump disappears with
slight F doping in $x$ = 0.05 sample, although the resistivity drop still
exists. The specific heat $C/T$ shows clear anomaly near $T_c$ for $x$ = 0.15
and 0.20 superconducting samples. Such anomaly has been absent in
LaO$_{1-x}$F$_x$FeAs. For the parent compound SmOFeAs, $C(T)$ shows a sharp
peak at 4.6 K, and with electron doping in $x$ = 0.15 sample, this peak shifts
to 3.7 K. It is interpreted that such a sharp peak results from the
antiferromagnetic ordering of Sm$^{3+}$ ions in this system, which mimics the
electron-doped high-$T_c$ cuprate Sm$_{2-x}$Ce$_x$CuO$_{4-\delta}$.
|
0804.3642v3
|
2008-05-08
|
On the peculiar properties of the narrow-line quasar PG 1543+489
|
We present the analysis of four XMM-Newton observations of the narrow-line
quasar PG 1543+489 at z=0.400 carried out over a rest-frame time-scale of about
three years. The X-ray spectrum is characterized by a broad, relativistic iron
K_alpha emission line and a steep photon index, which can be both explained by
a ionized reflection model, where the source of X-ray photons is presumably
very close to the black hole. If this were the case, strong light-bending
effects are expected, and actually they provide the most plausible explanation
for the large equivalent width (EW=3.1+/-0.8 keV in the source rest frame) of
the iron line. Although the light-bending model provides a good description of
the X-ray data of PG 1543+489, it is not possible to rule out an absorption
model, where obscuring matter partially covers the X-ray source. However, the
apparent lack of variations in the properties of the absorber over the
time-scale probed by our observations may indicate that this model is less
likely.
|
0805.1227v1
|
2008-05-19
|
A Brief Comment on the Low-Temperature Structure of LaOFeAs
|
In a recent paper [arXiv:0804.3569], Takatoshi Nomura {\it et al.} reported a
structural phase transition near 150 K in LaOFeAs and used space group "Cmma"
to describe their X-ray diffraction data. However, they did not discuss how
their proposed structure compares with the early neutron study by Cruz {\it et
al.}[arXiv:0804.0795] where the low temperature structure of LaOFeAs was
described by space group "P112/n". This caused some confusion, suggesting that
there may be some disagreement on the low temperature structure of LaOFeAs as
evidenced by several inquiries that we received. Here we show that the proposed
structures from x-ray and neutron diffraction are basically identical. The P2/c
(i.e., P112/n) cell becomes the primitive cell of the Cmma cell when the
z-coordinate of the oxygen and iron are assumed to be exactly 0 and 0.5 (these
numbers were reported to be -0.0057 and 0.5006 in neutron study). Our
first-principles total-energy calculations suggest that the oxygen and iron
atoms prefer to lie on the z=0 and 1/2 plane, respectively, supporting Cmma
symmetry. However it is more convenient to describe the structural distortion
in the primitive P2/c cell which makes it easier to see the connection between
the high (i.e., P4/nmm) and low temperature structures.
|
0805.2888v1
|
2008-06-05
|
Raman spectra in iron-based quaternary CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs
|
Raman spectra have been measured on iron-based quaternary
CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs with varying fluorine doping at
room temperatures. A group analysis has been made to clarify the optical modes.
Based on the first principle calculations, the observed phonon modes can be
assigned accordingly. In LaO$_{1-x}$F$_x$FeAs, the E$_g$ and A$_{1g}$ modes
related to the vibrations of La, are suppressed with increasing F doping.
However F doping only has a small effect on the E$_g$ and A$_{1g}$ modes of Fe
and As. The Raman modes of La and As are absent in rare-earth substituted
CeO$_{1-x}$F$_x$FeAs, and the E$_g$ mode of oxygen, corresponding to the
in-plane vibration of oxygen, moves to around 450 cm$^{-1}$ and shows a very
sharp peak. Electronic scattering background is low and electron-phonon
coupling is not evident for the observed phonon modes. Three features are found
above 500 cm$^{-1}$, which may be associated with multi-phonon process.
Nevertheless it is also possible that they are related to magnetic fluctuations
or interband transitions of d orbitals considering their energies.
|
0806.0885v1
|
2008-06-19
|
75As NQR/NMR Studies on Oxygen-deficient Iron-based Oxypnictide Superconductors LaFeAsO_{1-y} (y=0,0.25,0.4) and NdFeAsO_{0.6}
|
We report 75As-NQR/NMR studies on the oxygen-deficient iron(Fe)-based
oxypnictide superconductors LaFeAsO_{0.6} (T_c=28 K) along with the results on
LaFeAsO, LaFeAsO_{0.75}(T_c=20 K) and NdFeAsO_{0.6}(T_c=53 K). Nuclear
spin-lattice relaxation rate 1/T_1 of 75As NQR at zero field on LaFeAsO_{0.6}
has revealed a T^3 dependence below T_c upon cooling without the coherence peak
just below T_c, evidencing the unconventional superconducting state with the
line-node gap. We have found an intimate relationship between the nuclear
quadrupole frequencyof 75As and T_c for four samples used in this study. It
implies microscopically that the local configuration of Fe and As atoms is
significantly related to the T_c of the Fe-oxypnictide superconductors, namely,
the T_c can be enhanced up to 50 K when the local configuration of Fe and As
atoms is optimal, in which the band structure may be also optimized through the
variation of hybridization between As 4p orbitals and Fe 3d orbitals.
|
0806.3238v2
|
2008-07-03
|
XMM-Newton X-ray spectra of the SNR 0509-67.5: data and models
|
We report on X-ray observations of the supernova remnant 0509-67.5 in the
Large Magellanic Cloud with XMM-Newton X-ray observatory. We use the imaging
spectroscopy (EPIC) and Reflective Grating Spectrometer (RGS) data to
investigate properties of the remnant and its environment. The X-ray spectra
were analyzed with SPEX software package. In addition to this we performed a
numerical hydrodynamic simulation of the remnant. The EPIC data show prominent
Fe K line emission, but the deduced overall amount of iron in the shocked
ejecta is low. The data also show that the remnant has an asymmetric ejecta
structure: the bright southwest region of the remnant shows an overabundance of
metals. The analysis of the RGS spectrum shows that the remnant has a high
lines velocity broadening of ~5000 km/s. We found a hydrodynamical model for
the remnant with basic hydrodynamical and spectral parameters similar to the
observed ones. The data analysis show that the reverse shock just recently
reached iron layers of the ejecta. The brightness enhancement in the southwest
region could be a sign of an asymmetric explosion or it could be the result of
a density enhancement of the interstellar medium. We constructed numerical
models which are in good agreement with the observations, with circumstellar
density of 3e-25 g/cm^3, age of ~400 years, velocities of ~5000 km/s and an
electron to ion temperature ratio of 0.01.
|
0807.0579v1
|
2008-07-07
|
Local moment versus Kondo behavior of the 4f-electrons in rare-earth iron oxypnictides
|
We consider the role played by the 4f states in the rare-earth oxyarsenides
REOFeAs (RE=Ce,Pr,Nd) and the oxyphosphate CeOFeP, using a first-principles
technique that combines the local density approximation and dynamical
mean-field theory (LDA+DMFT). In the Pr and Nd compounds, the 4f states are
located well below and above the Fermi level E_F, and essentially do not
interact with the iron 3d bands located near E_F, resulting in local moment
behavior. In the Ce compounds, our results reveal a qualitatively different
picture, with the 3d-4f hybridization being sufficiently strong to give rise to
an observable Kondo screening of the local 4f moment. Our LDA+DMFT electronic
structure calculations allow us to estimate the Kondo temperature T_K for both
CeOFeP and CeOFeAs. For the phosphate, the order of magnitude of our estimate
is consistent with the experimental observation of T_K around 10 K. At ambient
pressure, T_K is found to be negligibly small for CeOFeAs. Under applied
hydrostatic pressure, we predict an exponential increase of T_K which reaches
values comparable to the superconducting T_c around 40 K at pressures above 10
GPa. We conjecture that the competition between the Kondo effect and
superconductivity may be at the origin of the monotonous decrease of T_c
observed in CeOFeAs under pressure. We argue that the quantitative aspects of
this competition are inconsistent with a weak-coupling BCS description of the
superconductivity in the oxyarsenides.
|
0807.1037v2
|
2008-07-08
|
Fe and N self-diffusion in amorphous FeN: A SIMS and neutron reflectivity study
|
Simultaneous measurement of self-diffusion of iron and nitrogen in amorphous
iron nitride (Fe86N14) using secondary ion mass spectroscopy (SIMS) technique
has been done. In addition neutron reflectivity (NR) technique was employed to
study the Fe diffusion in the same compound. The broadening of a tracer layer
of 57Fe8615N14 sandwiched between Fe86N14 layers was observed after isothermal
vacuum annealing of the films at different temperatures in SIMS measurements.
And a decay of the Bragg peak intensity after isothermal annealing was observed
in [Fe86N14/57Fe86N14]10 multilayers in NR. Strong structural relaxation of
diffusion coefficient was observed below the crystallization temperature of the
amorphous phase in both measurements. It was observed from the SIMS
measurements that Fe diffusion was about 2 orders of magnitude smaller compared
to nitrogen at a given temperature. The NR measurements reveal that the
mechanism of Fe self-diffusion is very similar to that in metal-metal type
metallic glasses. The structural relaxation time for Fe and N diffusion was
found comparable indicating that the obtained relaxation time essentially
pertain to the structural relaxation of the amorphous phase.
|
0807.1302v1
|
2008-07-08
|
Superconductivity induced by cobalt doping in iron-based oxyarsenides
|
Chemical doping has recently become a very important strategy to induce
superconductivity especially in complex compounds. Distinguished examples
include Ba-doped La$_2$CuO$_4$ (the first high temperature superconductor),
K-doped BaBiO$_3$, K-doped C$_{60}$ and Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O. The
most recent example is F-doped LaFeAsO, which leads to a new class of high
temperature superconductors. One notes that all the above dopants are
non-magnetic, because magnetic atoms generally break superconducting Cooper
pairs. In addition, the doping site was out of the (super)conducting structural
unit (layer or framework). Here we report that superconductivity was realized
by doping magnetic element cobalt into the (super)conducting-active
Fe$_2$As$_2$ layers in LaFe$_{1-x}$Co$_{x}$AsO. At surprisingly small Co-doping
level of $x$=0.025, the antiferromagnetic spin-density-wave transition in the
parent compound is completely suppressed, and superconductivity with $T_c\sim $
10 K emerges. With increasing Co content, $T_c$ shows a maximum of 13 K at
$x\sim 0.075$, and then drops to below 2 K at $x$=0.15. This result suggests
essential differences between previous cuprate superconductor and the present
iron-based arsenide one.
|
0807.1304v2
|
2008-07-24
|
Resonant Spin Excitation in the High Temperature Superconductor Ba0.6K0.4Fe2As2
|
The recent observations of superconductivity at temperatures up to 55K in
compounds containing layers of iron arsenide have revealed a new class of high
temperature superconductors that show striking similarities to the more
familiar cuprates. In both series of compounds, the onset of superconductivity
is associated with the suppression of magnetic order by doping holes and/or
electrons into the band leading to theories in which magnetic fluctuations are
either responsible for or strongly coupled to the superconducting order
parameter. In the cuprates, theories of magnetic pairing have been invoked to
explain the observation of a resonant magnetic excitation that scales in energy
with the superconducting energy gap and is suppressed above the superconducting
transition temperature, Tc. Such resonant excitations have been shown by
inelastic neutron scattering to be a universal feature of the cuprate
superconductors, and have even been observed in heavy fermion superconductors
with much lower transition temperatures. In this paper, we show neutron
scattering evidence of a resonant excitation in Ba0.6K0.4Fe2As2, which is a
superconductor below 38K, at the momentum transfer associated with magnetic
order in the undoped compound, BaFe2As2, and at an energy transfer that is
consistent with scaling in other strongly correlated electron superconductors.
As in the cuprates, the peak disappears at Tc providing the first experimental
confirmation of a strong coupling of the magnetic fluctuation spectrum to the
superconducting order parameter in the new iron arsenide superconductors.
|
0807.3932v1
|
2008-07-25
|
Element-Specific Phonon Density of States of Iron in LaFeAsO_{1-x}F_{x} and La_{1-x}Ca_{x}FePO
|
We have measured element-specific Fe-phonon densities of states (Fe-PDOS) of
LaFeAsO_{1-x}F_{x} (x = 0, 0.11) and La_{1-x}Ca_{x}FePO (x = 0.13) by using
nuclear resonant inelastic scattering of synchrotron radiation. The Fe-PDOS of
superconductor LaFeAsO_{0.89}F_{0.11} (Tc = 26 K) and that of
non-superconductor LaFeAsO have similar structures to both below Tc (15 K) and
above Tc (298 K) and, therefore, fluorine doping does not have notable effect
on the Fe-PDOS. As for the superconductor La_{0.87}Ca_{0.13}FePO (Tc = 5.4K),
the entire structure of Fe-PDOS resembles with that of LaFeAsO_{1-x}F_{x}, but
the energy of the highest peak is higher than that of LaFeAsO_{1-x}F_{x}. These
peaks are attributed to vibrational modes between Fe and pnicogen (As and P)
and the temperature-dependent energy shifts are observed for
LaFeAsO_{1-x}F_{x}. Observed Fe-PDOS of LaFeAsO_{1-x}F_{x} agrees well with an
previously calculated Fe-PDOS spectrum with a first-principles calculation and
shows the structural resemblance with an calculated Eliashberg function
#alpha^2F(x) giving small electron-phonon coupling. Therefore, our results
indicate that phonons are not the main contributors to the Tc superconductivity
of LaFeAsO_{1-x}F_{x}. From the experimental viewpoint, comparison of our
observed Fe-PDOS and an experimentally obtained bosonic glue spectrum will be
an important clue as to whether phonons are the main contributors to
superconductivity in iron-pnictide superconductors.
|
0807.3968v1
|
2008-07-29
|
Experimental Consequences of the S-wave $\cos(k_x) \cdot \cos(k_y)$ Superconductivity in the Iron-Pnictides
|
The experimental consequences of different order parameters in iron-based
superconductors are theoretically analyzed. We consider both nodeless and nodal
order parameters, with an emphasis on the $\cos(k_x)\cdot \cos(k_y)$ nodeless
order parameter recently derived by two of us. We analyze the effect of this
order parameter on the spectral function, density of states, tunneling
differential conductance, penetration depth, and the NMR spin relaxation time.
This extended s-wave symmetry has line-zeroes in between the electron and hole
pockets, but they do not intersect the two Fermi surfaces for moderate doping,
and the superconductor is fully gapped. However, this suggests several
quantitative tests: the exponential decay of the penetration depth weakens and
the density of states reveals a smaller gap upon electron or hole doping.
Moreover, the $\cos(k_x) \cdot \cos(k_y)$ superconducting gap is largest on the
smallest (hole) Fermi surface. For the $1/T_1$ NMR spin relaxation rate, the
inter-band contribution is consistent with the current experimental results,
including a (non-universal) $T^{3}$ behavior and the absence of a coherence
peak. However, the intra-band contribution is considerably larger than the
inter-band contributions and still exhibits a small enhancement in the NMR spin
relaxation rate right below $T_c$ in the clean limit.
|
0807.4572v2
|
2008-08-06
|
Temperature and Spatial Dependence of the Superconducting and Pseudogap of NdFeAsO0.86F0.14
|
The discovery of superconductivity with a critical temperature exceeding 55 K
in the iron-oxypnictides and related compounds has quite suddenly given the
community a new set of materials - breaking the tyranny of copper. This new
class of materials raises fundamental questions related to the origin of the
electron pairing in the superconducting state and to the similarity to
superconductivity in the cuprates. Here, we report spatially resolved
measurements using scanning tunneling microscopy/spectroscopy (STM/STS) of the
newly discovered iron-based layered superconductor NdFeAsO0.86F0.14 (Tc = 48 K)
as a function of temperature. The tunneling spectra at 17 K show a suppression
of spectral intensity within +/- 10 meV, indicative of the opening of the
superconducting gap (SG). Below Tc, the sample exhibits two characteristic gaps
- a large one (18 meV) and a small one (9 meV) - existing in different spatial
locations. Both gaps are closed above Tc at the bulk Tc, but only the small gap
can be fitted with a superconducting gap function. This gap displays a BCS -
like order parameter. Above Tc, at the same location where the small gap was
observed, a pseudogap (PG) opens abruptly at a temperature just above Tc and
closes at 120 K. In contrast to the cuprates, the SG and PG have competing
order parameters.
|
0808.0895v1
|
2008-08-18
|
(pi,pi)-electronic order in iron arsenide superconductors
|
The distribution of valence electrons in metals usually follows the symmetry
of an ionic lattice. Modulations of this distribution often occur when those
electrons are not stable with respect to a new electronic order, such as spin
or charge density waves. Electron density waves have been observed in many
families of superconductors[1-3], and are often considered to be essential for
superconductivity to exist[4]. Recent measurements[5-9] seem to show that the
properties of the iron pnictides[10, 11] are in good agreement with band
structure calculations that do not include additional ordering, implying no
relation between density waves and superconductivity in those materials[12-15].
Here we report that the electronic structure of Ba1-xKxFe2As2 is in sharp
disagreement with those band structure calculations[12-15], instead revealing a
reconstruction characterized by a (pi,pi) wave vector. This electronic order
coexists with superconductivity and persists up to room temperature.
|
0808.2454v3
|
2008-09-01
|
Atomic diffusion and mixing in old stars II. Observations of stars in the globular cluster NGC 6397 with VLT/FLAMES-GIRAFFE
|
Evolutionary trends in the surface abundances of heavier elements have
recently been identified in the globular cluster NGC 6397 ([Fe/H]=-2),
indicating the operation of atomic diffusion in these stars. Such trends
constitute important constraints for the extent to which diffusion modifies the
internal structure and surface abundances of solar-type, metal-poor stars. We
perform an independent check of the reality and size of abundance variations
within this metal-poor globular cluster. Observational data covering a large
stellar sample, located between the cluster turn-off point and the base of the
red giant branch, are homogeneously analysed. The spectroscopic data were
obtained with the medium-high resolution spectrograph FLAMES/GIRAFFE on
VLT-UT2. We derive independent effective-temperature scales from profile
fitting of Balmer lines and by applying colour-temperature calibrations to
Str\"omgren uvby and broad-band BVI photometry. An automated spectral analysis
code is used together with a grid of MARCS model atmospheres to derive stellar
surface abundances of Mg, Ca, Ti, and Fe. We identify systematically higher
iron abundances for more evolved stars. The turn-off point stars are found to
have 0.13dex lower surface abundances of iron compared to the coolest, most
evolved stars in our sample. There is a strong indication of a similar trend in
magnesium, whereas calcium and titanium abundances are more homogeneous. Within
reasonable error limits, the obtained abundance trends are in agreement with
the predictions of stellar structure models including diffusive processes
(sedimentation, levitation), if additional turbulent mixing below the outer
convection zone is included.
|
0809.0317v1
|
2008-10-02
|
The Delicate Electronic and Magnetic Structure of the LaOFePn System (Pn = pnictogen)
|
The occurrence of high temperature superconductivity, and the competition
with magnetism, in stoichiometric and doped LaOFeAs and isostructural
iron-oxypnictides is raising many fundamental questions about the electronic
structure and magnetic interactions in this class of materials. There are now
sufficient experimental data that it may be possible to identify the important
issues whose resolution will lead to the understanding of this system. In this
paper we address a number of the important issues. One important characteristic
is the Fe-As distance (or more abstractly the pnictogen (Pn) height $z$(Pn));
we present results for the effect of $z$(Pn) on the electronic structure,
energetics, and Fe magnetic moment. We also study LaOFeAs under pressure, and
investigate the effects of both electron and hole doping within the virtual
crystal approximation. The electric field gradients for all atoms in the
LaOFeAs compound are presented (undoped and doped) and compared with available
data. The observed $(\pi,\pi,\pi)$ magnetic order is studied and compared with
the computationally simpler $(\pi,\pi,0)$ order which is probably a very good
model in most respects. We investigate the crucial role of the pnictogen atom
in this class, and predict the structures and properties of the N and Sb
counterparts that have not yet been reported experimentally. At a certain
volume a gap opens at the Fermi level in LaOFeN, separating bonding from
antibonding bands and suggesting directions for a better simple understanding
of the seemingly intricate electronic structure of this system. Finally, we
address briefly on the possible effects of post-lanthanum rare earths, which
have been observed to enhance the superconducting critical temperature
substantially.
|
0810.0376v1
|
2008-10-09
|
Mottness underpins the anomalous optical response of Iron Pnictides
|
The recent discovery of high-temperature superconductivity (HTSC) in doped
Iron pnictides is the latest example of unanticipated behavior exhibited by
$d$- and $f$-band materials. The symmetry of the SC gap, along with the
mechanism of its emergence from the ``normal'' state is a central issue in this
context. Here, motivated by a host of experimental signatures suggesting strong
correlations in the Fe-pnictides, we undertake a detailed study of their normal
state. Focussing on symmetry-unbroken phases, we use the correlated band
structure method, LDA+DMFT, to study the one-particle responses of both ${\rm
LaO_{1-x}FeAsF_{x}}$ and ${\rm SmO_{1-x}FeAsF_{x}}$ in detail. Basing ourselves
on excellent quantitative agreement between LDA+DMFT and key experiments
probing the one-particle responses, we extend our study, undertaking the first
detailed study of their normal state electrodynamic response. In particular, we
propose that near-total normal state incoherence, resulting from strong, {\it
local} correlations in the Fe $d$-shell in Fe-pnictides, underpins the
incoherent normal state transport found in these materials, and discuss the
specific electronic mechanisms leading to such behavior. We also discuss the
implications of our work for the multi-band nature of the SC by studying the
pairing ``glue'' function, which we find to be an overdamped, electronic
continuum. Similarities and differences between cuprates and Fe-pnictides are
also touched upon. Our study supports the view that SC in Fe-pnictides arises
from a bad metallic, incoherent ``normal'' state that is proximate to a Mott
insulator.
|
0810.1607v1
|
2008-10-15
|
Coulomb repulsion and correlation strength in LaFeAsO from Density Functional and Dynamical Mean-Field Theories
|
LDA+DMFT (Local Density Approximation combined with Dynamical Mean-Field
Theory) computation scheme has been used to calculate spectral properties of
LaFeAsO -- the parent compound for new high-T$_c$ iron oxypnictides. Coulomb
repulsion $U$ and Hund's exchange $J$ parameters for iron 3d electrons were
calculated using \textit {first principles} constrained density functional
theory scheme in Wannier functions formalism. Resulting values strongly depend
on the number of states taken into account in calculations: when full set of
O-$2p$, As-$4p$, and Fe-3d orbitals with corresponding bands are included,
computation results in $U=3\div$4 eV and J=0.8 eV. In contrast to that when the
basis set is restricted to Fe-3d orbitals and bands only, computation gives
much smaller parameter values $F^0$=0.8 eV, $J$=0.5 eV. However, DMFT
calculations with both parameter sets and corresponding to them choice of basis
functions result in weakly correlated electronic structure that is in agreement
with experimental X-ray and photoemission spectra.
|
0810.2629v1
|
2008-10-19
|
Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors
|
This work is provoked by recent discovery of new class prototype systems
AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-Tc superconductors
(Tc=36K). Here we report ab initio LDA results for electronic structure of the
AFFeAs systems. We provide detailed comparison between electronic properties of
both new systems and reference LaOFeAs (La111) compound. In the vicinity of the
Fermi level all three systems have essentially the same band dispersions.
However for iron fluoride systems F(2p) states were found to be separated in
energy from As(4p) ones in contrast to La111, where O(2p) states strongly
overlaps with As(4p). Thus it should be more plausible to include only Fe(3d)
and As(4p) orbitals into a realistic noninteracting model than for La111.
Moreover Sr substitution with smaller ionic radius Ca in AFFeAs materials leads
to a lattice contruction and stronger Fe(3d)-As(4p) hybridization resulting in
smaller value of the density of states at the Fermi level in the case of Ca
compound. So to some extend Ca system reminds RE111 with later Rare Earths.
However Fermi surface of new fluorides is found to be nearly perfect
two-dimensional. Also we do not expect strong dependence of superconducting
properties with respect to different types of A substitutes.
|
0810.3377v1
|
2008-10-28
|
Mixing in Zero and Solar Metallicity Supernovae
|
Two-dimensional simulations of mixing and fall back in non-rotating massive
stars have been carried out using realistic initial models for the presupernova
star and assuming standard spherically symmetric explosions of 1.2*10^51ergs.
Stars of 15 and 25 solar masses with both primordial and solar composition were
modeled. The zero metallicity supernova progenitors were compact blue stars and
the amount of Rayleigh-Taylor induced mixing in them was greatly reduced
compared with what was seen in the red supergiants with solar metallicity. The
compact zero-metal stars also experienced more fallback than their solar
metallicity counterparts. As a result, the ejected nucleosynthesis from the two
populations was very different. For the simple explosion model assumed, low
metallicity stars ejected ejected too little iron and intermediate mass
elements even to explain the abundance patterns in the most iron-poor stars
found to date, suggesting that some important ingredient is missing. Rotation
is likely to alter these conclusions by producing a greater fraction of red
supergiants among Population III stars. The velocities of the heavy elements in
all models considered - both red and blue supergiants - were less than observed
in SN 1987A, suggesting that at least occasionally, asymmetric aspects of the
explosion mechanism and fallback play a major role in mixing.
|
0810.5142v1
|
2008-11-10
|
The intrinsic electronic phase diagram of iron-oxypnictide superconductors
|
We present the first comprehensive derivation of the intrinsic electronic
phase diagram of the iron-oxypnictide superconductors in the normal state based
on the analysis of the electrical resistivity $\rho$ of both
LaFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$ for a wide range of doping. Our
data give clear-cut evidence for unusual normal state properties in these new
materials. In particular, the emergence of superconductivity at low doping
levels is accompanied by distinct anomalous transport behavior in $\rho$ of the
normal state which is reminiscent of the spin density wave (SDW) signature in
the parent material. At higher doping levels $\rho$ of LaFeAsO$_{1-x}$F$_x$
shows a clear transition from this pseudogap-like behavior to Fermi liquid-like
behavior, mimicking the phase diagram of the cuprates. Moreover, our data
reveal a correlation between the strength of the anomalous features and the
stability of the superconducting phase. The pseudogap-like features become
stronger in SmFeAsO$_{1-x}$F$_x$ where superconductivity is enhanced and vanish
when superconductivity is reduced in the doping region with Fermi liquid-like
behavior.
|
0811.1601v3
|
2008-11-13
|
Electronic phase separation in the slightly underdoped iron pnictide superconductor Ba(1-x)K(x)Fe(2)As(2)
|
Here we present a combined study of the slightly underdoped novel pnictide
superconductor Ba(1-x)K(x)Fe(2)As(2) by means of X-ray powder diffraction,
neutron scattering, muon spin rotation (muSR), and magnetic force microscopy
(MFM). Commensurate static magnetic order sets in below Tm ~ 70 K as inferred
from the emergence of the magnetic (1 0 -3) reflection in the neutron
scattering data and from the observation of damped oscillations in the
zero-field-muSR asymmetry. Transverse-field muSR below Tc shows a coexistence
of magnetically ordered and non-magnetic states, which is also confirmed by MFM
imaging. We explain such coexistence by electronic phase separation into
antiferromagnetic and superconducting/normal state regions on a lateral scale
of several tens of nanometers. Our findings indicate that such mesoscopic phase
separation can be considered an intrinsic property of some iron pnictide
superconductors.
|
0811.2224v1
|
2008-11-13
|
On the importance of satellite lines to the He-like K ALPHA complex and the G ratio for calcium, iron, and nickel
|
New, more detailed calculations of the emission spectra of the He-like K
ALPHA complex of calcium, iron and nickel have been carried out using data from
both distorted-wave and R-matrix calculations. The value of the GD ratio (an
extended definition of the G ratio that accounts for the effect of resolved and
unresolved satellite lines) is significantly enhanced at temperatures below the
temperature of He-like maximum abundance. Furthermore it is shown that
satellite lines are important contributors to the GD ratio such that GD/G>1 at
temperatures well above the temperature of maximum abundance. These new
calculations demonstrate, with an improved treatment of the KLn (n>=3)
satellite lines, that K ALPHA satellite lines need to be included in models of
He like spectra even at relatively high temperatures. The excellent agreement
between spectra and line ratios calculated from R-matrix and distorted-wave
data also confirms the validity of models based on distorted-wave data for
highly charged systems, provided the effect of resonances are taken into
account as independent processes.
|
0811.2240v1
|
2009-01-15
|
Discovery of a broad iron line in the black-hole candidate Swift J1753.5-0127, and the disk emission in the low/hard state revisited
|
We analyzed simultaneous archival XMM-Newton and RXTE observations of the
X-ray binary and black hole candidate Swift J1753.5-0127. In a previous
analysis of the same data a soft thermal component was found in the X-ray
spectrum, and the presence of an accretion disk extending close to the
innermost stable circular orbit was proposed. This is in contrast with the
standard picture in which the accretion disk is truncated at large radii in the
low/hard state. We tested a number of spectral models and we found that several
of them fit the observed spectra without the need of a soft disk-like
component. This result implies that the classical paradigm of a truncated
accretion disk in the low/hard state can not be ruled out by these data. We
further discovered a broad iron emission line between 6 and 7 keV in these
data. From fits to the line profile we found an inner disk radius that ranges
between ~6-16 gravitational radii, which can be in fact much larger, up to ~250
gravitational radii, depending on the model used to fit the continuum and the
line. We discuss the implications of these results in the context of a fully or
partially truncated accretion disk.
|
0901.2255v1
|
2009-01-17
|
Electrostatic co-assembly of iron oxide nanoparticles and polymers : towards the generation of highly persistent superparamagnetic nanorods
|
A paradigm proposed recently by Boal et al. (A.K. Boal et al., Nature 404,
746-748, 2000) deals with the possibility to use inorganic nanoparticles as
building blocks for the design and fabrication of colloidal and supracolloidal
assemblies. It is anticipated that these constructs could be made of different
shapes, patterns and functionalities and could constitute the components of
future nanodevices including sensors, actuators or nanocircuits. Here we report
a protocol that allowed us to fabricate such nanoparticle aggregates. The
building blocks of the constructs were anionically coated iron oxide
nanocrytals (superparamagnetic, size 7 nm) and cationic-neutral block
copolymers. We have shown that the electrostatic interactions between charged
species can be controlled by tuning the ionic strength of the dispersion. Under
appropriate conditions, the control of electrostatics resulted in the
elaboration of spherical or elongated aggregates at the micrometer length
scale. The elongated aggregates were found to be rod-like, with diameters of a
few hundred nanometers and lengths between 1 and 50 micrometers. In addition to
their remarkable stiffness, the nanostructured rods were found to reorient
along with an externally applied magnetic field, in agreement with the laws of
superparamagnetism.
|
0901.2667v1
|
2009-01-18
|
Iron pnictides: Single crystal growth and effect of doping on structural, transport and magnetic properties
|
We demonstrate the preparation of large, free standing iron pnictide single
crystals with a size up to 20 x 10 x 1 mm3 using solvents in zirconia crucibles
under argon atmosphere. Transport and magnetic properties are investigated to
study the effect of potassium doping on the structural and superconducting
property of the compounds. The spin density wave (SDW) anomaly at Ts ~138 K in
BaFe2As2 single crystals from self-flux shifts to Ts ~85 K due to Sn solvent
growth. We show direct evidence for an incorporation of Sn on the Fe site. The
electrical resistivity data show a sharp superconducting transition temperature
Tc~38.5 K for the single crystal of Ba0.68K0.32Fe2As2. A nearly 100% shielding
fraction and bulk nature of the superconductivity for the single crystal were
confirmed by magnetic susceptibility data. A sharp transition Tc~25 K occurred
for the single crystal of Sr0.85K0.15Fe2As2. There is direct evidence for a
coexistence of the SDW and superconductivity in the low doping regime of
Sr1-xKxFe2As2 single crystals. Structural implications of the doping effects as
well as the coexistence of the two order parameters are discussed.
|
0901.2728v3
|
2009-02-01
|
Neutron Studies of the Iron-based Family of High TC Magnetic Superconductors
|
We review neutron scattering investigations of the crystal structures,
magnetic structures, and spin dynamics of the iron-based RFe(As,P)O (R=La, Ce,
Pr, Nd), (Ba,Sr,Ca)Fe2As2, and Fe1+x(Te-Se) systems. On cooling from room
temperature all the undoped materials exhibit universal behavior, where a
tetragonal-to-orthorhombic/monoclinic structural transition occurs, below which
the systems become antiferromagnets. For the first two classes of materials the
magnetic structure within the a-b plane consists of chains of parallel Fe spins
that are coupled antiferromagnetically in the orthogonal direction, with an
ordered moment typically less than one Bohr magneton. Hence these are itinerant
electron magnets, with a spin structure that is consistent with Fermi-surface
nesting and a very energetic spin wave bandwidth ~0.2 eV. With doping, the
structural and magnetic transitions are suppressed in favor of
superconductivity. Magnetic correlations are observed in the superconducting
regime, with a magnetic resonance that follows the superconducting order
parameter just like the cuprates. The rare-earth moments order
antiferromagnetically at low T like conventional magnetic-superconductors.
Pressure in CaFe2As2 transforms the system from a magnetically ordered
orthorhombic material to a collapsed non-magnetic tetragonal system. Tetragonal
Fe1+xTe transforms to a low T monoclinic structure at small x that changes to
orthorhombic at larger x, which is accompanied by a crossover from commensurate
to incommensurate magnetic order. Se doping suppresses the magnetic order.
|
0902.0091v2
|
2009-02-02
|
Doping dependent Irreversible Magnetic Properties of Ba(Fe1-xCox)2As2 Single Crystals
|
We discuss the irreversible magnetic properties of self-flux grown
Ba(Fe1-xCox)2As2 single crystals for a wide range of concentrations covering
the whole phase diagram from the underdoped to the overdoped regime, x=0.038,
0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized
by a magneto-optical method and show excellent spatial uniformity of the
superconducting state. The overall behavior closely follows classical Bean
model of the critical state. The field-dependent magnetization exhibits second
peak at a temperature and doping - dependent magnetic field, Hp. The evolution
of this fishtail feature with doping is discussed. Magnetic relaxation is
time-logarithmic and unusually fast. Similar to cuprates, there is an apparent
crossover from collective elastic to plastic flux creep above Hp. At high
fields, the field dependence of the relaxation rate becomes doping independent.
We discuss our results in the framework of the weak collective pinning and show
that vortex physics in iron-based pnictide crystals is much closer to high-Tc
cuprates than to conventional s-wave (including MgB2) superconductors.
|
0902.0201v1
|
2009-02-11
|
Non-exponential London penetration depth in Ba$_{1-}$K$_{x}$Fe$_{2}$As$_{2}$ single crystals
|
We have studied the in- and out-of-plane magnetic penetration depths in the
hole- doped iron based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$
($T_{c}\approx$ 30K). The study was performed on single crystals grown from
different fluxes and we find that the results are nearly the same. The in-plane
London penetration depth $\lambda_{ab}$ does not show exponential saturation at
low temperature, as would be expected from a fully gapped superconductor.
Instead, $\lambda_{ab}(T)$ shows a power-law behavior, $\lambda\propto T^{n}$
($n\approx 2$), down to $T\approx 0.02 T_{c}$, similar to the electron doped
Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. The penetration depth anisotropy
$\gamma_{\lambda}=\lambda_{c}(T)/\lambda_{ab}(T)$ increases upon cooling,
opposite to the trend observed in the anisotropy of the upper critical field,
$\gamma_{\xi}=H_{c2}^{\perp c}(0)/H_{c2}^{\parallel c}(0)$. These are universal
characteristics of both the electron and hole doped 122 systems, suggesting
unconventional superconductivity. The behavior of the in-plane superfluid
density $\rho_{ab}(T)$ is discussed in light of existing theoretical models
proposed for the iron pnictides superconductors.
|
0902.1804v1
|
2009-02-16
|
Electronic Structure of the BaFe$_2$As$_2$ Family of Iron Pnictides
|
We use high resolution angle-resolved photoemission spectroscopy to study the
band structure and Fermi surface topology of the BaFe$_2$As$_2$ iron pnictides.
We observe two electron bands and two hole bands near the X-point, $(\pi,\pi)$
of the Brillouin zone, in the paramagnetic state for different doping levels,
including electron-doped Ba(Co$_{0.06}$Fe$_{0.94}$)$_2$As$_2$, undoped
BaFe$_2$As$_2$, and hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. Among these
four bands, only the electron bands cross the Fermi level, forming two electron
pockets around X, while the hole bands approach but never reach the Fermi
level. We show that the band structure of the BaFe$_2$As$_2$ family matches
reasonably well with the prediction of LDA calculations after a
momentum-dependent shift and renormalization. Our finding resolves a number of
inconsistencies regarding the electronic structure of pnictides.
|
0902.2628v1
|
2009-03-11
|
Sub-surface convection zones in hot massive stars and their observable consequences
|
We study the convection zones in the outer envelope of hot massive stars
which are caused by opacity peaks associated with iron and helium ionization.
We determine the occurrence and properties of these convection zones as
function of the stellar parameters. We then confront our results with
observations of OB stars. A stellar evolution code is used to compute a grid of
massive star models at different metallicities. In these models, the mixing
length theory is used to characterize the envelope convection zones. We find
the iron convection zone (FeCZ) to be more prominent for lower surface gravity,
higher luminosity and higher initial metallicity. It is absent for luminosities
below about $10^{3.2}\Lsun$, $10^{3.9}\Lsun$, and $10^{4.2}$\Lsun$ for the
Galaxy, LMC and SMC, respectively. We map the strength of the FeCZ on the
Hertzsprung-Russell diagram for three metallicities, and compare this with the
occurrence of observational phenomena in O stars: microturbulence, non-radial
pulsations, wind clumping, and line profile variability. The confirmation of
all three trends for the FeCZ as function of stellar parameters by empirical
microturbulent velocities argues for a physical connection between
sub-photospheric convective motions and small scale stochastic velocities in
the photosphere of O- and B-type stars. We further suggest that clumping in the
inner parts of the winds of OB stars could be caused by the same mechanism, and
that magnetic fields produced in the FeCZ could appear at the surface of OB
stars as diagnosed by discrete absorption components in ultraviolet absorption
lines.
|
0903.2049v1
|
2009-03-17
|
Solar System formation
|
In this review, three major changes in our understanding of the early history
of the Solar System are presented.
1) Early differentiation: A few recent results support the idea that
protoplanet formation and differentiation occurred partly simultaneously than
CAI formation. First, some iron meteorites, eucrites, and angrites older than
the chondrules or even than the CAI have been found. Second, iron meteorites
could be debris of early disrupted differentiated planetesimals, scattered from
the terrestrial planet region to the Main Belt. Finally, chondrules contain
fragments of planetesimal material.
2) Earth and Moon: An equilibration mechanism explains the identical Oxygen
isotopic composition of the Earth and the Moon. In addition, it has been shown
that the Earth and the Moon mantles have the same 182^W anomaly, in contrast to
what was believed before. Consequently, the Moon forming impact should have
occurred after the extinction of the 182Hf radioactivity, about 60 Myr after
Solar System formation. This new datation is in agreement with new N-body
numerical simulations of the last phase of terrestrial planets formation, in
which giant impacts occur during about 100 Myr.
3) Giant planets and Nice model: The migration of the giant planets in the
protoplanetary disc can be prevented if the planets are in resonance, close to
each other. In the ``Nice model'', the 4 outer planets of the Solar System were
in a compact configuration after the dissipation of gaseous disc. A few hundred
million years later, a global instability drives the planets on their present
orbits, producing the Late Heavy Bombardment. In this frame, a lot of
characteristics of our Solar System can be explained.
|
0903.3008v1
|
2009-03-19
|
Classification of quantum phases for the star-lattice antiferromagnet via a projective symmetry group analysis
|
We study possible quantum ground states of the Heisenberg antiferromagnet on
the star lattice, which may be realized in the recently discovered polymeric
Iron Acetate,
Fe$_3$($\mu_3$-O)($\mu$-OAc)$_6$(H$_2$O)$_3$[Fe$_3$($\mu_3$-O)($\mu$-OAc)$_{7.5}$]$_2\cdot$
7H$_2$O. Even though the Fe$^{\rm III}$ moment in this material carries
spin-5/2 and the system eventually orders magnetically at low temperatures, the
magnetic ordering temperature is much lower than the estimated Curie-Weiss
temperature, revealing the frustrated nature of the spin interactions.
Anticipating that a lower spin analog of this material may be synthesized in
future, we investigate the effect of quantum fluctuations on the star-lattice
antiferromagnet using a large-$N$ Sp($N$) mean field theory and a projective
symmetry group analysis for possible bosonic quantum spin liquid phases. It is
found that there exist only two distinct gapped $Z_2$ spin liquid phases with
bosonic spinons for non-vanishing nearest-neighbor valence-bond-amplitudes. In
particular, the spin liquid phase which has a lower energy in the
nearest-neighbor exchange model can be stabilized for relatively higher spin
magnitudes. Hence it is perhaps a better candidate for the realization of
quantum spin liquid state. We also determine the magnetic ordering patterns
resulting from the condensation of the bosonic spinons in the two different
spin liquid phases. We expect these magnetic ordering patterns would directly
be relevant for the low temperature ordered phase of the Iron Acetate. The
phase diagram containing all of these phases and various dimerized states are
obtained for the nearest-neighbor exchange model and its implications are
discussed.
|
0903.3408v1
|
2009-04-28
|
Doping dependence of Normal-state Property in the Iron-based Oxypnictide Superconductors LaFeAsO1-y probed by 57Fe-NMR and 75As-NMR/NQR
|
We report systematic 57Fe-NMR and 75As-NMR/NQR studies on an underdoped
sample (T_c=20 K), an optimally doped sample (T_c=28 K), and an overdoped
sample (T_c=22 K) of oxygen-deficient iron (Fe)-based oxypnictide
superconductor LaFeAsO_{1-y}$. A microscopic phase separation between
superconducting domains and magnetic domains is shown to take place in the
underdoped sample, indicating a local inhomogeneity in association with the
density distribution of oxygen deficiencies. As a result, 1/T_1T in the normal
state of the superconducting domain decreases significantly upon cooling at
both the Fe and As sites regardless of the electron-doping level in
LaFeAsO_{1-y}. On the basis of this result, we claim that $1/T_1T$ is not
always enhanced by antiferromagnetic fluctuations close to an antiferromagnetic
phase in the underdoped superconducting sample. This contrasts with the
behavior in hole-doped Ba_{0.6}K_{0.4}Fe2As2(T_c= 38 K), which exhibits a
significant increase in $1/T_1T$ upon cooling. We remark that the crucial
difference between the normal-state properties of LaFeAsO_{1-y} and
Ba_{0.6}K_{0.4}Fe2As2 originates from the fact that the relevant Fermi surface
topologies are differently modified depending on whether electrons or holes are
doped into the FeAs layers.
|
0904.4301v2
|
2009-05-01
|
Vortex core states in a minimal two-band model for iron-based superconductors
|
The pairing symmetry is one of the major issues in the study of iron-based
superconductors. We adopt a minimal two-band tight-binding model with various
channels of pairing interaction, and derive a set of two-band Bogoliubov-de
Gennes (BdG) equations. The BdG equations are implemented in real space and
then solved self-consistently via exact diagonalization. In the uniform case,
we find that the $d_{x^2-y^2}$-wave pairing state is most favorable for a
nearest-neighbor pairing interaction while the $s_{x^2y^2}$-wave pairing state
is most favorable for a next-nearest-neighbor pairing interaction. The is
consistent with that reported by Seo {\em et al.} [Phys. Rev. Lett. {\bf 101},
206404 (2008)]. We then proceed to study the local electronic structure around
a magnetic vortex core for both $d_{x^2-y^2}$-wave and $s_{x^2y^2}$-wave
pairing symmetry in the mixed state. It is found from the local density of
states (LDOS) spectra and its spatial variation that the resonance core states
near the Fermi energy for the $d_{x^2-y^2}$-wave pairing symmetry are bound
while those for the $s_{x^2y^2}$-wave pairing symmetry can evolve from the
localized states into extended ones with varying electron filling factor.
Furthermore, by including an effective exchange interaction, the emergent
antiferromagnetic spin-density-wave (SDW) order can suppress the resonance core
states, which provides one possible avenue to understand the absence of
resonance peak as revealed by recent scanning tunneling microscopy experiment
(STM) by Yin {\em et al.} [Phys. Rev. Lett. {\bf 102}, 097002 (2009)].
|
0905.0014v2
|
2009-05-06
|
Impurity-induced bound states in iron-based superconductors with s-wave cos(kx)cos(ky) pairing symmetry
|
Using both the self-consistent Bogoliubov-de Gennes formulation and
non-self-consistent T-matrix approach, we perform a comprehensive investigation
of the in-gap bound states induced by a localized single impurity in iron-based
superconductors. We focus on studying signatures associated with the
unconventional sign-changed s-wave pairing symmetry. For a non-magnetic
impurity, we find that there are two in-gap bounds, symmetric with respect to
zero energy, only in the sign changed s-wave pairing state, not in the
sign-unchanged s-wave state, due to the existence of non-trivial Andreev bound
states caused by the sign change. For a magnetic impurity, we find that due to
the breakdown of the local time-reversal symmetry, there exist only bound state
solutions (with orbital degeneracy) carrying one of the electron-spin
polarizations around the impurity. As increasing the scattering strength, the
system undergoes a quantum phase transition (level crossing) from a
spin-unpolarized ground state to a spin-polarized one. While the results for
the magnetic impurity are qualitatively similar in both the sign-changed and
sign-unchanged s-wave superconducting states, the bound states in the first
case are more robust and there is no $\pi$ phase shift of the SC gap near the
impurity in the strong scattering regime.
|
0905.0734v2
|
2009-05-20
|
A new look on the nature of high-spin to low-spin transition in Fe2O3
|
Iron sesquioxide (Fe2O3) displays pressure and temperature induced spin and
structural transitions. Our calculations show that, density functional theory
(DFT), in the generalized gradient approximation (GGA) scheme, is capable of
capturing both the transitions. The ambient pressure corundum type phase
(hematite or alpha-Fe2O3), having R_3c symmetry, gets distorted by the
application of pressure and transforms to a distorted corundum type or
Rh2O3(II) phase with Pbcn symmetry, in agreement with recent experiments. GGA +
U calculations show the same trend but shift the transition pressures to higher
values. Experimentally, the onset of the structural transition begins in the
vicinity of the spin transition pressure and whether the system undergoes spin
transition in the corundum type (HP1) or in the Rh2O3(II) type (HP2) phase, is
still a controversial issue. With a relatively simple, but general, octahedral
structural parameter, Voct (the octahedral volume around iron ions), we show
that in order to acquire a low spin (LS) state from a high spin (HS) one, the
system does not necessarily need to change the crystal structure. Rather, the
spin transition is a phenomenon that concerns the cation octahedra and the spin
state of the system depends mainly on the value of Voct, which is governed by
two distinct equations of state, separated by a well defined volume gap, for
the HS and LS states respectively. Analysis of the results on the basis of
octahedral volume allows to sum up and bridge the gap between two experimental
results and thus provides a better description of the system in the region of
interest.
|
0905.3414v1
|
2009-06-01
|
A Compton-thick Wind in the High Luminosity Quasar, PDS 456
|
PDS 456 is a nearby (z=0.184), luminous (L_bol ~10^47 erg/s) type I quasar. A
deep 190 ks Suzaku observation in February 2007 revealed the complex, broad
band X-ray spectrum of PDS 456. The Suzaku spectrum exhibits highly
statistically significant absorption features near 9 keV in the quasar
rest--frame. We show that the most plausible origin of the absorption is from
blue-shifted resonance (1s-2p) transitions of hydrogen-like iron (at 6.97 keV
in the rest frame). This indicates that a highly ionized outflow may be present
moving at near relativistic velocities (~0.25c). A possible hard X-ray excess
is detected above 15 keV with HXD (at 99.8% confidence), which may arise from
high column density gas (Nh>10^24cm^-2) partially covering the X-ray emission,
or through strong Compton reflection. Here we propose that the iron K-shell
absorption in PDS 456 is associated with a thick, possibly clumpy outflow,
covering about 20% of $4\pi$ steradian solid angle. The outflow is likely
launched from the inner accretion disk, within 15-100 gravitational radii of
the black hole. The kinetic power of the outflow may be similar to the
bolometric luminosity of PDS 456. Such a powerful wind could have a significant
effect on the co-evolution of the host galaxy and its supermassive black hole,
through feedback.
|
0906.0312v1
|
2009-06-17
|
The Kondo effect in ferromagnetic atomic contacts
|
Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk,
electronic conduction in these materials takes place mainly through the $s$ and
$p$ electrons, whereas the magnetic moments are mostly in the narrow
$d$-electron bands, where they tend to align. This general picture may change
at the nanoscale because electrons at the surfaces of materials experience
interactions that differ from those in the bulk. Here we show direct evidence
for such changes: electronic transport in atomic-scale contacts of pure
ferromagnets (iron, cobalt and nickel), despite their strong bulk
ferromagnetism, unexpectedly reveal Kondo physics, that is, the screening of
local magnetic moments by the conduction electrons below a characteristic
temperature. The Kondo effect creates a sharp resonance at the Fermi energy,
affecting the electrical properties of the system;this appears as a Fano-Kondo
resonance in the conductance characteristics as observed in other artificial
nanostructures. The study of hundreds of contacts shows material-dependent
lognormal distributions of the resonance width that arise naturally from Kondo
theory. These resonances broaden and disappear with increasing temperature,
also as in standard Kondo systems. Our observations, supported by calculations,
imply that coordination changes can significantly modify magnetism at the
nanoscale. Therefore, in addition to standard micromagnetic physics, strong
electronic correlations along with atomic-scale geometry need to be considered
when investigating the magnetic properties of magnetic nanostructures.
|
0906.3135v1
|
2009-07-07
|
Doping dependence of heat transport in the iron-arsenide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$: from isotropic to strongly $k$-dependent gap structure
|
The temperature and magnetic field dependence of the in-plane thermal
conductivity $\kappa$ of the iron-arsenide superconductor
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ was measured down to $T \simeq 50$ mK and up to
$H = 15$ T as a function of Co concentration $x$ in the range 0.048 $ \leq x
\leq $ 0.114. In zero magnetic field, a negligible residual linear term in
$\kappa/T$ as $T \to 0$ at all $x$ shows that there are no zero-energy
quasiparticles and hence the superconducting gap has no nodes in the $ab$-plane
anywhere in the phase diagram. However, the field dependence of $\kappa$
reveals a systematic evolution of the superconducting gap with doping $x$, from
large everywhere on the Fermi surface in the underdoped regime, as evidenced by
a flat $\kappa (H)$ at $T \to 0$, to strongly $k$-dependent in the overdoped
regime, where a small magnetic field can induce a large residual linear term,
indicative of a deep minimum in the gap magnitude somewhere on the Fermi
surface. This shows that the superconducting gap structure has a strongly
$k$-dependent amplitude around the Fermi surface only outside the
antiferromagnetic/orthorhombic phase.
|
0907.1276v2
|
2009-09-09
|
Modeling the light curve of the transient SCP06F6
|
We consider simple models based on core collapse or pair-formation supernovae
to account for the light curve of the transient SCP06F6. A radioactive decay
diffusion model provides estimates of the mass of the required radioactive
nickel and the ejecta as functions of the unknown redshift. An opacity change
such as by dust formation or a recombination front may account for the rapid
decline from maximum. We particularly investigate two specific redshifts:
$z=0.143$, for which Gaensicke et al. (2008) have proposed that the
unidentified broad absorption features in the spectrum of SCP06F6 are C$_{2}$
Swan bands, and $z=0.57$ based on a crude agreement with the Ca H&K and UV
iron-peak absorption features that are characteristic of supernovae of various
types. The ejected masses and kinetic energies are smaller for a more tightly
constrained model invoking envelope recombination. We also discuss the
possibilities of circumstellar matter (CSM) shell diffusion and shock
interaction models. In general, optically-thick CSM diffusion models can fit
the data with the underlying energy coming from an energetic buried supernova.
Models in which the CSM is of lower density so that the shock energy is both
rapidly thermalized and radiated tend not to be self-consistent. We suggest
that a model of SCP06F6 worth futher exploration is one in which the redshift
is $\sim$ 0.57, the spectral features are Ca and iron peak elements, and the
light curve is powered by the diffusive release of a substantial amount of
energy from nickel decay or from an energetic supernova buried in the ejecta of
an LBV-like event.
|
0909.1798v2
|
2009-09-10
|
New identified (3H)4d - (3H)4f transitions of Fe II from UVES spectra of HR 6000 and 46 Aql
|
The analysis of the high-resolution UVES spectra of the CP stars HR 6000 and
46 Aql revealed the presence of an impressive number of unidentified lines, in
particular in the 5000 - 5400 A region. Because numerous 4d-4f transitions of
FeII lie in this spectral range, and because both stars are iron overabundant,
we investigated whether the unidentified lines can be due to FeII. ATLAS12
model atmospheres with parameters [13450K, 4.3] and [12560K, 3.8] were computed
for the individual abundances of HR 6000 and 46 Aql, respectively, in order to
use the stars as spectroscopic sources to identify FeII lines and to determine
FeII gf-values. After having identified several unknown lines in the stellar
spectra as due to (3H)4d - (3H)4f transitions of FeII, we derived stellar log
gf's for them by comparing observed and computed profiles. The energies of the
upper levels were assigned on the basis of both laboratory iron spectra and
predicted energy levels. We fixed 21 new levels of FeII with energies between
122910.9 cm^-1 and 123441.1 cm^-1. They allowed us to add 1700 new lines to the
Fe II line list in the range 810 - 15011 A.
|
0909.1936v1
|
2009-09-11
|
Direct Evidence for Outflow of Metal-enriched Gas Along the Radio Jets of Hydra A
|
Using deep Chandra observations of the Hydra A galaxy cluster, we examine the
metallicity structure near the central galaxy and along its powerful radio
source. We show that the metallicity of the intracluster medium is enhanced by
up to 0.2 dex along the radio jets and lobes compared to the metallicity of the
undisturbed gas. The enhancements extend from a radius of 20 kpc from the
central galaxy to a distance of ~ 120 kpc. We estimate the total iron mass that
has been transported out of the central galaxy to be between 2 x 10^7 M_sun and
7 x 10^7 M_sun which represents 10% - 20% of the iron mass within the central
galaxy. The energy required to lift this gas is roughly 1% to 5% of the total
energetic output of the AGN. Evidently, Hydra A's powerful radio source is able
to redistribute metal-enriched, low entropy gas throughout the core of the
galaxy cluster. The short re-enrichment time scale < 10^9 yr implies that the
metals lost from the central galaxy will be quickly replenished.
|
0909.2252v2
|
2009-09-22
|
Chemistry of atmospheres formed during accretion of the Earth and other terrestrial planets
|
We used chemical equilibrium and chemical kinetic calculations to model
chemistry of the volatiles released by heating different types of carbonaceous,
ordinary and enstatite chondritic material as a function of temperature and
pressure. Our results predict the composition of atmospheres formed by
outgassing during accretion of the Earth and other terrestrial planets.
Outgassing of CI and CM carbonaceous chondritic material produces H2O-rich
(steam) atmospheres in agreement with the results of impact experiments.
However, outgassing of other types of chondritic material produces atmospheres
dominated by other gases. Outgassing of ordinary (H, L, LL) and high iron
enstatite (EH) chondritic material yields H2-rich atmospheres with CO and H2O
being the second and third most abundant gases. Outgassing of low iron
enstatite (EL) chondritic material gives a CO-rich atmosphere with H2, CO2, and
H2O being the next most abundant gases. Outgassing of CV carbonaceous
chondritic material gives a CO2-rich atmosphere with H2O being the second most
abundant gas. Our results predict that the atmospheres formed during accretion
of the Earth and Mars were probably H2-rich unless the accreted material was
dominantly CI and CM carbonaceous chondritic material. We also predict
significant amounts of S, P, Cl, F, Na, and K in accretionary atmospheres at
high temperatures (1500-2500 K). Finally, our results may be useful for
interpreting spectroscopic observations of accreting extrasolar terrestrial
planets.
|
0909.4050v1
|
2009-10-12
|
CCD time-series photometry of the globular cluster NGC 5053: RR Lyrae, Blue Stragglers and SX Phoenicis stars revisited
|
We report the results of CCD $V$, $r$ and $I$ time-series photometry of the
globular cluster NGC 5053. New times of maximum light are given for the eight
known RR Lyrae stars in the field of our images and their periods are revised.
Their $V$ light curves were Fourier decomposed to estimate their physical
parameters. A discussion on the accuracy of the Fourier-based iron abundances,
temperatures, masses and radii is given. New periods are found for the 5 known
SX Phe stars and a critical discussion of their secular period changes is
offered. The mean iron abundance for the RR Lyrae stars is found to be [Fe/H]
$\sim -1.97 \pm 0.16$ and lower values are not supported by the present
analysis. The absolute magnitude calibrations of the RR Lyrae stars yield an
average true distance modulus of $16.12 \pm 0.04$ or a distance of $16.7 \pm
0.3$ kpc. Comparison of the observational CMD with theoretical isochrones
indicates an age of $12.5 \pm 2.0$ Gyrs for the cluster. A careful
identification of all reported Blue Stragglers (BS) and their $V,I$ magnitudes
leads to the conclusion that BS12, BS22, BS23 and BS24 are not BS. On the other
hand, three new BS are reported. Variability was found in seven BS, very likely
of the SX Phe type in five of them, and in one red giant star. The new SX Phe
stars follow established $PL$ relationships and indicate a distance in
agreement with the distance from the RR Lyrae stars.
|
0910.2068v1
|
2009-10-21
|
Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE
|
Non-local thermodynamic equilibrium (NLTE) line formation for neutral and
singly-ionized iron is considered through a range of stellar parameters
characteristic of cool stars. A comprehensive model atom for Fe I and Fe II is
presented. Our NLTE calculations support the earlier conclusions that the
statistical equilibrium (SE) of Fe I shows an underpopulation of Fe I terms.
However, the inclusion of the predicted high-excitation levels of Fe I in our
model atom leads to a substantial decrease in the departures from LTE. As a
test and first application of the Fe I/II model atom, iron abundances are
determined for the Sun and four selected stars with well determined stellar
parameters and high-quality observed spectra. Within the error bars, lines of
Fe I and Fe II give consistent abundances for the Sun and two metal-poor stars
when inelastic collisions with hydrogen atoms are taken into account in the SE
calculations. For the close-to-solar metallicity stars Procyon and $\beta$ Vir,
the difference (Fe II - Fe I) is about 0.1 dex independent of the line
formation model, either NLTE or LTE. We evaluate the influence of departures
from LTE on Fe abundance and surface gravity determination for cool stars.
|
0910.3997v1
|
2009-10-22
|
Point-contact spectroscopic studies on normal and superconducting AFe_2As_2-type iron-pnictide single crystals
|
Point-contact Andreev reflection spectroscopy (PCARS) is applied to
investigate the gap structure in iron pnictide single crystal superconductors
of the AFe_2As_2 (A=Ba, Sr) family ("Fe-122"). The observed point-contact
junction conductance curves, G(V), can be divided into two categories: one
where Andreev reflection is present for both (Ba_{0.6}K_{0.4})Fe_2As_2 and
Ba(Fe_{0.9}Co_{0.1})_2As_2, and the other with a V^{2/3} background conductance
universally observed extending even up to 100 meV for Sr_{0.6}Na_{0.4}Fe_2As_2
and Sr(Fe_{0.9}Co_{0.1})_2As_2. The latter is also observed in point-contact
junctions on the nonsuperconducting parent compound BaFe_2As_2. Mesoscopic
phase-separated coexistence of magnetic and superconducting orders is
considered to explain distinct behaviors in the superconducting samples. For
Ba_{0.6}K_{0.4}Fe_2As_2, double peaks due to Andreev reflection with
strongly-sloping background are frequently observed for point-contacts on
freshly-cleaved c-axis surfaces. If normalized by a background baseline and
analyzed by the Blonder-Tinkham-Klapwijk model, the data show a gap size
~3.0-4.0 meV with 2\Delta_0/k_BT_c ~ 2.0-2.6, consistent with the smaller gap
size reported in the LnFeAsO family ("Fe-1111"). For the
Ba(Fe_{0.9}Co_{0.1})_2As_2, G(V) curves typically display a zero-bias
conductance peak.
|
0910.4230v2
|
2009-11-01
|
Non-equilibrium ionization states in galaxy clusters
|
X-ray imaging observatories have revealed hydrodynamic structures with linear
scales ~ 10 kpc in clusters of galaxies, such as shock waves in the 1E0657-56
and A520 galaxy clusters and the hot plasma bubble in the MKW 3s cluster. The
future X-ray observatory IXO will resolve for the first time the metal
distribution in galaxy clusters at the these scales. Heating of plasmas by
shocks and AGN activities can result in non-equilibrium ionization states of
metal ions. We study the effect of the non-equilibrium ionization at linear
scales <50 kpc in galaxy clusters. A condition for non-equilibrium ionization
is derived by comparing the ionization time-scale with the age of hydrodynamic
structures. Modeling of non-equilibrium ionization when the plasma temperature
suddenly change is performed. An analysis of relaxation processes of the FeXXV
and FeXXVI ions by means of eigenvectors of the transition matrix is given. We
conclude that the non-equilibrium ionization of iron can occur in galaxy
clusters if the baryonic overdensity delta is smaller than 11.0/tau, where
tau<<1 is the ratio of the hydrodynamic structure age to the Hubble time. Our
modeling indicates that the emissivity in the helium-like emission lines of
iron increases as a result of deviation from the ionization equilibrium. A slow
process of helium-like ionic fraction relaxation was analyzed. A new way to
determine a shock velocity is proposed.
|
0911.0119v1
|
2009-11-17
|
Limits on spin determination from disc spectral fitting in GX 339-4
|
We attempt to constrain the black hole spin in GX 339-4 from spectral fitting
of disc dominated data using RXTE spectra from the three most recent outbursts.
We use the best current models for the disc emission, including full radiative
transfer through the photosphere rather than assuming that the intrinsic
emission from each radius has a (colour temperature corrected) blackbody
spectrum. The results strongly depend on the poorly known binary system
parameters, but we find a strict upper limit of a<0.9 for any distance greater
than 6 kpc, assuming that the orbital inclination is the same as that of the
inner disc. By contrast, the higher spin of 0.935 +/- 0.01 (statistical)
+/-0.01 (systematic) claimed from fitting the iron line profile in this object
requires that the inner disc is misaligned by over 20 degrees from the orbital
inclination. While some of these datasets are distorted by instrumental pileup,
the same spin/inclination constraints are derived from data which are not piled
up, so there is a real conflict between the two techniques to measure spin. We
argue that the disc spectral fits are more likely to be robust hence that there
are still issues to be understood in the iron line profile.
|
0911.3281v2
|
2009-12-05
|
Abundance Ratios in Stars vs. Hot Gas in Elliptical Galaxies: the Chemical Evolution Modeller Point of View
|
I will present predictions from chemical evolution model aimed at a
self-consistent study of both optical (i.e. stellar) and X-ray (i.e.gas)
properties of present-day elliptical galaxies. Detailed cooling and heating
processes in the interstellar medium (ISM) are taken into and allow a reliable
modelling of the SN-driven galactic wind. SNe Ia activity, in fact, may power a
galactic wind lasting for a considerable amount of the galactic lifetime, even
in the case for which the efficiency of energy transfer into the ISM per SN Ia
event is less than unity. The model simultaneously reproduces the
mass-metallicity, the colour-magnitude, the L_X - L_B and the L_X - T
relations, as well as the observed trend of the [Mg/Fe] ratio as a function of
sigma, by adopting the prescriptions of Pipino & Matteucci (2004) for the gas
infall and star formation timescales. The "iron discrepancy", namely the too
high predicted iron abundance in X-ray haloes of ellipticals compared to
observations, can be solved by taking into account the existence of dust. I
will make predictions on several abundance ratios in the ISM and compare them
with the most recent observations.
|
0912.0971v1
|
2009-12-11
|
Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2
|
The discovery of a new family of high Tc materials, the iron arsenides
(FeAs), has led to a resurgence of interest in superconductivity. Several
important traits of these materials are now apparent, for example, layers of
iron tetrahedrally coordinated by arsenic are crucial structural ingredients.
It is also now well established that the parent non-superconducting phases are
itinerant magnets, and that superconductivity can be induced by either chemical
substitution or application of pressure, in sharp contrast to the cuprate
family of materials. The structure and properties of chemically substituted
samples are known to be intimately linked, however, remarkably little is known
about this relationship when high pressure is used to induce superconductivity
in undoped compounds. Here we show that the key structural features in
BaFe2As2, namely suppression of the tetragonal to orthorhombic phase transition
and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same
behavior under pressure as found in chemically substituted samples. Using
experimentally derived structural data, we show that the electronic structure
evolves similarly in both cases. These results suggest that modification of the
Fermi surface by structural distortions is more important than charge doping
for inducing superconductivity in BaFe2As2.
|
0912.2376v1
|
2009-12-17
|
XMM-Newton Observations of X-ray Pulsar Cen X-3
|
We present the results of our study of X-ray pulsar Cen X-3 using XMM-Newton
observations. The light curve and the spectrum for this observations were built
and Fe triplet within 6.5-7 keV region was detected. The geometric model of
relativistic accretion disk for iron emission lines Fe I K alpha, Fe XXV and Fe
XXVI in 6.4-7.0 keV region was applied. The values of disc inclination, inner
and outer radii of the disc and mass of the central compact object (neutron
star) were obtained. Intensity variations of these lines during orbital motion
were also detected. The largest variation was detected for Fe I K alpha line,
that agrees with the results of other authors. These results conform the model
in which Fe I K alpha line forms in hot plasma of accretion disc and highly
ionized iron lines form in outer regions of binary system. Probably the most
interesting feature of Cen X-3 spectrum is Fe XXV triplet which was found by
Iaria et al. (2005) from Chandra data analysis. We did not find this triplet in
our analysis of XMM-Newton data and explain its absence by the insufficient
energy resolution of XMM-Newton instruments.
|
0912.3354v2
|
2009-12-22
|
XMM-Newton long-look observation of the narrow line Seyfert 1 galaxy PKS0558-504; I: Spectral analysis
|
We present results from the spectral analysis of a long XMM-Newton
observation of the radio-loud NLS1 galaxy PKS0558-504. The source is highly
variable, on all sampled time scales. We did not observe any absorption
features in either the soft or hard X-ray band. We found weak evidence for the
presence of an iron line at ~6.8 keV, which is indicative of emission from
highly ionized iron. The 2-10 keV band spectrum is well fitted by a simple
power law model, whose slope steepens with increasing flux, similar to what is
observed in other Seyferts as well. The soft excess is variable both in flux
and shape, and it can be well described by a low-temperature Comptonisation
model, whose slope flattens with increasing flux. The soft excess flux
variations are moderately correlated with the hard band variations, and we
found weak evidence that they are leading them by ~20 ksec. Our results rule
out a jet origin for the bulk of the X-ray emission in this object. The
observed hard band spectral variations suggest intrinsic continuum slope
variations, caused by changes in the "heating/cooling" ratio of the hot corona.
The low-temperature Comptonising medium, responsible for the soft excess
emission, could be a hot layer in the inner disc of the source, which appears
due to the fact that the source is accreting at a super-Eddington rate. The
soft excess flux and spectral variations could be caused by random variations
of the accretion rate.
|
0912.4416v1
|
2009-12-28
|
Warm absorber and truncated accretion disc in IRAS 05078+1626
|
X-ray observations of unabsorbed active galactic nuclei provide an
opportunity to explore the innermost regions of supermassive black hole
accretion discs. Our goal in this paper is to investigate the central
environment of a Seyfert 1.5 galaxy IRAS 05078+1626. We studied the
time-averaged spectrum obtained with the EPIC and RGS instruments. A power-law
continuum (photon index ~ 1.75) dominates the 2-10 keV energy range. A narrow
iron K alpha spectral line is clearly seen, presumably originating in a distant
torus, but no broad relativistic component was detected. However, the power law
and the iron K alpha line alone do not provide a satisfactory fit in the soft
X-ray band whose spectrum can be explained by the combination of three
components: a) a cold photoelectric absorber with column density ~ 10^(21)
cm^(-2). This gas could be located either in outer parts of the accretion disc,
at the rim of the torus or farther out in the host galaxy; b) a warm absorber
with high ionization parameter (log(xi) ~ 2.2) and column density ~ 10^(24)
cm^(-2); c) an ionized reflection where the reflecting gas could be either in
the inner wall of a warm absorber cone or in an ionized accretion disc. The
first X-ray spectroscopic measurement of IRAS05078+1626 unveils some of the
standard ingredients in Seyfert galaxies, such as a power-law primary
continuum, modified by reflection from the accretion disc and by the effect of
complex, multi-phase obscuration. However, data constrains the accretion disc,
if present, not to extend closer than to 60 gravitational radii from the black
hole.
|
0912.5165v2
|
2010-01-15
|
XMM-Newton unveils the complex iron K alpha region of Mrk 279
|
We present the results of a ~160 ks-long XMM-Newton observation of the
Seyfert 1 galaxy Mrk 279. The spectrum shows evidence of both broad and narrow
emission features. The Fe K alpha line may be equally well explained by a
single broad Gaussian (FWHM~10,000 km/s) or by two components: an unresolved
core plus a very broad profile (FWHM~14,000 km/s). For the first time we
quantified, via the "locally optimally emitting cloud" model, the contribution
of the broad line region (BLR) to the absolute luminosity of the broad
component of the Fe K alpha at 6.4 keV. We find that the contribution of the
BLR is only ~3%. In the two-line component scenario, we also evaluated the
contribution of the highly ionized gas component, which produces the FeXXVI
line in the iron K region. This contribution to the narrow core of the Fe K
alpha line is marginal <0.1%. Most of the luminosity of the unresolved,
component of Fe K alpha may come from the obscuring torus, while the very-broad
associated component may come from the accretion disk. However, models of
reflection by cold gas are difficult to test because of the limited energy
band. The FeXXVI line at 6.9 keV is consistent to be produced in a high column
density (N_H~10^23 cm^{-2}), extremely ionized (log\xi~5.5-7) gas. This gas may
be a highly ionized outer layer of the torus.
|
1001.2712v1
|
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