Search

Classical novae are known to demonstrate a supersoft X-ray source (SSS) state following outbursts, which is associated with residual thermonuclear burning on the white dwarf (WD) surface. During its all-sky survey (eRASS1), the eROSITA telescope onboard the Spectrum-Roentgen-Gamma observatory discovered a bright new SSS, whose position is consistent with the known classical nova AT 2018bej in the Large Magellanic Cloud. There were two eROSITA spectra obtained during eRASS1 and eRASS2 monitoring epochs and one XMM-Newton grating spectrum close to the eRASS1 epoch. We aim to describe the eROSITA and XMM-Newton spectra of AT 2018bej with our local thermodynamic equilibrium (LTE) atmosphere models. We focused on the evolution of the hot WD properties between the eRASS1 and eRASS2 epochs, especially on the change of the carbon abundance. A grid of LTE model atmosphere spectra were calculated for different values of the effective temperature (from $T_{\rm eff}= 525$ to $700\,\rm kK$), surface gravity (six values) and chemical composition with five different values of carbon and nitrogen abundances. Both eRASS1 and XMM $0.3-0.6$ keV spectral analyses yield a temperature of the WD of $T_{\rm eff}{\sim}\,600\, \rm kK$ and a WD radius of $8000-8700\,\rm km$. Simultaneous fitting of the eROSITA spectra for two epochs (eRASS1 and eRASS2) with a common WD mass parameter demonstrates a decrease in $T_{\rm eff}$ accompanied by an increase in the WD radius and a decrease in the carbon abundance. However, these changes are marginal and coincide within errors. The derived WD mass is estimated to be $1.05-1.15\, M_\odot$. We traced a minor evolution of the source on a half-year timescale accompanied by a decrease in carbon abundance and concluded that LTE model atmospheres can be used to analyse the available X-ray spectra of classical novae during their SSS stage., Comment: 12 pages, 16 figures, accepted for publication to A&A

We present a comparative analysis of photometric observations of the cataclysmic variable TT Ari in its bright state, obtained by the TESS orbital observatory in 2021 and 2023 and by ground-based amateur telescopes in 2022. The light curves from 2021 and 2022 are dominated by modulations with a period slightly shorter than the orbital one (negative superhumps), 0.13292 and 0.13273 d respectively. In 2023, much stronger modulations appeared on a much longer time scale of a few days with an amplitude of up to 0.5 mag, compared to 0.2 mag in 2021. The negative superhump variability with the period of 0.1338 d was also found in the 2023 observations, but the significance of these negative superhumps is much lower than in the previous seasons. Less significant additional modulations with a period exceeding the orbital one (positive superhumps) were detected in 2021 and 2022. Their periods were 0.15106 and 0.1523 d, respectively. We also found a previously unnoticed periodic signal corresponding to the orbital period of 0.13755 d in the TESS observations in 2021. Theoretical models of tidal precession of an elliptical disk predict a decrease in the precession period (and an increase in the positive superhumps period) with increasing disk radius, which is consistent with the observed photometric behavior of the system. It enables us to estimate the mass ratio of the components in TT Ari to be $q$ in the range 0.24-0.29. The tilted disk precession model predicts a period of nodal precession whose value is in general agreement with observations., Comment: 13 pages, 15 figures, significantly revised after the referee report, accepted for publication in A&A

Supersoft X-ray sources (SSSs) are accreting white dwarfs (WDs) with stable or recurrent thermonuclear burning on their surfaces. High-resolution X-ray spectra of such objects are rather complex, often consist of several components, and are difficult to interpret accurately. The main emission source is the hot surface of the WD and the emergent radiation can potentially be described by hot WD model atmospheres. We present a new set of such model atmosphere spectra computed in the effective temperature range from $100\rm\,kK$ to $1000\rm\,kK$, for eight values of surface gravity and three different chemical compositions. These compositions correspond to the solar one as well as to the Large and Small Magellanic Clouds, with decreased heavy element abundances, at one-half and one-tenth of the solar value. The presented model grid covers a broad range of physical parameters and, thus, it can be applied to a wide range of objects. It is also publicly available in XSPEC~format. As an illustration, we applied it here for the interpretation of \textit{Chandra} and XMM grating spectra of two classical SSSs, namely, CAL 83 (RX J0543.5$-$6823) and RX J0513.9$-$6951. The obtained effective temperatures and surface gravities of $T_{\rm eff} \approx 560$ kK, $\log g \approx 8.6-8.7$, and $T_{\rm eff} \approx 630\,{\rm kK}, \log g \approx 8.5-8.6$, respectively, are in a good agreement with previous estimations for both sources. The derived WD~mass estimations are within $1.1-1.4\,M_\odot$ for CAL 83 and $1.15-1.4\,M_\odot$ for RX J0513.9$-$6951. The mass of the WD in CAL $83$ is consistent with the mass predicted from the respective model of recurrent thermonuclear burning., Comment: 12 pages, 15 figures, accepted for publication to A&A

Within the magnetospheric radius, the geometry of accretion flow in X-ray pulsars is shaped by a strong magnetic field of a neutron star. Starting at the magnetospheric radius, accretion flow follows field lines and reaches the stellar surface in small regions located close to the magnetic poles of a star. At low mass accretion rates, the dynamic of the flow is determined by gravitational attraction and rotation of the magnetosphere due to the centrifugal force. At the luminosity range close to the Eddington limit and above it, the flow is additionally affected by the radiative force. We construct a model simulating accretion flow dynamics over the magnetosphere, assuming that the flow strictly follows field lines and is affected by gravity, radiative and centrifugal forces only. The magnetic field of a NS is taken to be dominated by the dipole component of arbitrary inclination with respect to the accretion disc plane. We show that accretion flow becomes unstable at high mass accretion rates and tends to fluctuate quasi-periodically with a typical period comparable to the free-fall time from the inner disc radius. The inclination of a magnetic dipole with respect to the disc plane and strong anisotropy of X-ray radiation stabilise the mass accretion rate at the poles of a star, but the surface density of material covering the magnetosphere fluctuates even in this case., Comment: 14 pages, 13 figures, submitted to MNRAS

X Persei is a persistent low-luminosity X-ray pulsar of period of $\sim$835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above $10^{13}$ G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3-8 keV band varied from several to $\sim$20 per cent over the pulse with a positive correlation with the pulsed X-ray flux. The polarization angle shows significant variation and makes two complete revolutions during the pulse period resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008$-$57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered with its value increasing from 0% at $\sim$2 keV to 30% at 8 keV., Comment: 11 pages, 7 figures, submitted to MNRAS

We analyse new optical spectroscopic, direct-image and X-ray observations of the recently discovered a high proper motion cataclysmic variable V1838 Aql. The data were obtained during its 2013 superoutburst and its subsequent quiescent state. An extended emission around the source was observed up to 30 days after the peak of the superoutburst, interpreted it as a bow--shock formed by a quasi-continuous outflow from the source in quiescence. The head of the bow--shock is coincident with the high--proper motion vector of the source ($v_{\perp}=123\pm5$ km s$^{-1}$) at a distance of $d=202\pm7$ pc. The object was detected as a weak X-ray source ($0.015\pm0.002$ counts s$^{-1}$) in the plateau of the superoutburst, and its flux lowered by two times in quiescence (0.007$\pm$0.002 counts s$^{-1}$). Spectroscopic observations in quiescence we confirmed the orbital period value $P_{\rm{orb}}=0.0545\pm 0.0026$ days, consistent with early-superhump estimates, and the following orbital parameters: $\gamma= -21\pm3$ km s$^{-1}$ and $K_1 = 53\pm3$ km s$^{-1}$. The white dwarf is revealed as the system approaches quiescence, which enables us to infer the effective temperature of the primary $T_{eff}=11,600\pm400$K. The donor temperature is estimated $\lesssim 2200$K and suggestive of a system approaching the period minimum. Doppler maps in quiescence show the presence of the hot spot in HeI line at the expected accretion disc-stream shock position and an unusual structure of the accretion disc in H$\alpha$., Comment: 14 pages, 8 figures, accepted manuscript to MNRAS

The X-ray spectra of the neutron stars located in the centers of supernova remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models. These fits yield plausible neutron star sizes for the known or estimated distances to these supernova remnants. The evidence in favor of the presence of a pure carbon envelope at the neutron star surface is rather indirect and is based on the assumption that the emission is generated uniformly by the entire stellar surface. Although this assumption is supported by the absence of pulsations, the observational upper limit on the pulsed fraction is not very stringent. In an attempt to quantify this evidence, we investigate the possibility that the observed spectrum of the neutron star in HESS J1731-347 is a combination of the spectra produced in a hydrogen atmosphere of the hotspots and of the cooler remaining part of the neutron star surface. The lack of pulsations in this case has to be explained either by a sufficiently small angle between the neutron star spin axis and the line of sight, or by a sufficiently small angular distance between the hotspots and the neutron star rotation poles. As the observed flux from a non-uniformly emitting neutron star depends on the angular distribution of the radiation emerging from the atmosphere, we have computed two new grids of pure carbon and pure hydrogen atmosphere model spectra accounting for Compton scattering. Using new hydrogen models, we have evaluated the probability of a geometry that leads to a pulsed fraction below the observed upper limit to be about 8.2 %. Such a geometry thus seems to be rather improbable but cannot be excluded at this stage., Comment: 8 pages, 14 figures. Accepted for publication in A&A

We report extensive 3-yr multiwavelength observations of the WZ Sge-type dwarf nova SSS J122221.7-311525 during its unusual double superoutburst, the following decline and in quiescence. The second segment of the superoutburst had a long duration of 33 d and a very gentle decline with a rate of 0.02 mag/d, and it displayed an extended post-outburst decline lasting at least 500 d. Simultaneously with the start of the rapid fading from the superoutburst plateau, the system showed the appearance of a strong near-infrared excess resulting in very red colours, which reached extreme values (B-I~1.4) about 20 d later. The colours then became bluer again, but it took at least 250 d to acquire a stable level. Superhumps were clearly visible in the light curve from our very first time-resolved observations until at least 420 d after the rapid fading from the superoutburst. The spectroscopic and photometric data revealed an orbital period of 109.80 min and a fractional superhump period excess <0.8 per cent, indicating a very low mass ratio q<0.045. With such a small mass ratio the donor mass should be below the hydrogen-burning minimum mass limit. The observed infrared flux in quiescence is indeed much lower than is expected from a cataclysmic variable with a near-main-sequence donor star. This strongly suggests a brown-dwarf-like nature for the donor and that SSS J122221.7-311525 has already evolved away from the period minimum towards longer periods, with the donor now extremely dim., Comment: 24 pages, 19 figures, Matches to MNRAS published version, typos corrected

Observed hard X-ray spectra of intermediate polars are determined mainly by the accretion flow velocity at the white dwarf surface, which is normally close to the free-fall velocity. This allows to estimate the white dwarf masses as the white dwarf mass-radius relation M-R and the expected free-fall velocities at the surface are well known. This method is widely used, however, derived white dwarf masses M can be systematically underestimated because the accretion flow is stopped at and re-accelerates from the magnetospheric boundary R_m, and therefore, its velocity at the surface will be lower than free-fall.To avoid this problem we computed a two-parameter set of model hard X-ray spectra, which allows to constrain a degenerate M - R_m dependence. On the other hand, previous works showed that power spectra of accreting X-ray pulsars and intermediate polars exhibit breaks at the frequencies corresponding to the Keplerian frequencies at the magnetospheric boundary. Therefore, the break frequency \nu_b in an intermediate polar power spectrum gives another relation in the M - R_m plane. The intersection of the two dependences allows, therefore, to determine simultaneously the white dwarf mass and the magnetospheric radius. To verify the method we analyzed the archival Suzaku observation of EX Hya obtaining M /M_sun= 0.73 \pm 0.06 and R_ m / R = 2.6 \pm 0.4 consistent with the values determined by other authors. Subsequently, we applied the same method to a recent NuSTAR observation of another intermediate polar GK~Per performed during an outburst and found M/M_sun = 0.86 \pm 0.02 and R_ m / R = 2.8 \pm 0.2. The long duration observations of GK Per in quiescence performed by Swift/BAT and INTEGRAL observatories indicate increase of magnetosphere radius R_m at lower accretion rates., Comment: 12 pages, 17 figures, submitted to Astronomy and Astrophysics

The central compact object in the supernova remnant HESS J1731-347 appears to be the hottest observed isolated cooling neutron star. The cooling theory of neutron stars enables one to explain observations of this star by assuming the presence of strong proton superfluidity in the stellar core and the existence of the surface heat blanketing envelope which almost fully consists of carbon. The cooling model of this star is elaborated to take proper account of the neutrino emission due to neutron-neutron collisions which is not suppressed by proton superfluidity. Using the results of spectral fits of observed thermal spectra for the distance of 3.2 kpc and the cooling theory for the neutron star of age 27 kyr, new constraints on the stellar mass and radius are obtained which are more stringent than those derived from the spectral fits alone., Comment: 9 pages, 5 figures, MNRAS, accepted

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 M$_{\odot}$sun 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 $\odot$) abundances is able to reproduce the line ratios by reducing the cooling rate at temperatures lower than $\sim 10^{7.3}$ 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 corrections

We present the results of the pulse phase- and luminosity-resolved spectroscopy of the transient X-ray pulsar V0332+53, performed for the first time in a wide luminosity range (1-40)x10^{37} erg/s during a giant outburst observed by the RXTE observatory in Dec 2004 - Feb 2005. We characterize the spectra quantitatively and built the detailed "three-dimensional" picture of spectral variations with pulse phase and throughout the outburst. We show that all spectral parameters are strongly variable with the pulse phase, and the pattern of this variability significantly changes with luminosity directly reflecting the associated changes in the structure of emission regions and their beam patterns. Obtained results are qualitatively discussed in terms of the recently developed reflection model for the formation of cyclotron lines in the spectra of X-ray pulsars., Comment: 12 pages, 10 figures, 3 tables, accepted for publication in MNRAS

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 tables

Context: The CCO candidate in the center of the supernova remnant shell HESS J1731-347 / G353.6-0.7 shows no pulsations and exhibits a blackbody-like X-ray spectrum. If the absence of pulsations is interpreted as evidence for the emitting surface area being the entire neutron star surface, the assumption of the measured flux being due to a blackbody emission translates into a source distance that is inconsistent with current estimates of the remnant's distance. Aims: With the best available observational data, we extended the pulse period search down to a sub-millisecond time scale and used a carbon atmosphere model to describe the X-ray spectrum of the CCO and to estimate geometrical parameters of the neutron star. Methods: To search for pulsations we used data of an observation of the source with XMM-Newton performed in timing mode. For the spectral analysis, we used earlier XMM-Newton observations performed in imaging mode, which permits a more accurate treatment of the background. The carbon atmosphere models used to fit the CCO spectrum are computed assuming hydrostatic and radiative equilibria and take into account pressure ionization and the presence of spectral lines. Results: Our timing analysis did not reveal any pulsations with a pulsed fraction above ~8% down to 0.2 ms. This finding further supports the hypothesis that the emitting surface area is the entire neutron star surface. The carbon atmosphere model provides a good fit to the CCO spectrum and leads to a normalization consistent with the available distance estimates of the remnant. The derived constraints on the mass and radius of the source are consistent with reasonable values of the neutron star mass and radius. After the CCO in Cas A, the CCO in HESS J1731-347 / G353.6-0.7 is the second object of this class for which a carbon atmosphere model provides a consistent description of X-ray emission., Comment: 6 pages, 5 figures, accepted for publication in Astronomy&Astrophysics

A model with a condensed iron surface and partially ionized hydrogen-thin atmosphere allows us to fit simultaneously the observed general spectral shape and the broad absorption feature (observed at 0.3 keV) in different spin phases of the isolated neutron star RBS 1223. We constrain some physical properties of the X-ray emitting areas, i.e. the temperatures (Tpole1 ~ 105eV, Tpole2 ~99eV), magnetic field strengths Bpole1 ~ Bpole2 ~ 8.6x10^13G) at the poles, and their distribution parameters (a1 ~ 0.61, a2 ~ 0.29, indicating an absence of strong toroidal magnetic field component). In addition, we are able to place some constraints on the geometry of the emerging X-ray emission and the gravitational redshift (z ~0.16+0.03-0.01) of the isolated neutron star RBS 1223., Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and Opportunities after 80 years", J. van Leeuwen (ed.); 3 pages, 3 figures

Results of photometric and spectroscopic investigations of the recently discovered eclipsing cataclysmic variable star HBHA 4705-03 are presented. The emission spectra of the system show broad hydrogen and helium emission lines. The bright spots with an approximately zero velocity components are found in the Doppler maps for the hydrogen and ionized helium lines. The disc structure is more prominent in the maps for the neutral helium lines. The masses of the components (M_WD = 0.54 \pm 0.10 M_sun and M_RD = 0.45 \pm 0.05 M_sun), and the orbit inclination (i = 71.8 \pm 0.^7 deg) were estimated using the radial velocity light curve and the eclipse width. The modeling of the light curve allows us to evaluate the bright spot parameters and the mass accretion rate (\dot M \approx 2 10^{17} g s^{-1})., Comment: 4 pages, 4 figures, to be published in the proceedings of the 18th European White Dwarf Workshop

Theoretical spectra of X-ray bursting neutron star (NS) model atmospheres are widely used to determine the basic NS parameters such as their masses and radii. We construct accurate NS atmosphere models using for the first time an exact treatment of Compton scattering via the integral relativistic kinetic equation. We also compare the results with the previous calculations based on the Kompaneets operator. We solve the radiation transfer equation together with the hydrostatic equilibrium equation accounting exactly for the radiation pressure by electron scattering. We thus construct a new set of plane-parallel atmosphere models in LTE for hot NSs. The models were computed for six chemical compositions (pure H, pure He, solar H/He mix with various heavy elements abundances Z = 1, 0.3, 0.1, and 0.01 Z_sun, and three log g = 14.0, 14.3, and 14.6. For each chemical composition and log g, we compute more than 26 model atmospheres with various luminosities relative to the Eddington luminosity L_Edd computed for the Thomson cross-section. The maximum relative luminosities L/L_Edd reach values of up to 1.1 for high gravity models. The emergent spectra of all models are redshifted and fitted by diluted blackbody spectra in the 3--20 keV energy range appropriate for the RXTE/PCA. We also compute the color correction factors f_c. The radiative acceleration g_rad in our luminous, hot-atmosphere models is significantly smaller than in corresponding models based on the Kompaneets operator, because of the Klein-Nishina reduction of the electron scattering cross-section, and therefore formally "super-Eddington" model atmospheres do exist. The differences between the new and old model atmospheres are small for L / L_Edd < 0.8. For the same g_rad / g, the new f_c are slightly larger (by approximately 1%) than the old values., Comment: 16 pages, 16 figures, A&A, in press. Tables containing spectra and color corrections are available by downloading the source file via Other formats field

We present the results of our photometric and spectroscopic studies of the new eclipsing cataclysmic variable star 1RXS J180834.7+101041. Its spectrum exhibits double-peaked hydrogen and helium emission lines. The Doppler maps constructed from hydrogen lines show a nonuniform distribution of emission in the disk similar to that observed in IP Peg. This suggests that the object can be a cataclysmic variable with tidal density waves in the disk. We have determined the component masses (M_WD =0.8 \pm 0.22 M_sun and M_RD =0.14 \pm 0.02 M_sun) and the binary inclination (i =78 \pm 1.5 deg) based on well-known relations between parameters for cataclysmic variable stars. We have modeled the binary light curves and showed that the model of a disk with two spots is capable of explaining the main observed features of the light curves., Comment: 22 pages, 9 figures, 2 tables, published in Astronomy Letters, 2011, 37, 845-857

The data from all observations of RBS 1223 (1RXS J130848.6+212708) conducted by XMM-Newton EPIC pn with the same instrumental setup in 2003-2007 were combined to form spin-phase resolved spectra. A number of complex models of neutron stars with strongly magnetized ($B_{pole} 10^{13}-10^{14}$ G) surface, with temperature and magnetic field distributions around magnetic poles, and partially ionized hydrogen thin atmosphere above it have been implemented into the X-ray spectral fitting package XSPEC for simultaneous fitting of phase-resolved spectra. A Markov-Chain-Monte-Carlo (MCMC) approach is also applied to verify results of fitting and estimating in multi parameter models. The spectra in different rotational phase intervals and light curves in different energy bands with high S/N ratio show a high complexity. The spectra can be parameterized with a Gaussian absorption line superimposed on a blackbody spectrum, while the light curves with double-humped shape show strong dependence of pulsed fraction upon the energy band (13%-42%), which indicates that radiation emerges from at least two emitting areas. A model with condensed iron surface and partially ionized hydrogen thin atmosphere above it allows us to fit simultaneously the observed general spectral shape and the broad absorption feature observed at 0.3 keV in different spin phases of RBS 1223. It allowed to constrain some physical properties of X-ray emitting areas, i.e. the temperatures ($T_{p1} ~ 105$ eV, $T_{p2} ~ 99$ eV), magnetic field strengths ($B_{p1}= B_{p2} ~ 8.6x10^{13}$G) at the poles, and their distributions parameters ($a_{1} 0.61, a_{2} 0.29$, indicating an absence of strong toroidal magnetic field component). In addition, it puts some constraints on the geometry of the emerging X-ray emission and gravitational redshift ($z=0.16^{+0.03}_{-0.01}$) of RBS 1223., Comment: 7 pages, 7 figures, Astronomy and Astrophysics accepted

The spectral energy distribution (SED) of the TT Ari system, which is well known from published IUE and optical photometric observations, was modeled by a steady-state accretion \alpha-disc around a white dwarf. Parameters of the system were derived from time-resolved optical spectral observations in the bright state that we obtained in Sep. 1998. The radial velocity semi-amplitude of the white dwarf (33.8 +/- 2.5 km/s) and corresponding mass function (f(M) = 5.5 +/- 1.2 *10^{-4}~ M_sun) were derived from the motion of the emission components of Balmer lines. The mass ratio q (\approx 0.315) was evaluated from the fractional period excess of the superhump period over the orbital period \epsilon (\approx 0.085), and a secondary mass range (0.18 - 0.38 M_sun) was estimated from the orbital period. Therefore, the white dwarf mass range is 0.57 - 1.2 M_sun and the inclination angle of the system to the line of sight is 17 - 22.5 degrees. The adopted distance to the system is 335 +/- 50 pc. To fit the observed SED it is necessary to add a thermal spectrum with T \approx 11600 K and luminosity \approx 0.4 L_disk to the accretion disc spectrum. This combined spectrum successfully describes the observed Balmer lines absorption components. Formally the best fit of the HeI 4471 line gives minimum masses of the components (M_RD = 0.18 M_sun and M_WD = 0.57 M_sun), with the corresponding inclination angle i = 22.1 deg and mass-accretion rate \dot M = 2.6 * 10^{17} g/s., Comment: 4 pages, 3 figures, conference "European White Dwarf Workshop, 2010", Tuebingen, Germany

Results of photometric and spectroscopic investigations of the recently discovered disc cataclysmic variable star 1RXS J180834.7+101041 are presented. Emission spectra of the system show broad double peaked hydrogen and helium emission lines. Doppler maps for the hydrogen lines demonstrate strongly non-uniform emissivity distribution in the disc, similar to that found in IP Peg. It means that the system is a new cataclysmic variable with a spiral density wave in the disc. Masses of the components (M_WD = 0.8 +/- 0.22 M_sun and M_RD = 0.14 +/- 0.02 M_sun), and the orbit inclination (i = 78 +/- 1.5 deg) were estimated using the various well-known relations for cataclysmic variables., Comment: 4 pages, 3 figures, conference "European White Dwarf Workshop, 2010", Tuebingen, Germany

Extending the population synthesis method to isolated young cooling white dwarfs we are able to confront our model assumptions with observations made in ROSAT All-Sky Survey (Fleming et al., 1996). This allows us to check model parameters such as evolution of spectra and separation of heavy elements in DA WD envelopes. It seems like X-ray spectrum temperature of these objects is given by the formula T_{X-ray} = min(T_eff, T_max). We have obtained DA WD's birth rate and upper limit of the X-ray spectrum temperature: DA birth rate $= 0.61\times 10^{-12}$ in cubic parsec per year and T_max = 41000 K. These values are in good correspondence with values obtained by other authors (Liebert et al., 2004; Wolff et al., 1996). From this fact we also conclude that our population synthesis method is applicable to the population of close-by isolated cooling white dwarfs as well as to the population of the isolated cooling neutron stars., Comment: 4 pages, 3 figures, conference "European White Dwarf Workshop, 2010", Tubingen, Germany

We present an analysis of time-resolved, medium resolution optical spectroscopic observations of UX UMa in the blue (3920-5250 A) and red (6100-7200 A) wavelength ranges, that were obtained in April 1999 and March 2008 respectively. The observed characteristics of our spectra indicate that UX UMa has been in different states during those observations. The blue spectra are very complex. They are dominated by strong and broad single-peaked emission lines of hydrogen. The high-excitation lines of HeII 4686 and the Bowen blend are quite strong as well. All the lines consist of a mixture of absorption and emission components. Using Doppler tomography we have identified four distinct components of the system: the accretion disc, the secondary star, the bright spot from the gas stream/disc impact region, and the unique compact area of absorption in the accretion disc seen as a dark spot in the lower-left quadrant of the tomograms. In the red wavelength range, both the hydrogen (H_alpha) and neutral helium (HeI 6678 and HeI 7065) lines were observed in emission and both exhibited double-peaked profiles. Doppler tomography of these lines reveals spiral structure in the accretion disc, but in contrast to the blue wavelength range, there is no evidence for either the dark spot or the gas stream/disc impact region emission, while the emission from the secondary star is weak. During the observations in 1999, UX UMa showed many of the defining properties of the SW Sex stars. However, all these features almost completely disappeared in 2008. We have also estimated the radial velocity semi-amplitudes K_1 and K_2 and evaluated the system parameters of UX UMa. These estimates are inconsistent with previous values derived by means of analysis of WD eclipse features in the light curve in the different wavelength ranges., Comment: 16 pages, 9 figures, Accepted for publication by MNRAS

We present the results of an in-depth study of the long-period X-ray pulsar GX 301-2. Using archival data of INTEGRAL, RXTE ASM, and CGRO BATSE, we study the spectral and timing properties of the source. Comparison of our timing results with previously published work reveals a secular decay of the orbital period at a rate of \simeq -3.25 \times 10^{-5} d yr^{-1}, which is an order of magnitude faster than for other known systems. We argue that this is probably result either of the apsidal motion or of gravitational coupling of the matter lost by the optical companion with the neutron star, although current observations do not allow us to distinguish between those possibilities. We also propose a model to explain the observed long pulse period. We find that a very strong magnetic field B \sim 10^{14} G can explain the observed pulse period in the framework of existing models for torques affecting the neutron star. We show that the apparent contradiction with the magnetic field strength B_{CRSF} \sim 4 \times 10^{12} G derived from the observed cyclotron line position may be resolved if the line formation region resides in a tall accretion column of height \sim 2.5 - 3 R_{NS}. The color temperature measured from the spectrum suggests that such a column may indeed be present, and our estimates show that its height is sufficient to explain the observed cyclotron line position., Comment: 10 pages, 7 figures, accepted for publication in Astronomy and Astrophysics. Referee comments are implemented.

Observed X-ray spectra of some isolated magnetized neutron stars display absorption features, sometimes interpreted as ion cyclotron lines. Modeling the observed spectra is necessary to check this hypothesis and to evaluate neutron star parameters.We develop a computer code for modeling magnetized neutron star atmospheres in a wide range of magnetic fields (10^{12} - 10^{15} G) and effective temperatures (3 \times 10^5 - 10^7 K). Using this code, we study the possibilities to explain the soft X-ray spectra of isolated neutron stars by different atmosphere models. The atmosphere is assumed to consist either of fully ionized electron-ion plasmas or of partially ionized hydrogen. Vacuum resonance and partial mode conversion are taken into account. Any inclination of the magnetic field relative to the stellar surface is allowed. We use modern opacities of fully or partially ionized plasmas in strong magnetic fields and solve the coupled radiative transfer equations for the normal electromagnetic modes in the plasma. Spectra of outgoing radiation are calculated for various atmosphere models: fully ionized semi-infinite atmosphere, thin atmosphere, partially ionized hydrogen atmosphere, or novel "sandwich" atmosphere (thin atmosphere with a hydrogen layer above a helium layer. Possibilities of applications of these results are discussed. In particular, the outgoing spectrum using the "sandwich" model is constructed. Thin partially ionized hydrogen atmospheres with vacuum polarization are shown to be able to improve the fit to the observed spectrum of the nearby isolated neutron star RBS 1223 (RX J1308.8+2127)., Comment: Accepted for publications in Astronomy and Astrophysics, 9 pages, 12 figures