Search

We have carried out a deep survey of the M81 field in the 25–60 keV energy band based on long-term (2003–2023) INTEGRAL observations. A record sensitivity of 0.16 mCrab at a detection significance of has been achieved in the central part of the field owing to the long accumulated exposure (19.2 Ms). The total area of the survey is 1004 deg at a sensitivity level better than 0.72 mCrab. We have produced a catalog of sources detected at a significance level higher than . It contains 51 objects most of which are active galactic nuclei (AGNs). The median redshift of the Seyfert galaxies in the catalog is . Six sources have not been detected previously in any of the X-ray surveys. According to the available indirect data, all of them and two more sources that have already been entered previously into the INTEGRAL survey catalogs can also be AGNs, including those with strong internal absorption. [ABSTRACT FROM AUTHOR]

Transonic (with Mach number ) motion of a pulsar relative to the external medium can help its compact pulsar wind nebula develop a double-torus X-ray morphology. The double-torus structure can reverberate as a whole under the dynamic pressure of the external flow. For a flow aligned with the symmetry axis of the nebula, the response of the double-torus is uniform in azimuth. For a misaligned flow, the leeward sides of the tori respond with some delay relative to their windward sides. The delay can cause a curious swaying in the short midsection of the leeward jet of the compact X-ray nebula. Within the framework of the relativistic magnetohydrodynamical model of a pulsar wind nebula we study the dynamics of the nebular outflows contributing to the swaying of the jet. When applied to the Vela X-ray nebula, the model allows us to naturally relate two distinct phenomena, the swaying of the bright midsection of the Vela lee jet and the reverberation of its double-torus. [ABSTRACT FROM AUTHOR]

We describe the methods of the SRGz system for the physical identification of eROSITA point X-ray sources from photometric data in the DESI Legacy Imaging Surveys footprint. We consider the models included in the SRGz system (version 2.1) that have allowed us to obtain accurate measurements of the cosmological redshift and class of an X-ray object (quasar/galaxy/star) from multiwavelength photometric sky surveys (DESI LIS, SDSS, Pan-STARRS, WISE, eROSITA) for 87 of the entire eastern extragalactic region ( , ). An important feature of the SRGz system is that its data handling model (identification, classification, photo-z algorithms) is based entirely on heuristic machine learning approaches. For a standard choice of SRGz parameters the optical counterpart identification completeness (recall) in the DESI LIS footprint is (with an optical counterpart selection precision of ); the classification completeness (recall) of X-ray sources without optical counterparts in DESI LIS is ( precision). A high quality of the photometric classification of X-ray source optical counterparts is achieved in SRGz: photometric classification completeness (recall) for extragalactic objects (a quasar or a galaxy) and stars on a test sample of sources with SDSS spectra and GAIA astrometric stars. We present an analysis of the importance of various photometric features for the optical identification and classification of eROSITA X-ray sources. We have shown that the infrared (IR) magnitude , the X-ray/optical(IR) ratios, the optical colors (for example, ), and the IR color () as well as the color distances introduced by us play a significant role in separating the classes of X-ray objects. We use the most important photometric features to interpret the SRGz predictions in this paper. The accuracy of the SRGz photometric redshifts (from DESI LIS, SDSS, Pan-STARRS, and WISE photometric data) has been tested in the Stripe82X field on a sample of 3/4 of the optical counterparts of eROSITA point X-ray sources (for which spectroscopic measurements are available in Stripe82X): (the normalized median absolute deviation of the prediction) and (the fraction of catastrophic outliers). The presented photo-z results for eROSITA X-ray sources in the Stripe82X field are more than a factor of 2 better in both metrics ( and ) than the photo-z results of other groups published in the Stripe82X catalog. [ABSTRACT FROM AUTHOR]

This paper is an extension of the study by Khamitov et al. (2022) with regard to the catalog and the astrophysical interpretation of the imitation of significant proper motions in active galactic nuclei (AGNs) and quasars based on data from the Gaia space observatory. We present a sample of SRG/eROSITA X-ray sources in the eastern Galactic hemisphere () having significant proper motions in the Gaia EDR3 measurements with the confirmed extragalactic nature of the objects. The catalog consists of 248 extragalactic sources with spectroscopically measured redshifts. The catalog includes all of the objects available in the SIMBAD database and coincident with the identified optical counterpart within 0.5 arcsec. Eighteen sources with spectroscopically measured redshifts from observations with the Russian–Turkish 1.5-m telescope RTT-150 (Khamitov et al. 2022) have been additionally included in the catalog. The sources in the catalog are AGNs of various types (Sy1, Sy2, LINER), quasars, radio galaxies, and star-forming galaxies. The imitation of significant proper motions can be explained (by the VIM effect previously known in astrometry) by the presence of transient events on the line of sight in the vicinity of AGNs and quasars (within the Gaia optical resolution element). Among such astrophysical events are supernova outbursts, tidal disruption events in binary AGNs, the variability of high-mass supergiants, the presence of OB associations against the background of AGNs with a variable brightness, etc. The model of outbursts with a fast rise–exponential decay profile allows the variable positional parameters of most sources observed in Gaia to be described. This approach can be used as an independent way of detecting transient events in the vicinity of AGNs (on scales of several hundred parsecs in the plane of the sky) based on data from the SRG/eROSITA catalogs of X-ray sources and the optical Gaia catalog. [ABSTRACT FROM AUTHOR]

Pulsar wind nebulae (PWNe) show complex multiscale X-ray morphology with a wide variety of structures in form of tori, jets, rings, arcs, and knots. The global torus-jet morphology of PWNe is well described in terms of modern relativistic MHD models. Meanwhile, the nature of some of the finer features remains to be elucidated. These include the chain of knots in the "inner ring" in the Crab PWN and the chain of knots around the southeastern jet in the Vela PWN. Recently, the knots in Crab have been interpreted as giant plasmoids. These arise due to magnetic reconnection in the equatorial current sheet just after the shock wave terminating the pulsar wind and responsible for the appearance of the "inner ring" feature. In this work, we aim to find out whether the knots in Vela could have been formed in the same way. Within the frame of numerical relativistic MHD model, we show that in Vela-type nebulae—double-torus objects in transonic motion relative to their surroundings—conditions for strong reconnection and hence for giant plasmoids are formed at the outskirts of the X-ray nebula, where compact regions of high magnetization occur regularly on both sides of the equatorial belt. In the projection onto the celestial plane, these regions line up along two narrow coaxial "outer rings," of which the windward is larger than the leeward. Due to the difference in size of the rings, their far halves must be very close to each other if the nebula is seen from its leeward side at the same angle as Vela. In this case, giant plasmoids from the far side of the nebula should be concentrated in the vicinity a narrow arcuate stripe bounded by these halves. [ABSTRACT FROM AUTHOR]

X-ray morphology of pulsar wind nebulae (PWNe) reflects the structure of underlying plasma outflows. This study aims to reveal the nature of one fine X-ray feature in images of the Vela nebula. The feature is shaped like a narrow bar, oriented across the south-eastern jet of the nebula. It is located in the jet at the beginning of its bright middle section (stretching from to from the pulsar) and has an angular size of . Within the framework of relativistic MHD modeling, we show that such a bar may be a characteristic feature of Vela-like objects-transonic PWNe with a double-torus X-ray structure seen from the leeward side. Our modeling indicates the shock-wave origin for the bar. The shock is formed in the jet due to its dynamical interaction with two regular toroidal vortices in the lee hemisphere of the nebula. Namely, with large-scale circulation at middle latitudes, and with smaller-scale recirculation within the polar funnel of the pulsar wind termination shock. The former vortex almost dams the lee jet in its far downstream (so the bright middle section of the jet is limited in length by the axial size of the vortex) and the latter chokes the jet at its very beginning. The shock underlying the bar feature occurs immediately after the breakthrough of the supersonic jet through the recirculation. The shock is strong, so it propagates from the jet's body into the surrounding plasma, and it is quasi-stationary–it can disappear and reappear again at the same place. [ABSTRACT FROM AUTHOR]

Using long-term INTEGRAL hard X-ray observations, we have put upper limits on the bolometric luminosity ( erg s ) of the nuclei of 72 nearby dwarf galaxies located in the M81 field. This has allowed us to constrain the fraction of galaxies in which the central black hole accretes matter at a rate above 0.1 of the critical one. Under simple assumptions about the distributions of black hole masses and accretion rates this fraction does not exceed 9 . Reliable estimates of the central black hole masses are required to obtain more stringent constraints on the fraction of AGNs. [ABSTRACT FROM AUTHOR]

We present the results of our study of the X-ray pulsar IGR J21343+4738 based on NuSTAR, Swift, and SRG observations in the wide energy range 0.3–79 keV. The absence of absorption features in the energy spectra of the source, both averaged and phase-resolved ones, has allowed us to estimate the upper and lower limits on the magnetic field of the neutron star in the binary system, and G, respectively. Our spectral and timing analyses have shown that IGR J21343+4738 has all properties of a quasi-persistent X-ray pulsar with a pulsation period of s and a luminosity erg s . Our analysis of the long-term variability of the object in X-rays has confirmed the possible orbital period of the binary system days previously detected in the optical range. [ABSTRACT FROM AUTHOR]

We present the results of our search for highly variable active galactic nuclei (AGNs) the X-ray flux from which changed by more than an order of magnitude during the SRG/eROSITA all-sky survey. Using the eROSITA data obtained in the period from December 2019 to February 2022, we have found 1325 sources the X-ray flux from which in the 0.3–2.3 keV energy band changed by more than a factor of 10 at a confidence level of at least 99.73 . Of them, 635 objects have been classified as AGNs or AGN candidates. We describe the procedure of searching for highly variable sources and the selection of extragalactic objects among them and describe the statistical properties of the produced catalog. We provide a catalog of 49 sources for which a statistically significant flux in their low state was detected. For the latter we provide their light curves and X-ray spectra and discuss in detail the most interesting of them. [ABSTRACT FROM AUTHOR]

Based on a comparison of the SRG/eROSITA catalog of X-ray active stars and the Gaia catalog, we have obtained a sample of 502 peculiar objects for which Gaia, on the one hand, detects statistically significant parallaxes or proper motions and, on the other hand, records signs of a nonzero optical extent. On the –color diagram these objects are significantly separated from the bulk of the stars and are located in the region typical for active galactic nuclei (AGNs). According to the SIMBAD database, of them are AGNs and galaxies with spectroscopic redshifts and only are confirmed objects in our Galaxy. Our spectroscopic observations of 19 unidentified objects with the RTT-150 telescope have shown that 18 of them are AGNs at redshifts 0.01–0.3 and one object is an M star in our Galaxy. We discuss various scenarios for the appearance of such peculiar objects. [ABSTRACT FROM AUTHOR]

We present a method for the optical cross-match of sources detected in wide-field X-ray sky surveys. For this purpose, we have constructed and trained a neural network model to characterize the photometric attributes of the populations of optical counterparts of X-ray sources and optical field objects. The photometric information processing result is used for the probabilistic cross-match of X-ray sources with optical DESI LIS sources. The efficiency of the method is illustrated using the deep Lockman Hole survey with the eROSITA telescope of the SRG observatory as an example. To estimate the accuracy of the method, we have produced a validation sample based on the Chandra and XMM-Newton catalogs of X-ray sources. The cross-match precision in our method reaches 94 for the entire Lockman Hole X-ray catalog and 97 for sources with a flux erg cm s . We discuss the further development of the cross-match model and the steps needed for its application to the SRG/eROSITA all-sky survey data. [ABSTRACT FROM AUTHOR]

Close binary systems consisting of two neutron stars (BNS) emit gravitational waves, that allow them to merge on timescales shorter than Hubble time. It is widely believed, that NS–NS mergers in such systems power short gamma-ray bursts (GRB). Several mechanisms which could lead to electromagnetic energy release prior to a merger have been proposed. We estimate the ability to observe the possible pre-burst emission with telescopes of Spectrum–Roentgen–Gamma. We also investigate first such event, GRB210919A, which fell into the field of view of the SRG telescopes less than two days before the burst. [ABSTRACT FROM AUTHOR]

We present the results of our study of the transient X-ray pulsar GRO J2058+42 in a wide energy range in a state with a luminosity erg s . We have found that the pulse profile of the source, along with the pulse fraction, changed significantly in comparison with the previous NuSTAR observations performed when the pulsar was brighter approximately by a factor of 10. The position of the cyclotron line at keV in a narrow phase interval is consistent with the observations in the high state. Spectral analysis has shown that at high luminosities erg s the spectrum has a shape typical of accreting pulsars, whereas a two-component model should be used to describe the spectrum when the luminosity drops approximately by an order of magnitude, to erg s . This behavior fits into the model in which the low-energy part of the spectrum is formed in a hot spot, while the high-energy one is formed as a result of resonant Compton scattering by the infalling matter in the accretion channel above the neutron star surface. [ABSTRACT FROM AUTHOR]

During accretion-induced outbursts, low-mass X-ray binaries demonstrate a complex behavior accompanied by a rapid change in their spectral and timing characteristics in the entire range of the electromagnetic spectrum — from radio to hard X-rays. So far there is no complete, self-consistent model describing all of the observed phenomena, including the rapid changes in visible and near-infrared fluxes and the correlation of these changes with the X-ray emission, while the observed evolution of outbursts is described in terms of the transition between characteristic states. Based on quasi-simultaneous optical, infrared, and X-ray observations, we show that the sharp decline in optical brightness near the low/hard-to-high/soft state transition is accompanied by an increase in the frequency of the observed X-ray quasi-periodic oscillations. This behavior can be explained in terms of the model in which both X-ray and optical emissions are produced by the synchrotron mechanism in a hot flow near a compact object. [ABSTRACT FROM AUTHOR]

We study the distribution of accretion rates onto supermassive black holes in AGNs of the local Universe () based on near-infrared and hard X-ray surveys (2MASS and Swift/BAT). Using sufficiently accurate black hole mass estimates has allowed the Eddington ratio to be reliably estimated approximately for half of the objects in the AGN sample; a rougher estimate from the correlation of with the galaxy stellar mass has been used for the remaining ones. As a result, for a wide range of galaxy masses, , including the most massive galaxies in the local Universe, we have shown that the distribution above is described by a power law with -independent parameters and falls off with a characteristic slope up to the Eddington limit (), where there is evidence for a break. In addition, there is evidence that at the dependence has a lower slope or flattens out. The mean characteristic growth time of supermassive black holes at the present epoch has been estimated. It has turned out to depend weakly on the galaxy stellar mass and to exceed the lifetime of the Universe, but by no more than one order of magnitude. The mean duty cycle of supermassive black holes (the fraction of objects with ) in the local Universe has been estimated. It also depends weakly on and is 0.2–1 . On the whole, these results obtained for the present epoch confirm the trends pointed out in previous studies for the earlier Universe, refining the parameters of the dependence at . The revealed universal (weakly dependent on the galaxy stellar mass) pattern of the dependence probably stems from the fact that, at present, the episodes of mass accretion onto supermassive black holes are associated mainly with stochastic processes in galactic nuclei rather than with global galaxy evolution processes. [ABSTRACT FROM AUTHOR]

We present the results of our spectral and timing analysis of the emission from the transient X-ray pulsar XTE J1946+274 based on the simultaneous NuSTAR and Swift/XRT observations in the broad energy range 0.3–79 keV carried out in June 2018 during a bright outburst. Our spectral analysis has confirmed the presence of a cyclotron absorption line at an energy keV in both averaged and phase-resolved spectra of the source. Phase-resolved spectroscopy has also allowed the variation in spectral parameters with neutron star rotation phase, whose period is s, to be studied. The energy of the cyclotron line is shown to change significantly (from to keV) on the scale of a pulse, with the line width and optical depth also exhibiting variability. The observed behavior of the cyclotron line parameters can be interpreted in terms of the model of the reflection of emission from a small accretion column (the source's luminosity at the time of its observations was erg s ) off the neutron star surface. The equivalent width of the iron line has been found to also change significantly with pulse phase. The time delay between the pulse and equivalent width profiles can be explained by the reflection of neutron star emission from the outer accretion disk regions. [ABSTRACT FROM AUTHOR]

We continue the study begun in Karasev et al. (2018) and present the results of our optical identifications of four hard X-ray sources from the INTEGRAL sky surveys. Having first improved the positions of these objects in the sky with the X-ray telescope (XRT) of the Swift observatory, we have identified their counterparts using optical and infrared sky survey data. Then, we have obtained optical spectra for the putative counterparts with the RTT-150 Russian-Turkish telescope and the AZT-33IK telescope. This has allowed the nature of the objects under study to be established. The sources IGR J11079+7106 and IGR J12171+7047 have turned out to be extragalactic in nature and be Seyfert 1 and 2 galaxies, respectively, with the second object being characterized by a large absorption column density. The source IGR J18165−3912 is most likely an intermediate polar with a very high luminosity. The fourth source, IGR J20596+4303, is a chance superposition of two objects—a Seyfert 2 galaxy and a cataclysmic variable. [ABSTRACT FROM AUTHOR]

The possibility of conducting a Galactic plane survey at energies above 5 keV with the ART-XC telescope onboard the Spectrum-Röntgen-Gamma (SRG) observatory during the satellite's flight to the Lagrange point L2 is considered. One of the possible fields for such a survey is proposed. We show that it will be possible to detect ∼100 cataclysmic variables with luminosities 1031−1034 erg s−1 in a survey with an area of 10 deg2 and a duration of 10 days. Such a representative sample may allow the luminosity function and other properties of the population of cataclysmic variables in the Galaxy to be refined. [ABSTRACT FROM AUTHOR]

We present the results of our optical identifications of several hard X-ray sources from the INTEGRAL all-sky survey obtained over 14 years of observations. Having improved the positions of these objects in the sky with the X-ray telescope (XRT) of the Swift observatory and the XMMNewton observatory, we have identified their counterparts using optical and infrared sky survey data. We have obtained optical spectra for more than half of the objects from our sample with the RTT-150 and AZT-33IK telescopes, which have allowed us to establish the nature of the objects and to measure their redshifts. Six sources are shown to be extragalactic in origin and to belong to Seyfert 1 and 2 galaxies (IGR J01017+6519, IGR J08215-1320, IGR J08321-1808, IGR J16494-1740, IGR J17098-2344, IGR J17422-2108); we have failed to draw definitive conclusions about the nature of four more objects (IGR J11299-6557, IGR J14417-5533, IGR J18141-1823, IGR J18544+0839), but, judging by circumstantial evidence, they are most likely also extragalactic objects. For one more object (IGR J18044-1829) no unequivocal identification has been made. [ABSTRACT FROM AUTHOR]

To choose the best strategy for conducting a deep extragalactic survey with the ART-XC X-ray telescope onboard the Spectrum-Ro¨ ntgen-Gamma (SRG) observatory and to estimate the expected results, we have simulated the observations of a 1.1° × 1.1° field in the 5-11 and 8-24 keV energy bands. For this purpose, we have constructed a model of the active galactic nuclei (AGN) population that reflects the properties of the X-ray emission from such objects. The photons that"arrived" from these sources were passed through a numerical model of the telescope, while the resulting data were processed with the standard ART-XC data processing pipeline. We show that several hundred AGNs at redshifts up to z ≈ 3 will be detected in such a survey over 1.2 Ms of observations with the expected charged particle background levels. Among them there will be heavily obscured AGNs, which will allow a more accurate estimate of the fraction of such objects in the total population to be made. Source confusion is expected at fluxes below 2 × 10−14 erg s−1 cm−2 (5-11 keV). Since this value can exceed the source detection threshold in a deep survey at low particle background levels, it may turn out to be more interesting to conduct a survey of larger area (several square degrees) but smaller depth, obtaining a sample of approximately four hundred bright AGNs as a result. [ABSTRACT FROM AUTHOR]

The discovery of a type I X-ray burst from the faint unidentified transient source IGR J17445-2747 in the Galactic bulge by the JEM-X telescope onboard the INTEGRAL observatory is reported. Type I bursts are believed to be associated with thermonuclear explosions of accreted matter on the surface of a neutron star with a weak magnetic field in a low-mass X-ray binary. Thus, this observation allows the nature of this source to be established. [ABSTRACT FROM AUTHOR]

During the scanning observations of the Galactic center region in late August-September 2016 we detected a new (third) outburst of the historical X-ray nova GRS 1739-278, a presumed black hole in a low-mass X-ray binary. This was reported in the Astronomer's Telegrams (Mereminskiy et al. 2016). In this paper we present the results of INTEGRAL and Swift observations of the outburst development. According to these observations, the flux from the source in the hard X-ray band (20-60 keV) rose from ~11 (September 3) to ~30 mCrab (September 14), was at the attained level for ~8 days, and then returned to ~15 mCrab. The spectrum of the source taken at its peak brightness in the energy range 0.5-150 keV could be fitted by a single power law with a photon index of 1.86 ± 0.07 distorted only by photoabsorption corresponding to the hydrogen column density log ( N ) = 22.37 under the assumption of a solar abundance. This means that the source at this time was in the low/hard state. Infrared observations with the RTT-150 telescope near the X-ray brightness peak of the source revealed no emission down to $$22_ \cdot ^m 0$$ (in the r' band) and $$20_ \cdot ^m 9$$ (in the i' band). At the time of writing the paper we do not yet know whether this outburst ended or only its initial stage was observed. If it ended, then based on the light curve and spectra, we can state that it was a 'failed' outburst, i.e., the amount of accreted matter in this episode was insufficient to reach the high or very high state with a soft blackbody component in the spectrum characteristic of developed outbursts. [ABSTRACT FROM AUTHOR]

Based on data from the SWIFT, INTEGRAL, MAXI/ISS orbital observatories, and the ground-based RTT-150 telescope, we have investigated the broadband (from the optical to the hard X-ray bands) spectrum of the X-ray nova MAXI J1828-249 and its evolution during the outburst of the source in 2013-2014. The optical and infrared emissions from the nova are shown to be largely determined by the extension of the power-law component responsible for the hard X-ray emission. The contribution from the outer cold regions of the accretion disk, even if the X-ray heating of its surface is taken into account, turns out to be moderate during the source's 'high' state (when a soft blackbody emission component is observed in the X-ray spectrum) and is virtually absent during its 'low' ('hard') state. This result suggests that much of the optical and infrared emissions from such systems originates in the same region of main energy release where their hard X-ray emission is formed. This can be the Compton or synchro-Compton radiation from a high-temperature plasma in the central accretion disk region puffed up by instabilities, the synchrotron radiation from a hot corona above the disk, or the synchrotron radiation from its relativistic jets. [ABSTRACT FROM AUTHOR]

An X-ray survey of the Galactic center region with a radius of ~20◦ has been performed using the data obtained with the JEM-X telescope onboard the INTEGRAL observatory over ~10 years of observations (2003-2013). The exposure at the field center directly toward the Galactic center has reached 4.8 Ms. We have constructed sky maps in the 5-10 and 10-25 keV energy bands and compiled a catalog of detected sources. Together with 83 sources revealed on the integral sky maps, it includes 22 transients that are absent on them but are confidently detected during outbursts with a duration of several days. One of the persistent sources, IGR J17452-2909, has never been observed previously. In contrast to the catalog of sources detected in the harder energy band by the IBIS/ISGRI telescope onboard the INTEGRAL observatory, most of the sources in this catalog are low-mass X-ray binaries (73 of the 105 sources) and only 18 + 3 are high-mass X-ray binaries and cataclysmic variables. Out of the Galactic sources, there are also the black hole candidate XTE J1652-453, the peculiar X-ray burster XMMJ174457-2850.3, and the soft gamma repeater SGR 1806-20 in the catalog; out of the extragalactic sources, there are three active galactic nuclei and a galaxy cluster (Oph CL). The nature of four sources, including the newly discovered one, still remains unknown. We have constructed the luminosity function for the low-mass X-ray binaries from the catalog and considered other statistical properties of their sample. [ABSTRACT FROM AUTHOR]

The model of a nonstationary α-disk with allowance made for the irradiation and vertical convection in the outer accretion disk, where hydrogen is partially ionized, is considered. Allowance for the energy release through turbulent viscosity is shown to be important in the mixing-length convection theory in accretion disks. This model has been used to explain the optical and X-ray light curves of the X-ray nova A0620-00. The turbulent viscosity parameter of the accretion disk has been estimated, α = 0.5−0.6, to explain the luminosity decay rate on the descending branch of the X-ray light curve for the A0620-00 1975 outburst. The secondary luminosity maximum on the light curves is explained by assuming an additional injection of matter into the accretion disk from the optical companion. [ABSTRACT FROM AUTHOR]

Results of the study of the X-ray nova SWIFT J174510.8-262411 by the INTEGRAL, SWIFT, and VLA observatories in September-October 2012 at the early outburst phase are presented. Attention is focused on analyzing the power spectrum of X-ray flux fluctuations of the source in which a powerful quasi-periodic oscillation (QPO) peak has been detected. The evolution of the QPO peak parameters with time has been traced; a correlation between the QPO frequency, low-frequency noise (LFN) parameters, X-ray and radio fluxes, and fractional polarization of radio emission has been revealed. The fractal properties of the source's light curves in the standard X-ray band (<10 keV) are shown to change with QPO peak frequency. In the hard X-ray band (20-80 keV), the source's light curves have no fractal properties, despite the larger QPO and LFN amplitude than that in the standard X-ray band. The observational results can be explained by assuming that the source's X-ray emission is formed in a hot plasma cloud surrounding a black hole, while the QPOs are produced at its boundary, at the place of contact with the accretion disk; their frequency reflects the Keplerian rotation of matter at the corresponding radius. [ABSTRACT FROM AUTHOR]

This paper is devoted to determining the nature of the hard X-ray source IGR J17463-2854 located toward the Galactic bulge. Using data from the INTEGRAL and Chandra X-ray observatories, we show that five point X-ray sources with approximately identical fluxes in the 2-10 keV energy band are detected in the error circle of the object under study. In addition, significant absorption at low energies has been detected in the spectra of all these sources. Based on data from the VVV (VISTA/ESO) infrared Galactic Bulge Survey, we have unambiguously identified three of the five sources, determined the J, H, and K magnitudes of the corresponding stars, and obtained upper limits on the fluxes for the remaining two sources. Analysis of the color-magnitude diagrams has shown that one of these objects most likely belongs to a class of rarely encountered objects, symbiotic binary systems (several tens are known with certainty), i.e., low-mass binary systems consisting of a white dwarf and a red giant. It is important to note that all our results were obtained using improved absorption values and an extinction law differing in this direction from the standard one. [ABSTRACT FROM AUTHOR]

We present the results of the the identification of six objects from the ASCA Galactic center and Galactic plane surveys: AX J173548-3207, AX J173628-3141, AX J1739.5-2910, AX J1740.4-2856, AX J1740.5-2937, and AX J1743.9-2846. Chandra, XMM-Newton, and XRT/Swift X-ray data have been used to improve the positions of the optical counterparts to these sources. Thereafter, we have carried out a series of spectroscopic observations of the established optical counterparts at the RTT-150 telescope. Analysis of X-ray and optical spectra as well as photometric measurements in a wide wavelength range based on optical and infrared catalogs has allowed the nature of the program sources to be determined. Two X-ray objects have been detected in the error circle of AX J173628-3141: one is a coronally active G star and the other may be a symbiotic star, a red giant with an accreting white dwarf. Three sources (AX J1739.5-2910, AX J1740.5-2937, AX J1743.9-2846) have turned out to be active G-K stars, presumably RS CVn objects, one (AX J1740.4-2856) is an M dwarf, and another one (AX J173548-3207) most likely a low-mass X-ray binary in its low state. The distances and corresponding luminosities of the sources in the soft X-ray band (0.5-10 keV) have been estimated; analysis of deep INTEGRAL Galactic center observations has not revealed a statistically significant flux at energies >20 keV from any of them. [ABSTRACT FROM AUTHOR]

We present the first results of our search for cataclysmic variables among the point X-ray sources detected in a part of the sky survey with an area of 400 sq. deg. performed on the basis of data from the ROSAT telescope (400d), for which photometric data from the Sloan Digital Sky Survey are also available. The necessary optical observations of the candidates for cataclysmic variables have been carried out with the 1.5-m Russian-Turkish telescope (RTT-150). We show that one of the four investigated objects (400D J0019126+220733) is a cataclysmic variable, two objects are quasars at redshifts z ∼ 2, and the nature of one object remains unclarified; however, we can exclude it from the list of possible candidates for cataclysmic variables: its spectrum contains no characteristic emission features. [ABSTRACT FROM AUTHOR]

The evolution of symbiotic X-ray binaries in the Galaxy is studied by the population synthesis method. We show that allowance for the nonstationarity of the regime of quasi-spherical subsonic accretion from the stellar wind of a giant onto slowly rotating neutron stars in these sources allows their observed positions on the neutron star spin period-X-ray luminosity diagramto be described in a wide range of stellar wind parameters. The derived distributions of sources in orbital periods, neutron star spin periods, and X-ray luminosities can be used to analyze the observations of Galactic sources in the range of luminosities ∼10-10 erg s in the planned SRG/eROSITA all-sky survey. [ABSTRACT FROM AUTHOR]

Results of a fractal analysis of the X-ray light curves from accreting black holes in low-mass binary systems are presented for the object GX 339-4 as an example. The fractal dimension of the light curves is shown to be strongly dependent on the presence of quasi-periodic oscillations (QPOs) in the observations. A correlation between the fractal dimension of the light curves and the frequency of the QPO peak has been revealed. A method supplementary to a Fourier analysis that allows the pattern of accretion disk emission as a function of time scales to be investigated is proposed. The results of this analysis can be explained if the accretion disk is separated by the QPO region into two zones with different emission parameters. [ABSTRACT FROM AUTHOR]

Results of quasi-simultaneous SWIFT and RTT-150 observations for the X-ray nova SWIFT J174510.8-262411 in May-June 2013 at the decaying phase of its outburst are presented. It is shown that the nova spectrum can be fitted in a very wide energy range (from the infrared z and i bands to hard X-rays) by a single power law attenuated due to absorption but without any traces of the presence of a soft (blackbody) component. The presence of such a component is suggested by the generally accepted models of disk accretion onto a black hole in a binary system. The observation of a single power-law spectrum may imply that synchrotron radiation from the source's relativistic jets makes a major contribution to its flux or that the accretion disk is everywhere hot, optically thin, and radiates nonthermally. [ABSTRACT FROM AUTHOR]

We present the results of measurements of the total X-ray flux from the Andromeda galaxy (M31) in the 3-100 keV band based on data from the RXTE/PCA, INTEGRAL/ISGRI, and SWIFT/BAT space experiments. We show that the total emission from the galaxy has a multicomponent spectrum whose main characteristics are specified by binaries emitting in the optically thick and optically thin regimes. The galaxy's luminosity at energies 20-100 keV gives about 6% of its total luminosity in the 3-100 keV band. The emissivity of the stellar population in M31 is L ∼ 1.1 × 10 erg s M in the 2-20 keV band and L ∼ 8 × 10 erg s M in the 20-100 keV band. Since low-mass X-ray binaries at high luminosities pass into a soft state with a small fraction of hard X-ray emission, the detection of individual hard X-ray sources in M31 requires a sensitivity that is tens of times better (up to 10 erg s cm) than is needed to detect the total hard X-ray emission from the entire galaxy. Allowance for the contribution from the hard spectral component of the galaxy changes the galaxy's effective Compton temperature approximately by a factor of 2, from ∼1.1 to ∼2.1 keV. [ABSTRACT FROM AUTHOR]

The evolution of the family of binaries with a low-mass star and a compact neutron star companion (low-mass X-ray binaries (LMXBs) with neutron stars) ismodeled by the method of population synthesis. Continuous Roche-lobe filling by the optical star in LMXBs is assumed to be maintained by the removal of orbital angular momentum from the binary by a magnetic stellar wind from the optical star and the radiation of gravitational waves by the binary. The developed model of LMXB evolution has the following significant distinctions: (1) allowance for the effect of the rotational evolution of a magnetized compact remnant on themass transfer scenario in the binary, (2) amore accurate allowance for the response of the donor star to mass loss at the Roche-lobe filling stage. The results of theoretical calculations are shown to be in good agreement with the observed orbital period-X-ray luminosity diagrams for persistent Galactic LMXBs and their X-ray luminosity function. This suggests that the main elements of binary evolution, on the whole, are correctly reflected in the developed code. It is shown that most of the Galactic bulge LMXBs at luminosities L > 10 erg s should have a post-main-sequence Roche-lobe-filling secondary component (low-mass giants). Almost all of the models considered predict a deficit of LMXBs at X-ray luminosities near ∼10 erg s due to the transition of the binary from the regime of angular momentum removal by a magnetic stellar wind to the regime of gravitational waves (analogous to the widely known period gap in cataclysmic variables, accreting white dwarfs). At low luminosities, the shape of the model luminosity function for LMXBs is affected significantly by their transient behavior-the accretion rate onto the compact companion is not always equal to the mass transfer rate due to instabilities in the accretion disk around the compact object. The best agreement with observed binaries is achieved in the models suggesting that heavy neutron stars with masses 1.4-1.9 M can be born. [ABSTRACT FROM AUTHOR]

The discovery of a new X-ray source, IGR J18175-2419, that flared up for a short (∼1 h) time on September 26, 2012, by the INTEGRAL observatory is reported. The results of the source's localization and the spectral/timing analysis of its X-ray emission are presented. The source may turn out to be yet another representative of the population of fast X-ray transients, which is the focus of attention due to the identification of their optical counterparts with early-type supergiants. [ABSTRACT FROM AUTHOR]

Increasing the identification completeness of sources from new X-ray sky surveys is a necessary condition for further works on analyzing the formation and long-term evolution of star systems in our Galaxy. Infrared observations of several sources selected from Galactic plane surveys as candidates for low-mass X-ray binaries with the IRSF telescope at the South African Astronomical Observatory are presented. The infrared fluxes have been reliably measured from five of the eight sources (4U 1556-60, 4U 1708-40, AX J165901-4208, IGR J16287-5021, IGR J17350-2045, AX J171922-3703, SAX J1712.6-3739, 4U 1705-32). One of the objects (AX J165901-4208) may be a candidate for symbiotic X-ray binaries, i.e., binaries in which the companion of a relativistic object is a giant star. The distances have been estimated for three sources and the orbital periods have been estimated for two. [ABSTRACT FROM AUTHOR]

Four hard X-ray sources from the INTEGRAL and Swift catalogs have been identified. X-ray and optical spectra have been obtained for each of the objects being studied by using data from the INTEGRAL, Swift, ROSAT, and Chandra X-ray observatories as well as observations with the RTT-150 and AZT-33IK optical telescopes. Two sources (SWIFT J1553.6+2606 and SWIFT J1852.2+8424) are shown to be extragalactic in nature: the first is a quasar, while the recordedX-ray flux from the second is the total emission from two Seyfert 1 galaxies at redshifts 0.1828 and 0.2249. The source IGR J22534+6243 resides in our Galaxy and is an X-ray pulsar with a period of ∼46.674 s that is a member of a high-mass X-ray binary with a Be star. The nature of yet another Galactic source, SWIFT J1852.8+3002, is not completely clear and infrared spectroscopy is needed to establish it. [ABSTRACT FROM AUTHOR]

Details of the observations of a new (second) outburst of the X-ray transientMAXI J1836-194 discovered late in August 2011, a suspected black hole in a low-mass binary system, with the instruments of the SWIFT and INTEGRAL orbiting observatories are presented. The outburst was weaker than the first one; the source had a power-law spectrum in a wide X-ray (0.3-400 keV) energy range without any clear evidence for the presence of a soft (blackbody) component related to the emission from the outer accretion disk regions. This shows that the outburst was a 'failed' one: the source did not pass through the sequence of spectral states characteristic of X-ray novae. The observed optical emission from the source whose variability was strongly correlated with its X-ray variability seems to have also been an extension of the power-law spectrum. Spectrum uniformity is, on the whole, unusual for other sources containing a black hole and raises the question about the nature of the emission from MAXI J1836-194 (disk or jet). [ABSTRACT FROM AUTHOR]

We present the results of Chandra and XMM-Newton observations for six hard X-ray sources (IGR J12134-6015, IGR J18293-1213, IGR J18219-1347, IGR J17350-2045, IGR J18048-1455, XTE J1901+014) from the INTEGRAL all-sky survey. Based on these observations, we have improved significantly the localization accuracy of the objects and, therefore, have managed to identify their optical counterparts. Using data from the publicly available 2MASS and UKIDSS infrared sky surveys as well as data from the SOFI/NTT telescope (European Southern Observatory), we have determined the magnitudes of the optical counterparts, estimated their types and (in some cases) the distances to the program objects. A triplet of iron lines with energies of 6.4, 6.7, and 6.9 keV has been detected in the X-ray spectrum of IGR J18048-1455; together with the detection of pulsations with a period of ∼1440 s from this source, this has allowed it to be classified as a cataclysmic variable, most likely an intermediate polar. In addition, broadband X-ray spectra of IGR J12134-6015 and IGR J17350-2045 in combination with infrared and radio observations suggest an extragalactic nature of these objects. The source IGR J18219-1347 presumably belongs to the class of high-mass X-ray binaries. [ABSTRACT FROM AUTHOR]

We present the results of our optical identifications of four hard X-ray sources from the Swift all-sky survey. We obtained optical spectra for each of the program objects with the 6-m BTA telescope (Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz), which allowed their nature to be established. Two sources (SWIFT J2237.2+6324 and SWIFT J2341.0+7645) are shown to belong to the class of cataclysmic variables (suspected polars or intermediate polars). The measured emission line width turns out to be fairly large ( FWHM ∼ 15-25 Å), suggesting the presence of extended, rapidly rotating ( v ≃ 400-600 km s) accretion disks in the systems. Apart from line broadening, we have detected a change in the positions of the line centroids for SWIFT J2341.0+7645, which is most likely attributable to the orbital motion of the white dwarf in the binary system. The other two program objects (SWIFT J0003.3+2737 and SWIFT J0113.8+2515) are extragalactic in origin: the first is a Seyfert 2 galaxy and the second is a blazar at redshift z = 1.594. Apart from the optical spectra, we provide the X-ray spectra for all sources in the 0.6-10 keV energy band obtained from XRT/Swift data. [ABSTRACT FROM AUTHOR]

We present results of optical identifications of six hard X-ray sources from the INTEGRAL and Swift all-sky surveys (IGR J03249+4041, SWIFT J1449.5+8602, SWIFT J1542.0-1410, IGR J17009+3559, IGR J18151-1052, IGR J18538-0102). Our optical observations were performed in 2009-2011 with the 6-m BTA telescope (Special Astrophysical Observatory, Nizhnii Arkhyz, Russia) and the 1.5-m RTT-150 telescope (Turkish National Observatory, Antalya, Turkey). The optical spectra obtained for each of the program sources have allowed us to establish the nature of the objects and to measure their redshifts from the positions of emission and absorption lines. Five sources are shown to be extragalactic-four of them are identified with Seyfert 1 or 2 galaxies and the fifth source belongs to the class of X-ray-bright, optically normal galaxies (XBONGs). The sixth object (IGR J18151-1052) is located in our Galaxy and is an X-ray binary (XRB), a suspected cataclysmic variable. Apart from the optical spectra, we provide the X-ray spectra for five sources in the 0.6-10 keV energy band obtained from XRT/Swift data. [ABSTRACT FROM AUTHOR]

In the course of our search for the optical identifications of ROSAT X-ray sources, we have found a highly variable object with a very unusual behavior on long time scales, rare color indices, and a high X-ray-to-optical flux ratio. We present the archival light curve from the Catalina Sky Survey, optical spectroscopy from RTT150, and time-resolved photometry from the Astrotel-Caucasus telescope. The object appears to be a magnetic cataclysmic variable (a polar) with an orbital period P = 3.20 h. [ABSTRACT FROM AUTHOR]

a result of applying the original optical variability search method on publicly available data, we have found eight new cataclysmic variables and two possible Optically Violent Variable quasars among the previously unidentified X-ray sources in the ROSAT catalog. We describe the search method and present the characteristics of the newly identified variable stars. The obtained results demonstrate the large potential of the concept of Virtual Observatory for identifying new objects of astrophysical interest. [ABSTRACT FROM AUTHOR]

The problem of disk accretion onto the surface of a neutron star with a weak magnetic field at a luminosity exceeding several percent of the Eddington one is reduced to the problem of the braking of a hypersonic flow with a velocity that is 0.4-0.5 of the speed of light above the base of the spreading layer-a dense atmosphere made up of previously fallen matter. We show that turbulent braking in the Prandtl-Karman model with universally accepted coefficients for terrestrial conditions and laboratory experiments and a ladder of interacting gravity waves in a stratified quasi-exponential atmosphere at standard Richardson numbers lead to spin-up of the massive zone that extends to the 'ocean' made up of a plasma with degenerate electrons. Turbulent braking in the ocean at the boundary with the outer solid crust reduces the rotation velocity to the solid-body rotation velocity of the star. This picture should lead to strong heating of deep atmospheric layers and to the switch-off of the explosive helium burning mechanism. Obviously, a more efficient mechanism for the dissipation of a strong azimuthal flow in the atmosphere should operate in X-ray bursters. We show that a giant solitary gravity wave in the atmosphere can lead to energy dissipation and to a sharp, abrupt decrease in azimuthal velocity in fairly rarefied atmospheric layers above the zone of explosive helium burning nuclear reactions. We discuss the reasons why this wave that has no direct analog in the Earth's atmosphere and ocean appears and its stability. We pose the question as to whether neutron stars with massive atmospheres spun up to high velocities by accreting matter from a disk can exist among the observed Galactic X-ray sources. [ABSTRACT FROM AUTHOR]

Recently, it has been shown that the cataclysmic variable EI UMa should be attributed to intermediate polars on the basis of its X-ray spectrum. However, no short-period oscillations with a high degree of coherence typical of intermediate polars could be detected in this star. Therefore, it was suggested to call it a “hidden” intermediate polar. Using a multichannel photometer and a 70-cm telescope, we performed observations of EI UMa for 60 h. Analysis of the observations has clearly revealed a brightness oscillation with a period of 769.63±0.10 s and a mean semiamplitude of 0008 completely coherent during one month of our observations that was detectable during each of the 13 observing nights. On the basis of this detection, EI UMa should be considered a classical intermediate polar rather than a hidden one. In addition, we have managed to detect the orbital variability of EI UMa for the first time. The orbital light curve is extremely interesting, showing evidence of partial eclipses of the accretion flow. [ABSTRACT FROM AUTHOR]

Details of the discovery of a new X-ray source, IGR J18462-0223, on October 12, 2007, during a short (several hours), intense (∼35 mCrab at the peak) outburst of hard radiation by the IBIS/ISGRI gamma-ray telescope onboard the INTEGRAL observatory are given. The detection of another earlier outburst from this source occurred on April 28, 2006, in the archival data of the telescope is reported. We present the results of the source’s localization and our spectral/timing analysis of the observational data. The source may turn out to be yet another representative of the continuously growing population of fast X-ray transients, which are the focus of attention because of the identification of their optical counterparts with early-type supergiants. [ABSTRACT FROM AUTHOR]

We present the results of our optical identification of the hard X-ray source IGR J18257-0707 through its spectrophotometric observations with the optical RTT-150 telescope. The accurate position of the X-ray source determined using Chandra observations has allowed this source to be confidently associatedwith a faint optical object ( m R ≈ 20.4) whose optical spectrumexhibits a broad H α emission line at redshift z = 0.037. Thus, the source IGR J18257-0707 is a type-1 Seyfert galaxy at redshift z = 0.037. [ABSTRACT FROM AUTHOR]

Results of the optical identification of the hard X-ray source IGR J08390-4833 recently discovered in the INTEGRAL all-sky survey are presented. We show that the source is most likely a cataclysmic variable, i.e., an accreting white dwarf in a binary. Analysis of its optical light curve clearly reveals intrinsic variability on timescales of the order of an hour or longer. However, the short time of the source’s optical observations does not allow a definitive conclusion about the periodicity of the detected variability to be reached. Further optical and X-ray observations are required for a more accurate classification of the source. [ABSTRACT FROM AUTHOR]

We describe the cooling theory for isolated neutron stars that are several tens of years old. Their cooling differs greatly from the cooling of older stars that has been well studied in the literature. It is sensitive to the physics of the inner stellar crust and even to the thermal conductivity of the stellar core, which is never important at later cooling stages. The absence of observational evidence for the formation of a neutron star during the explosion of Supernova 1987A is consistent with the fact that the star was actually born there. It may still be hidden in the dense center of the supernova remnant. If, however, the star is not hidden, then it should have a low thermal luminosity (below ∼1034 erg s−1) and a short internal thermal relaxation time (shorter than 13 yr). This requires that the star undergo intense neutrino cooling (e.g., via the direct Urca process) and have a thin crust with strong superfluidity of free neutrons and/or an anomalously high thermal conductivity. [ABSTRACT FROM AUTHOR]

The results of optical identifications of five hard X-ray sources in the Galactic plane from the INTEGRAL all-sky survey are presented. The X-ray data on one source (IGR J20216+4359) are published for the first time. The optical observations were performed with the 1.5-m RTT-150 telescope (Turkish National Observatory, Antalya, Turkey) and the 6-m BTA telescope (Special Astrophysical Observatory, Nizhny Arkhyz, Russia). A blazar, three Seyfert galaxies, and a high-mass X-ray binary are among the identified sources. [ABSTRACT FROM AUTHOR]