298 research outputs found
Model atmospheres of X-ray bursting neutron stars
We present an extended set of model atmospheres and emergent spectra of X-ray
bursting neutron stars in low mass X-ray binaries. Compton scattering is taken
into account. The models were computed in LTE approximation for six different
chemical compositions: pure hydrogen and pure helium atmospheres, and
atmospheres with a solar mix of hydrogen and helium and various heavy elements
abundances: Z = 1, 0.3, 0.1, and 0.01 Z_sun, for three values of gravity, log g
=14.0, 14.3, and 14.6 and for 20 values of relative luminosity l = L/L_Edd in
the range 0.001 - 0.98. The emergent spectra of all models are fitted by
diluted blackbody spectra in the observed RXTE/PCA band 3 - 20 keV and the
corresponding values of color correction factors f_c are presented. We also
show how to use these dependencies to estimate the neutron star's basic
parameters.Comment: 2 pages, 1 figure, conference "Astrophysics of Neutron Stars - 2010"
in honor of M. Ali Alpar, Izmir, Turke
Modeling the EUV spectra of optically thick boundary layers of dwarf novae in outburst
Here we compute detailed model spectra of recently published optically thick
one-dimensional radial baundary layer (BL) models in cataclysmic variables and
compare them with observed soft X-ray/extreme ultraviolet (EUV) spectra of
dwarf novae in outburst. Every considered BL model is divided into a number of
rings, and for each ring, a structure model along the vertical direction is
computed using the stellar-atmosphere method. The ring spectra are then
combined into a BL spectrum taking Doppler broadening and limb darkening into
account. Two sets of model BL spectra are computed, the first of them consists
of BL models with fixed white dwarf (WD) mass (1 M_sun) and various relative WD
angular velocities (0.2, 0.4, 0.6 and 0.8 break-up velocities), while the other
deals with a fixed relative angular velocity (0.8 break-up velocity) and
various WD masses (0.8, 1, and 1.2 M_sun). The model spectra show broad
absorption features because of blending of numerous absorption lines, and
emission-like features at spectral regions with only a few strong absorption
lines. The model spectra are very similar to observed soft X-ray/EUV spectra of
SS Cyg and U Gem in outburst. The observed SS Cyg spectrum could be fitted by
BL model spectra with WD masses 0.8 - 1 M_sun and relative angular velocities
0.6 - 0.8 break up velocities. These BL models also reproduce the observed
ratio of BL luminosity and disk luminosity. The difference between the observed
and the BL model spectra is similar to a hot optically thin plasma spectrum and
could be associated with the spectrum of outflowing plasma with a mass loss
rate compatible with the BL mass accretion rate. The suggested method of
computing BL spectra seems very promising and can be applied to other BL models
for comparison with EUV spectra of dwarf novae in outburst.Comment: Accepted for publication in A&A, 14 pages, 13 figures, 4 table
On the compactness of the isolated neutron star RX J0720.4-3125
The data from all observations of RX J0720.4-3125 conducted by XMM-Newton
EPIC-pn with the same instrumental setup in 2000-2012 were reprocessed to form
a homogenous data set of solar barycenter corrected photon arrival times
registered from RX J0720.4-3125. A Bayesian method for the search, detection,
and estimation of the parameters of an unknown-shaped periodic signal was
employed as developed by Gregory & Loredo (1992).
A number of complex models (single and double peaked) of light curves from
pulsating neutron stars were statistically analyzed. The distribution of phases
for the registered photons was calculated by folding the arrival times with the
derived spin-period and the resulting distribution of phases approximated with
a mixed von Mises distribution, and its parameters were estimated by using the
Expected Maximization method. Spin phase-resolved spectra were extracted, and a
number of highly magnetized atmosphere models of an INS were used to fit
simultaneously, the results were verified via an MCMC approach. The
phase-folded light curves in different energy bands with high S/N ratio show a
high complexity and variations depending on time and energy.
They can be parameterized with a mixed von Mises distribution, i.e. with
double-peaked light curve profile showing a dependence of the estimated
parameters (mean directions, concentrations, and proportion) upon the energy
band, indicating that radiation emerges from at least two emitting areas.
The genuine spin-period of the isolated neutron star RX J0720-3125 derived as
more likely is twice of that reported in the literature (16.78s instead of
8.39s).
The gravitational redshift of RX J0720.4-3125 was determined to
and the compactness was estimated to
.Comment: Comments: 19 pages, 15 figures and 5 tables, Astronomy and
Astrophysics accepted. arXiv admin note: text overlap with arXiv:1108.389
Absorption Features in Spectra of Magnetized Neutron Stars
The X-ray spectra of some magnetized isolated neutron stars (NSs) show
absorption features with equivalent widths (EWs) of 50 - 200 eV, whose nature
is not yet well known. To explain the prominent absorption features in the soft
X-ray spectra of the highly magnetized (B ~ 10^{14} G) X-ray dim isolated NSs
(XDINSs), we theoretically investigate different NS local surface models,
including naked condensed iron surfaces and partially ionized hydrogen model
atmospheres, with semi-infinite and thin atmospheres above the condensed
surface. We also developed a code for computing light curves and integral
emergent spectra of magnetized neutron stars with various temperature and
magnetic field distributions over the NS surface. We compare the general
properties of the computed and observed light curves and integral spectra for
XDINS RBS\,1223 and conclude that the observations can be explained by a thin
hydrogen atmosphere above the condensed iron surface, while the presence of a
strong toroidal magnetic field component on the XDINS surface is unlikely. We
suggest that the harmonically spaced absorption features in the soft X-ray
spectrum of the central compact object (CCO) 1E 1207.4-5209 (hereafter 1E 1207)
correspond to peaks in the energy dependence of the free-free opacity in a
quantizing magnetic field, known as quantum oscillations. To explore observable
properties of these quantum oscillations, we calculate models of hydrogen NS
atmospheres with B ~ 10^{10} - 10^{11} G (i.e., electron cyclotron energy
E_{c,e} ~ 0.1 - 1 keV) and T_eff = 1 - 3 MK. Such conditions are thought to be
typical for 1E 1207. We show that observable features at the electron cyclotron
harmonics with EWs \approx 100 - 200 eV can arise due to these quantum
oscillations.Comment: 4 pages, 3 figures, conference "Astrophysics of Neutron Stars - 2010"
in honor of M. Ali Alpar, Izmir, Turke
Vertical Structure of the Outer Accretion Disk in Persistent Low-Mass X-Ray Binaries
We have investigated the influence of X-ray irradiation on the vertical
structure of the outer accretion disk in low-mass X-ray binaries by performing
a self-consistent calculation of the vertical structure and X-ray radiation
transfer in the disk. Penetrating deep into the disk, the field of scattered
X-ray photons with energy \,keV exerts a significant influence on
the vertical structure of the accretion disk at a distance
\,cm from the neutron star. At a distance \,cm,
where the total surface density in the disk reaches
\,g\,cm, X-ray heating affects all layers of an
optically thick disk. The X-ray heating effect is enhanced significantly in the
presence of an extended atmospheric layer with a temperature
\,K above the accretion disk. We have derived
simple analytic formulas for the disk heating by scattered X-ray photons using
an approximate solution of the transfer equation by the Sobolev method. This
approximation has a \,% accuracy in the range of X-ray photon
energies \,keV.Comment: 19 pages, 8 figures, published in Astronomy Letter
Influence of Compton scattering on the broad-band X-ray spectra of intermediate polars
Context. The majority of cataclysmic variables observed in the hard X-ray energy band are intermediate polars where the magnetic field is strong enough to channel the accreting matter to the magnetic poles of the white dwarf. A shock above the stellar surface heats the gas to fairly high temperatures (10-100 keV). The post-shock region cools mostly via optically thin bremsstrahlung.Aims. We investigate the influence of Compton scattering on the structure and the emergent spectrum of the post-shock region. We also study the effect it has on the mass of the white dwarfs obtained from fitting the observed X-ray spectrum of intermediate polars. Methods. We construct the model of the post-shock region taking Compton scattering into account. The radiation transfer equation is solved in the plane-parallel approximation. The feedback of Compton scattering on the structure of the post-shock region is also accounted for. A set of the post-shock region model spectra for various white dwarf masses is calculated.Results. We find that Compton scattering does not change the emergent spectra significantly for low accretion rates or low white dwarf masses. However, it becomes important at high accretion rates and high white dwarf masses. The time-averaged, broad-band X-ray spectrum of intermediate polar V709 Cas obtained by the RXTE and and INTEGRAL observatories is fitted using the set of computed spectral models. We obtained the white dwarf mass of 0.90 ± 0.02 M⊙ and 0.88 ± 0.02 M⊙ using models with Compton scattering taken into account and without it, respectively. © 2008 ESO
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