597 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
Relation between the X-ray and Optical Luminosities in Binary Systems with Accreting Nonmagnetic White Dwarfs
We investigate the relation between the optical (g-band) and X-ray (0.5-10
keV) luminosities of accreting nonmagnetic white dwarfs. According to the
present-day counts of the populations of star systems in our Galaxy, these
systems have the highest space density among the close binary systems with
white dwarfs. We show that the dependence of the optical luminosity of
accreting white dwarfs on their X-ray luminosity forms a fairly narrow
one-parameter curve. The typical half-width of this curve does not exceed
0.2-0.3 dex in optical and X-ray luminosities, which is essentially consistent
with the amplitude of the aperiodic flux variability for these objects. At
X-ray luminosities Lx~1e32 erg/sec or lower, the optical g-band luminosity of
the accretion flow is shown to be related to its X-ray luminosity by a factor
~2-3. At even lower X-ray luminosities (Lx~1e30 erg/sec), the contribution from
the photosphere of the white dwarf begins to dominate in the optical spectrum
of the binary system and its optical brightness does not drop below Mg~13-14.
Using the latter fact, we show that in current and planned X-ray sky surveys,
the family of accreting nonmagnetic white dwarfs can be completely identified
to the distance determined by the sensitivity of an optical sky survey in this
region. For the Sloan Digital Sky Survey (SDSS) with a limiting sensitivity
m_g~22.5, this distance is ~400-600 pcComment: 7 pages, 3 figures, published in Astronomy Letter
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
Testing the cooling flow model in the intermediate polar EX Hydrae
We use the best available X-ray data from the intermediate polar EX Hydrae to
study the cooling-flow model often applied to interpret the X-ray spectra of
these accreting magnetic white dwarf binaries. First, we resolve a
long-standing discrepancy between the X-ray and optical determinations of the
mass of the white dwarf in EX Hya by applying new models of the inner disk
truncation radius. Our fits to the X-ray spectrum now agree with the white
dwarf mass of 0.79 Msun determined using dynamical methods through
spectroscopic observations of the secondary. We use a simple isobaric cooling
flow model to derive the emission line fluxes, emission measure distribution,
and H-like to He-like line ratios for comparison with the 496 ks Chandra High
Energy Transmission Grating observation of EX Hydrae. We find that the H/He
ratios are not well reproduced by this simple isobaric cooling flow model and
show that while H-like line fluxes can be accurately predicted, fluxes of
lower-Z He-like lines are significantly underestimated. This discrepancy
suggests that some extra heating mechanism plays an important role at the base
of the accretion column, where cooler ions form. We thus explored more complex
cooling models including the change of gravitational potential with height in
the accretion column and a magnetic dipole geometry. None of these
modifications to the standard cooling flow model are able to reproduce the
observed line ratios. While a cooling flow model with subsolar (0.1 )
abundances is able to reproduce the line ratios by reducing the cooling rate at
temperatures lower than K, the predicted line-to-continuum
ratios are much lower than observed. We discuss and discard mechanisms such as
photoionization, departures from constant pressure, resonant scattering,
different electron-ion temperatures, and Compton cooling. [Abridged]Comment: Accepted in Astronomy & Astrophysics, modified version after referee
comments and proof correction
Theoretical And Experimental Studies Of Collision-Induced Electronic Energy Transfer From v=0-3 Of The E(0g+) Ion-Pair State Of Br2: Collisions With He And Ar
Collisions of Br(2), prepared in the E(0(g)(+)) ion-pair (IP) electronic state, with He or Ar result in electronic energy transfer to the D, D(\u27), and beta IP states. These events have been examined in experimental and theoretical investigations. Experimentally, analysis of the wavelength resolved emission spectra reveals the distribution of population in the vibrational levels of the final electronic states and the relative efficiencies of He and Ar collisions in promoting a specific electronic energy transfer channel. Theoretically, semiempirical rare gas-Br(2) potential energy surfaces and diabatic couplings are used in quantum scattering calculations of the state-to-state rate constants for electronic energy transfer and distributions of population in the final electronic state vibrational levels. Agreement between theory and experiment is excellent. Comparison of the results with those obtained for similar processes in the IP excited I(2) molecule points to the general importance of Franck-Condon effects in determining vibrational populations, although this effect is more important for He collisions than for Ar collisions
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