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Supersoft X-ray sources (SSS) are thought to be accreting white dwarfs (WDs) in close binary systems, with thermonuclear burning on their surfaces. The SSS RX J0513.9-6951 in the Large Magellanic Cloud (LMC) exhibits cyclic variations between optical low and high states, which are anti-correlated with its X-ray flux. This behaviour is believed to result from the periodic expansion and contraction of the WD due to variations in the accretion rate in the system. We analyse eight high-resolution XMM and six grating Chandra spectra of RX J0513.9-6951 with our grid of model atmosphere spectra of hot WDs computed under the assumption of local thermodynamic equilibrium. Our aim is to test a contraction model of the source variability by tracking the evolution of the WD properties. The used grid of hot WD model atmospheres spans a wide range of effective temperatures ($T_{\rm eff}=100-1000\,\rm kK$ in steps of $25\,\rm kK$) and eight values of surface gravity $\log g$. The LMC chemical composition was assumed. The obtained fitting parameters ($T_{\rm eff}$, $\log g$, and bolometric luminosity $L$) evolve on the $T_{\rm eff}- \log g$ and $T_{\rm eff}- L$ planes. This evolution follows the model tracks of WDs with masses of $1.05-1.15\,M_{\odot}$ and thermonuclear burning on the surface. The analysis has showed that the optical brightness of the system is lower when the WD is larger, more luminous, and more effectively illuminates the accretion disc. These results contradict the contraction model, which predicts the opposite behaviour of the source. We use a model, that assumes that the far UV/soft X-ray flux is reprocessed into the optical band due to multiple scattering in the cloud system above the accretion disc. More significant illumination can lead to rarefying of the cloud slab, thereby reducing the reprocessing efficiency and making the source fainter in the optical band., Comment: 10 pages, 8 figures, 4 tables, accepted for publication in A&A

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 report on the discovery of the new accreting millisecond X-ray pulsar SRGAJ144459.2-604207 using the SRG/ART-XC data. The source was observed twice in February 2024 during the declining phase of the outburst. Timing analysis revealed a coherent signal near 447.8~Hz modulated by the Doppler effect due to the orbital motion. The derived parameters for the binary system are consistent with the circular orbit with a period of $\sim5.2$~h. The pulse profiles of the persistent emission, showing a sine-like part during half a period with a plateau in between, can well be modelled by emission from two circular spots partially eclipsed by the accretion disk. Additionally, during our 133~ks exposure observations, we detected 19 thermonuclear X-ray bursts. All bursts have similar shapes and energetics, and do not show any signs of photospheric radius expansion. The burst rate decreases linearly from one per $\sim$1.6~h at the beginning of observations to one per $\sim$2.2~h at the end and anticorrelates with the persistent flux. Spectral evolution during the bursts is consistent with the models of the neutron star atmospheres heated by accretion and imply a neutron star radius of 11--12~km and the distance to the source of 8--9~kpc. We also detected pulsations during the bursts and showed that the pulse profiles differ substantially from those observed in the persistent emission. However, we could not find a simple physical model explaining the pulse profiles detected during the bursts., Comment: 8 pages, 13 figures, 1 table

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

The eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930000 entries detected in the most sensitive 0.2-2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3-5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We show that the number counts of X-ray sources in eRASS1 are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) of F_{0.5-2 keV} > 5 \times 10^{-14}$ erg\,s$^{-1}$\,cm$^{-2}$, we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1-2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided., Comment: 39 pages, 23 figures. Accepted for publication in A&A. Accompanying eROSITA-DE Data Release 1

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

One of the models explaining the high luminosity of pulsing ultra-luminous X-ray sources (pULXs) was suggested by Mushtukov et al. (2015). They showed that the accretion columns on the surfaces of highly magnetized neutron stars can be very luminous due to opacity reduction in the high magnetic field. However, a strong magnetic field leads also to amplification of the electron-positron pairs creation. Therefore, increasing of the electron and positron number densities compensates the cross-section reduction, and the electron scattering opacity does not decrease with the magnetic field magnification. As a result, the maximum possible luminosity of the accretion column does not increase with the magnetic field. It ranges between 10$^{40} - 10^{41}$ erg s$^{-1}$ depending only slightly on the magnetic field strength., Comment: 3 pages, 2 figures, subm. to Proc. IAU Symp. 363, poster presentation

The 840 kyr old pulsar PSR J1957+5033, detected so far only in $\gamma$- and X-rays, is a nearby and rather cool neutron star with a temperature of 0.2--0.3 MK, a distance of $\la$1 kpc, and a small colour reddening excess $E(B-V) \approx 0.03$. These properties make it an ideal candidate to detect in the optical to get additional constraints on its parameters. We thus performed the first deep optical observations of the pulsar with the 10.4-meter Gran Telescopio Canarias in the $g'$ band and found its possible counterpart with $g'=27.63\pm 0.26$. The counterpart candidate position is consistent with the X-ray coordinates of the pulsar within the 0.5 arcsec accuracy. Assuming that this is the real counterpart, we analysed the pulsar X-ray spectrum together with the derived optical flux density. As a result, we found that the thermal emission from the bulk surface of the cooling neutron star can significantly contribute to its optical flux. Our multi-wavelength spectral analysis favours the pulsar nature of the detected optical source, since it provides physically adequate parameters of the pulsar emission. We show that the optical data can provide new constraints on the pulsar temperature and distance., Comment: 7 pages, 3 figures, published in MNRAS

We analyze new XMM-Newton and archival Chandra observations of the middle-aged $\gamma$-ray radio-quiet pulsar J1957+5033. We detect, for the first time, X-ray pulsations with the pulsar spin period of the point-like source coinciding by position with the pulsar. This confirms the pulsar nature of the source. In the 0.15--0.5 keV band, there is a single pulse per period and the pulsed fraction is $\approx18\pm6$ per cent. In this band, the pulsar spectrum is dominated by a thermal emission component that likely comes from the entire surface of the neutron star, while at higher energies ($\gtrsim0.7$ keV) it is described by a power law with the photon index $\Gamma \approx 1.6$. We construct new hydrogen atmosphere models for neutron stars with dipole magnetic fields and non-uniform surface temperature distributions with relatively low effective temperatures. We use them in the spectral analysis and derive the pulsar average effective temperature of $\approx(2-3)\times10^5$ K. This makes J1957+5033 the coldest among all known thermally emitting neutron stars with ages below 1 Myr. Using the interstellar extinction--distance relation, we constrain the distance to the pulsar in the range of 0.1--1 kpc. We compare the obtained X-ray thermal luminosity with those for other neutron stars and various neutron star cooling models and set some constraints on latter. We observe a faint trail-like feature, elongated $\sim 8$ arcmin from J1957+5033. Its spectrum can be described by a power law with a photon index $\Gamma=1.9\pm0.5$ suggesting that it is likely a pulsar wind nebula powered by J1957+5033., Comment: 14 pages, 13 figures, published in MNRAS

We investigate the absorption and emission features in observations of GX 301-2 detected with Insight-HXMT/LE in 2017-2019. At different orbital phases, we found prominent Fe Kalpha, Kbeta and Ni Kalpha lines, as well as Compton shoulders and Fe K-shell absorption edges. These features are due to the X-ray reprocessing caused by the interaction between the radiation from the source and surrounding accretion material. According to the ratio of iron lines Kalpha and Kbeta, we infer the accretion material is in a low ionisation state. We find an orbital-dependent local absorption column density, which has a large value and strong variability around the periastron. We explain its variability as a result of inhomogeneities of the accretion environment and/or instabilities of accretion processes. In addition, the variable local column density is correlated with the equivalent width of the iron Kalpha lines throughout the orbit, which suggests that the accretion material near the neutron star is spherically distributed., Comment: 10 pages, 5 figures, 2 tables, accepted for publication in MNRAS

The hard X-ray spectrum of magnetic cataclysmic variables can be modelled to provide a measurement of white dwarf mass. This method is complementary to radial velocity measurements, which depend on the (typically rather uncertain) binary inclination. Here we present results from a Legacy Survey of 19 magnetic cataclysmic variables with NuSTAR. We fit accretion column models to their 20-78 keV spectra and derive the white dwarf masses, finding a weighted average $\bar{M}_{\rm WD}=0.77\pm0.02$ $M_{\odot}$, with a standard deviation $\sigma=0.10$ $M_{\odot}$, when we include the masses derived from previous NuSTAR observations of seven additional magnetic cataclysmic variables. We find that the mass distribution of accreting magnetic white dwarfs is consistent with that of white dwarfs in non-magnetic cataclysmic variables. Both peak at a higher mass than the distributions of isolated white dwarfs and post-common-envelope binaries. We speculate as to why this might be the case, proposing that consequential angular momentum losses may play a role in accreting magnetic white dwarfs and/or that our knowledge of how the white dwarf mass changes over accretion-nova cycles may also be incomplete., Comment: 14 pages. 5 figures in main paper, 1 figure in appendix. Accepted for publication in MNRAS

We report on our analysis of the 2019 outburst of the X-ray accreting pulsar 4U 1901+03 observed with Insight-HXMT and NICER. Both spectra and pulse profiles evolve significantly in the decaying phase of the outburst. Dozens of flares are observed throughout the outburst. They are more frequent and brighter at the outburst peak. We find that the flares, which have a duration from tens to hundreds of seconds, are generally brighter than the persistent emission by a factor of $\sim$ 1.5. The pulse profile shape during the flares can be significantly different than that of the persistent emission. In particular, a phase shift is clearly observed in many cases. We interpret these findings as direct evidence of changes of the pulsed beam pattern, due to transitions between the sub- and super-critical accretion regimes on a short time scale. We also observe that at comparable luminosities the flares' pulse profiles are rather similar to those of the persistent emission. This indicates that the accretion on the polar cap of the neutron star is mainly determined by the luminosity, i.e., the mass accretion rate., Comment: 11 pages, 8 figures, accepted for publication in MNRAS

Type I X-ray bursts in GS 1826-24, and in several other systems, may induce cooling of the hot inner accretion flow that surrounds the bursting neutron star. Given that GS 1826-24 remained persistently in the hard state over the period 2003-2008 and presented regular bursting properties, we stacked the spectra of the X-ray bursts detected by INTEGRAL (JEM-X and ISGRI) and XMM-Newton (RGS) during that period to study the effect of the burst photons on the properties of the Comptonizing medium. The extended energy range provided by these instruments allows the simultaneous observation of the burst and persistent emission spectra. We detect an overall change in the shape of the persistent emission spectrum in response to the burst photon shower. For the first time, we observe simultaneously a drop in the hard X-ray emission, together with a soft X-ray excess with respect to the burst blackbody emission. The hard X-ray drop can be explained by burst-induced coronal cooling, while the bulk of the soft X-ray excess can be described by fitting the burst emission with an atmosphere model, instead of a simple blackbody model. Traditionally, the persistent emission was assumed to be invariant during X-ray bursts, and more recently to change only in normalization but not in spectral shape; the observed change in the persistent emission level during X-ray bursts may thus trigger the revision of existing neutron star mass-radius constraints, as the derived values rely on the assumption that the persistent emission does not change during X-ray bursts. The traditional burst fitting technique leads to up to a 10% overestimation of the bolometric burst flux in GS 1826-24, which significantly hampers the comparisons of the KEPLER and MESA model against this 'textbook burster'.

The aim of this work is to study the possible effects and biases on the radius constraints for rotation-powered millisecond pulsars when using Thomson approximation to describe electron scattering in the atmosphere models, instead of using exact formulation for Compton scattering. We compare the differences between the two models in the energy spectrum and angular distribution of the emitted radiation. We also analyse a self-generated synthetic phase-resolved energy spectrum, based on Compton atmosphere and the most X-ray luminous rotation-powered millisecond pulsars observed by the Neutron star Interior Composition ExploreR (NICER). We derive constraints for the neutron star parameters using both the Compton and Thomson models. The results show that the method works by reproducing the correct parameters with the Compton model. However, biases are found in size and the temperature of the emitting hot spot, when using the Thomson model. The constraints on the radius are still not significantly changed, and therefore the Thomson model seems to be adequate if we are interested only in the radius measurements using NICER., Comment: 6 pages, 9 figures, published in A&A

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

Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in low-mass X-ray binaries take place during hard persistent states of the systems. Spectral evolution of these bursts is well described by the atmosphere model of a passively cooling neutron star when the burst luminosity is high enough. The observed spectral evolution deviates from the model predictions when the burst luminosity drops below a critical value of 20-70% of the maximum luminosity. We suggest that these deviations are induced by the additional heating of the accreted particles. We present a method for computation of the neutron star atmosphere models heated by accreted particles assuming that their energy is released via Coulomb interactions with electrons. We compute the temperature structures and the emergent spectra of the atmospheres of various chemical compositions and investigate the dependence of the results on the other model parameters. We show that the heated atmosphere develops the hot (20--100 keV) corona-like surface layer cooled by Compton scattering, and the deeper, almost isothermal optically thick region with a temperature of a few keV. The emergent spectra deviate strongly from those of undisturbed neutron star atmospheres, with the main differences being the presence of a high-energy tail and a strong excess in the low-energy part of the spectrum. They also lack the iron absorption edge, which is visible in the spectra of undisturbed low-luminosity atmospheres with solar chemical composition. Using the computed spectra, we obtained the dependences of the dilution and color-correction factors as functions of relative luminosities for pure helium and solar abundance atmospheres. We show that the helium model atmosphere heated by accretion corresponding to 5% of the Eddington luminosity describes well the late stages of the X-ray bursts in 4U 1820-30., Comment: 14 pages, 19 figures, published version, some misprints were fixed

We report on the analysis of XMM-Newton observations of the central compact object CXOU J160103.1-513353 located in the center of the non-thermally emitting supernova remnant (SNR) G330.2+1.0. The X-ray spectrum of the source is well described with either single-component carbon or two-component hydrogen atmosphere models. In the latter case, the observed spectrum is dominated by the emission from a hot component with a temperature ~3.9MK, corresponding to the emission from a hotspot occupying ~1% of the stellar surface (assuming a neutron star with mass M = 1.5M$_\odot$, radius of 12 km, and distance of ~5 kpc as determined for the SNR). The statistics of the spectra and obtained upper limits on the pulsation amplitude expected for a rotating neutron star with hot spots do not allow us to unambiguously distinguish between these two scenarios. We discuss, however, that while the non-detection of the pulsations can be explained by the unfortunate orientation in CXOU J160103.1-513353, this is not the case when the entire sample of similar objects is considered. We therefore conclude that the carbon atmosphere scenario is more plausible., Comment: accepted in A&A

Neutron stars are the densest, directly observable stellar objects in the universe and serve as unique astrophysical laboratories to study the behavior of matter under extreme physical conditions. This book chapter is devoted to describing how electromagnetic observations, particularly at X-ray, optical and radio wavelengths, can be used to measure the mass and radius of neutron stars and how this leads to constraints on the equation of state of ultra-dense matter. Having accurate theoretical models to describe the astrophysical data is essential in this effort. We will review different methods to constrain neutron star masses and radii, discuss the main observational results and theoretical developments achieved over the past decade, and provide an outlook of how further progress can be made with new and upcoming ground-based and space-based observatories., Comment: To appear as a chapter in the White Book of "NewCompStar" European COST Action MP1304. Accepted by the editorial board in February 2018; 61 pages, 17 figures, 1 table

[Abridged] We present more than 4 years of Swift X-ray observations of the 2013 superoutburst, subsequent decline and quiescence of the WZ Sge-type dwarf nova SSS J122221.7-311525 (SSS122222) from 6 days after discovery. Only a handful of WZ Sge-type dwarf novae have been observed in X-rays, and until recently GW Lib was the only binary of this type with complete coverage of an X-ray light curve throughout a superoutburst. We collected extensive X-ray data of a second such system to understand the extent to which the unexpected properties of GW Lib are common to the WZ Sge class. We analysed the X-ray light curve and compared it with the behaviour of superhumps which were detected in the optical light curve. We also performed spectral analysis of the data. The results were compared with the properties of GW Lib, for which new X-ray observations were also obtained. SSS122222 was variable and around five times brighter in 0.3-10 keV X-rays during the superoutburst than in quiescence, mainly because of a significant strengthening of a high-energy component of the X-ray spectrum. The post-outburst decline of the X-ray flux lasted at least 500 d. The data show no evidence of the expected optically thick boundary layer in the system during the outburst. SSS122222 also exhibited a sudden X-ray flux change in the middle of the superoutburst, which occurred exactly at the time of the superhump stage transition. A similar X-ray behaviour was also detected in GW Lib. This result demonstrates a relationship between the outer disc and the white dwarf boundary layer for the first time, and suggests that models for accretion discs in high mass ratio accreting binaries are currently incomplete. The very long decline to X-ray quiescence is also in strong contrast to the expectation of low viscosity in the disc after outburst., Comment: 12 pages, 6 figures. Accepted for publication in A&A (abstract abridged). This paper is a sequel to arXiv:1701.03134

Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs) in low-mass X-ray binary systems can be used to constrain NS masses and radii. Most previous work of this type has set these constraints using Planck function fits as a proxy: both the models and the data are fit with diluted blackbody functions to yield normalizations and temperatures which are then compared against each other. Here, for the first time, we fit atmosphere models of X-ray bursting NSs directly to the observed spectra. We present a hierarchical Bayesian fitting framework that uses state-of-the-art X-ray bursting NS atmosphere models with realistic opacities and relativistic exact Compton scattering kernels as a model for the surface emission. We test our approach against synthetic data, and find that for data that are well-described by our model we can obtain robust radius, mass, distance, and composition measurements. We then apply our technique to Rossi X-ray Timing Explorer observations of five hard-state X-ray bursts from 4U 1702-429. Our joint fit to all five bursts shows that the theoretical atmosphere models describe the data well but there are still some unmodeled features in the spectrum corresponding to a relative error of 1-5% of the energy flux. After marginalizing over this intrinsic scatter, we find that at 68% credibility the circumferential radius of the NS in 4U 1702-429 is R = 12.4+-0.4 km, the gravitational mass is M=1.9+-0.3 Msun, the distance is 5.1 < D/kpc < 6.2, and the hydrogen mass fraction is X < 0.09., Comment: 15 pages, 11 figures, submitted to A&A

Thermonuclear (type-I) X-ray bursts, observed from neutron star (NS) low-mass X-ray binaries (LMXB), provide constraints on NS masses and radii and consequently the equation of state of NS cores. In such analyses various assumptions are made without knowing if they are justified. We have analyzed X-ray burst spectra from the LMXB 4U 1608-52, with the aim of studying how the different persistent emission components react to the bursts. During some bursts in the soft spectral state we find that there are two variable components; one corresponding to the burst blackbody component and another optically thick Comptonized component. We interpret the latter as the spreading layer between the NS surface and the accretion disc, which is not present during the hard state bursts. We propose that the spectral changes during the soft state bursts are driven by the spreading layer that could cover almost the entire NS in the brightest phases due to the enhanced radiation pressure support provided by the burst, and that the layer subsequently returns to its original state during the burst decay. When deriving the NS mass and radius using the soft state bursts two assumptions are therefore not met: the NS is not entirely visible and the burst emission is reprocessed in the spreading layer, causing distortions of the emitted spectrum. For these reasons the NS mass and radius constraints using the soft state bursts are different compared to the ones derived using the hard state bursts., Comment: 13 pages, 9 figures, accepted for publication in MNRAS

The aim of this paper is to investigate the transition of a strongly magnetized neutron star into the accretion regime with very low accretion rate. For this purpose we monitored the Be-transient X-ray pulsar GRO J1008-57 throughout a full orbital cycle. The current observational campaign was performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV) between two subsequent Type I outbursts in January and September 2016. The expected transition to the propeller regime was not observed. However, the transitions between different regimes of accretion were detected. In particular, after an outburst the source entered a stable accretion state characterised by the accretion rate of ~10^14-10^15 g/s. We associate this state with accretion from a cold (low-ionised) disc of temperature below ~6500 K. We argue that a transition to such accretion regime should be observed in all X-ray pulsars with certain combination of the rotation frequency and magnetic field strength. The proposed model of accretion from a cold disc is able to explain several puzzling observational properties of X-ray pulsars., Comment: 8 pages, 3 figures, 1 table, accepted by A&A

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 $z=0.205_{-0.003}^{+0.006}$ and the compactness was estimated to $(M/M_{Sun})/R(km)=0.105 \pm 0.002 $., Comment: Comments: 19 pages, 15 figures and 5 tables, Astronomy and Astrophysics accepted. arXiv admin note: text overlap with arXiv:1108.3897

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

We present a study of the evolution of the optical and X-ray fluxes during outbursts of two short-period cataclysmic variables, the confirmed intermediate polar CC Scl and the intermediate polar candidate FS Aur. We found that the X-ray and optical light curves are well correlated in both objects, although the amplitudes of outbursts in X-rays are smaller than those in the optical. The ratio of the outburst amplitudes in X-rays and the optical in both objects is close to ~0.6. This is significantly higher than was observed during the outburst of the non-magnetic dwarf nova U Gem, in which this ratio was only ~0.03. The obtained data also suggest that the dependence between the X-ray and optical fluxes must steepen significantly toward very low accretion rates and very low fluxes. Similarities in the behaviour of CC Scl and FS Aur indicate strongly the magnetic nature of the white dwarf in FS Aur., Comment: 6 pages, 4 figures. To appear in the ASP Conference Series proceedings of "20th European Workshop on White Dwarfs" held on July 25-29, 2016 at the University of Warwick, UK

When neutron stars (NS) accrete gas from low-mass binary companions, explosive nuclear burning reactions in the NS envelope fuse hydrogen and helium into heavier elements. The resulting thermonuclear (type-I) X-ray bursts produce energy spectra that are fit well with black bodies, but a significant number of burst observations show deviations from Planck spectra. Here we present our analysis of RXTE/PCA observations of X-ray bursts from the NS low-mass X-ray binary HETE J1900.1-2455. We have discovered that the non-Planckian spectra are caused by photo-ionization edges. The anti-correlation between the strength of the edges and the colour temperature suggests that the edges are produced by the nuclear burning ashes that have been transported upwards by convection and become exposed at the photosphere. The atmosphere model fits show that occasionally the photosphere can consist entirely of metals, and that the peculiar changes in black body temperature and radius can be attributed to the emergence and disappearance of metals in the photosphere. As the metals are detected already in the Eddington-limited phase, it is possible that a radiatively driven wind ejects some of the burning ashes into the interstellar space., Comment: Accepted to MNRAS Letters

We present our study of the central point source CXOU J181852.0-150213 in the young Galactic supernova remnant (SNR) G15.9+0.2 based on the recent ~90 ks Chandra observations. The point source was discovered in 2005 in shorter Chandra observations and was hypothesized to be a neutron star associated with the SNR. Our X-ray spectral analysis strongly supports the hypothesis of a thermally emitting neutron star associated with G15.9+0.2. We conclude that the object belongs to the class of young cooling low-magnetized neutron stars referred to as central compact objects (CCOs). We modeled the spectrum of the neutron star with a blackbody spectral function and with our hydrogen and carbon neutron star atmosphere models, assuming that the radiation is uniformly emitted by the entire stellar surface. Under this assumption, only the carbon atmosphere models yield a distance that is compatible with a source located in the Galaxy. In this respect, CXOU J181852.0-150213 is similar to two other well-studied CCOs, the neutron stars in Cas A and in HESS J1731-347, for which carbon atmosphere models were used to reconcile their emission with the known or estimated distances., Comment: Accepted for publication in Astronomy&Astrophysics

Symbiotic stars are a heterogeneous class of interacting binaries. Among them, RT Cru has been classified as prototype of a subclass that is characterised by hard X-ray spectra extending past ~20 keV. We analyse ~8.6 Ms of archival INTEGRAL data collected in the period 2003-2014, ~140 ks of Swift/XRT data, and a Suzaku observation of 39 ks, to study the spectral X-ray emission and investigate the nature of the compact object. Based on the 2MASS photometry, we estimate the distance to the source of 1.2-2.4 kpc. The X-ray spectrum obtained with Swift/XRT, JEM-X, IBIS/ISGRI, and Suzaku data is well fitted by a cooling flow model modified by an absorber that fully covers the source and two partial covering absorbers. Assuming that the hard X-ray emission of RT Cru originates from an optically thin boundary layer around a non-magnetic white dwarf, we estimated a mass of the WD of about 1.2 M_Sun. The mass accretion rate obtained for this source might be too high for the optically thin boundary layer scenario. Therefore we investigate other plausible scenarios to model its hard X-ray emission. We show that, alternatively, the observed X-ray spectrum can be explained with the X-ray emission from the post-shock region above the polar caps of a magnetised white dwarf with mass ~0.9-1.1 M_Sun., Comment: Accepted for publication in Astronomy & Astrophysics. 6 pages

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

We present the results of the monitoring programmes performed with the Swift/XRT telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition to the propeller regime in two well known X-ray pulsars 4U 0115+63 and V 0332+53 during their giant outbursts in 2015. Such transitions were detected at the threshold luminosities of $(1.4\pm0.4)\times10^{36}$ erg s$^{-1}$ and $(2.0\pm0.4)\times10^{36}$ erg s$^{-1}$ for 4U 0115+63 and V 0332+53, respectively. Spectra of the sources are shown to be significantly softer during the low state. In both sources, the accretion at rates close to the aforementioned threshold values briefly resumes during the periastron passage following the transition into propeller regime. The strength of the dipole component of the magnetic field required to inhibit the accretion agrees well with estimates based on the position of the cyclotron lines in their spectra, thus excluding presence of a strong multipole component of the magnetic field in the vicinity of the neutron star., Comment: 7 pages, 4 figures, accepted by A&A

Model spectra of neutron star atmospheres are nowadays widely used to fit the observed thermal X-ray spectra of neutron stars. This fitting is the key element in the method of the neutronstar radius determination. Here, we present the basic assumptions used for the neutron star atmosphere modeling as well as the main qualitative features of the stellar atmospheres leading to the deviations of the emergent model spectrum from blackbody. We describe the properties of two of our model atmosphere grids: (i) pure carbon atmospheres for relatively cool neutron stars (1--4 MK) and (ii) hot atmospheres with Compton scattering taken into account. The results obtained by applying these grids to model the X-ray spectra of the central compact object in supernova remnant HESS 1731-347, and two X-ray bursting neutron stars in low-mass X-ray binaries, 4U 1724-307 and 4U 1608-52, are presented. Possible systematic uncertainties associated with the obtained neutron star radii are discussed., Comment: 20 pages, 20 figures. Accepted for publication in the European Physical Journal A topical issue on "Exotic Matter in Neutron Stars" . Based on the invited talk in MIAPP Workshop "The Many Faces of Neutron Stars", Garching, 24 August - 18 September 2015

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

The cooling phase of thermonuclear (type-I) X-ray bursts can be used to constrain the neutron star (NS) compactness by comparing the observed cooling tracks of bursts to accurate theoretical atmosphere model calculations. By applying the so-called cooling tail method, where the information from the whole cooling track is used, we constrain the mass, radius, and distance for three different NSs in low-mass X-ray binaries 4U 1702-429, 4U 1724-307, and SAX J1810.8-260. Care is taken to only use the hard state bursts where it is thought that only the NS surface alone is emitting. We then utilize a Markov chain Monte Carlo algorithm within a Bayesian framework to obtain a parameterized equation of state (EoS) of cold dense matter from our initial mass and radius constraints. This allows us to set limits on various nuclear parameters and to constrain an empirical pressure-density relation for the dense matter. Our predicted EoS results in NS radius between 10.5-12.8 km (95% confidence limits) for a mass of 1.4 $M_{\odot}$. Due to systematic errors and uncertainty in the composition these results should be interpreted as lower limits for the radius., Comment: 23 pages, 12 figures, revised version, accepted for publication in A&A

Superbursts are hours-long X-ray flares attributed to the thermonuclear runaway burning of carbon-rich material in the envelope of accreting neutron stars. By studying the details of the X-ray light curve, properties of carbon combustion can be determined. In particular, we show that the shape of the rise of the light curve is set by the the slope of the temperature profile left behind by the carbon flame. We analyse RXTE/PCA observations of 4U 1636-536 and separate the direct neutron star emission from evolving photoionized reflection and persistent spectral components. This procedure results in the highest quality light curve ever produced for the superburst rise and peak, and interesting behaviour is found in the tail. The rising light curve between 100 and 1000 seconds is inconsistent with the idea that the fuel burned locally and instantaneously everywhere, as assumed in some previous models. By fitting improved cooling models, we measure for the first time the radial temperature profile of the superbursting layer. We find $d\ln T/d\ln P=1/4$. Furthermore, 20% of the fuel may be left unburned. This gives a new constraint on models of carbon burning and propagation in superbursts., Comment: 9 pages, 5 figures, accepted for publication in MNRAS

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 an analysis of BeppoSAX observations of the unique transient bursting X-ray pulsar GRO J1744-28. The observations took place in March 1997 during the decay phase of the outburst. We find that the persistent broadband X-ray continuum of the source is consistent with a cutoff power law typical for the accreting pulsars. We also detect the fluorescence iron line at 6.7 keV and an absorption feature at ~4.5 keV, which we interpret as a cyclotron line. The corresponding magnetic field strength in the line forming region is ~3.7 x 10^11 G. Neither line is detected in the spectra of the bursts. However, additional soft thermal component with kT ~2 keV was required to describe the burst spectrum. We briefly discuss the nature of this component and argue that among other possibilities it might be connected with thermonuclear flashes at the neutron star surface which accompany the accretion-powered bursts in the source.

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

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of thermonuclear X-ray bursts on accreting neutron stars. For a summary, we refer to the paper., Comment: White Paper in Support of the Mission Concept of the Large Observatory for X-ray Timing

Context: Central Compact Objects (CCOs) in supernova remnants are isolated thermally emitting neutron stars (NSs). They are most probably characterized by a magnetic field strength that is roughly two orders of magnitude lower than that of most of the radio and accreting pulsars. The thermal emission of CCOs can be modeled to obtain constraints on the physical parameters of the star such as its mass, radius, effective temperature, and chemical composition. Aims: The CCO in HESS, J1731-347 is one of the brightest objects of this class. We present our analysis of two new XMM-Newton observations of the source which increase the total exposure time of the by a factor of five compared to the analyses presented before. Methods: We use our numerical spectral models for carbon and hydrogen atmospheres to fit the spectrum of the CCO. From our fits, we derive constraints on the physical parameters of the emitting star. We also use the new data to derive new upper limits on pulsations and to confirm the absence of a long-term variability. Results: The analysis shows that atmosphere spectral models are clearly preferred by the fit over the blackbody function. Under the assumption that the X-ray emission is uniformly produced by the entire star surface, hydrogen atmosphere models lead to uncomfortably large distances of the CCO, above 7-8 kpc. On the other hand, the carbon atmosphere model formally excludes distances above 5-6 kpc and is compatible with the source located in the Scutum-Crux (3 kpc) or Norma-Cygnus (4.5 kpc) Galactic spiral arm. We provide and discuss the corresponding confidence contours in the NS mass-radius plane. The measured effective temperature indicates that the NS is exceptionally hot for the estimated age of 30 kyr. We discuss possible cooling scenarios to explain this property, as well as possible additional constraints on the star mass and radius from cooling theory., Comment: Accepted for publication in Astronomy & Astrophysics; a small error in the last section is corrected

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

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 pc, Comment: 7 pages, 3 figures, published in Astronomy Letters

The accuracy of measuring the basic parameters of neutron stars is limited in particular by uncertainties in chemical composition of their atmospheres. For example, atmospheres of thermally - emitting neutron stars in supernova remnants might have exotic chemical compositions, and for one of them, the neutron star in CasA, a pure carbon atmosphere has recently been suggested by Ho & Heinke (2009). To test such a composition for other similar sources, a publicly available detailed grid of carbon model atmosphere spectra is needed. We have computed such a grid using the standard LTE approximation and assuming that the magnetic field does not exceed 10^8 G. The opacities and pressure ionization effects are calculated using the Opacity Project approach. We describe the properties of our models and investigate the impact of the adopted assumptions and approximations on the emergent spectra., Comment: 8 pages, 11 figures; accepted for publication in ApJS

To examine the previously claimed fast cooling of the Central Compact Object (CCO) in the Cas A supernova remnant (SNR), we analyzed two Chandra observations of this CCO, taken in a setup minimizing instrumental spectral distortions. We fit the two CCO X-ray spectra from 2006 and 2012 with hydrogen and carbon neutron star atmosphere models. The temperature and flux changes in the 5.5 years between the two epochs depend on the adopted constraints on the fitting parameters and the uncertainties of the effective area calibrations. If we allow a change of the equivalent emitting region size, R_Em, the effective temperature remains essentially the same. If R_Em is held constant, the best-fit temperature change is negative, but its statistical significance ranges from 0.8sigma to 2.5sigma, depending on the model. If we assume that the optical depth of the ACIS filter contaminant in 2012 was +/-10% different from its default calibration value, the significance of the temperature drop becomes 0.8sigma to 3.1sigma, for the carbon atmospheres with constant R_Em. Thus, we do not see a statistically significant temperature drop in our data, but the involved uncertainties are too large to firmly exclude the previously reported fast cooling. Our analysis indicate a decrease of 4%-6% (1.9-2.9sigma significance) for the absorbed flux in the energy range 0.6-6keV between 2006 and 2012, most prominent in the 1.4-1.8 keV energy range. It could be caused by unaccounted changes of the detector response or contributions from unresolved SNR material along the line of sight to the CCO., Comment: 20 pages, 19 figures, 3 tables; accepted for publication in ApJ

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