arXiv subjects

I. F. Bikmaev

Publications and source records attributed to I. F. Bikmaev.

At least 19 recordsLinked to original sources

arXiv2026

Study of Massive OBA Stars with X-ray Emission

A study of the physical parameters of a sample of 15 OBA-type stars with detected X-ray emission from the Spektr-RG/eROSITA telescope is presented. While X-ray emission from cool stars (spectral types F-G-K-M) originates in their near-surface regions, namely in the chromosphere and corona, the origin of X-ray emission in OBA stars requires case-by-case analysis since isolated OBA stars are not intrinsic X-ray emitters. In this work, we derive the fundamental parameters of the stars in our sample, including the effective temperature $T_{\mathrm{eff}}$ and surface gravity $\log g$, based on spectral energy distribution fitting and optical spectroscopy obtained with the 1.5-m Russian-Turkish telescope RTT-150. An additional analysis of the $\mathrm{H}α$ line profiles allows us to identify possible mechanisms responsible for the observed X-ray emission, including non-stationary stellar winds, interactions in circumstellar material, and coronal emission from hidden cool companions. We find that the X-ray emission in eight stars, with typical luminosities in the range $\log L_{\mathrm{X}} = 28.5$-$30.0$, is most likely associated with hidden late-type companions.

astro-ph.SR↗

arXiv2026

SRG/eROSITA-SDSS view on the relation between X-ray and UV emission for quasars

Motivated by the idea of using quasars as standardizable candles for cosmology, we examine the relation between X-ray (at 2 keV, $L_{\rm 2keV}$) and ultraviolet (at 2500 Angstrom, $L_{\rm 2500}$) monochromatic luminosities of quasars using a sample of 2414 X-ray sources from the SRG/eROSITA all-sky survey cross-matched with the Sloan Digital Sky Survey data release 16 quasar catalogue (SDSS DR16Q), at redshifts between 0.5 and 2.5. These objects are bright both in X-rays and in the optical, so that the sample is characterized by nearly 100% statistical completeness. We have developed a new method for determining the $L_{\rm 2keV}-L_{\rm 2500}$ relation, which consistently takes into account (i) X-ray and UV flux limited object selection, (ii) X-ray and UV variability of quasars, and (iii) the decreasing space density of quasars with increasing luminosity. Assuming a linear relation between $l_{\rm X}\equiv\log(L_{\rm 2keV}/[{\rm erg\,s^{-1}\,Hz^{-1}}])$ and $l_{\rm UV}\equiv\log(L_{\rm 2500}/[{\rm erg\,s^{-1}\,Hz^{-1}}])$, we find the slope, $γ=0.69\pm0.02$ (hereafter all uncertainties are quoted at the 68% confidence level), and normalization, $l_{\rm X}=26.47\pm0.02$ at $l_{\rm UV}=30.5$, of the $L_{\rm 2keV}$ ($L_{\rm 2500}$) dependence. These values are not substantially different from the results of previous studies. A key novel aspect of our work is allowance for intrinsic scatter (which adds to the dispersion induced by quasar variability and flux measurement uncertainties) of the $L_{\rm 2keV}-L_{\rm 2500}$ relation in both variables, i.e. in X-ray and UV luminosity. The intrinsic X-ray scatter ($σ^2_{\rm intX}=0.063\pm0.005$) strongly dominates over the UV one ($σ^2_{\rm intUV}=0.002^{+0.003}_{-0.002}$). Further studies should seek to explain this behaviour in terms of accretion onto supermassive black holes and orientation of quasars with respect to the observer.

astro-ph.HE↗

arXiv2025

Is the Fermi source 4FGL J1824.2+1231 a transitional millisecond pulsar?

Transitional millisecond pulsars (tMSPs) in tight binary systems represent an important evolutionary link between low-mass X-ray binaries and radio millisecond pulsars. To date, only three confirmed tMSPs and a few candidates have been discovered. Most of them are gamma-ray sources. For this reason, searching for multiwavelength counterparts to unassociated Fermi gamma-ray sources can help to find new tMSPs. Here we investigate whether the unassociated gamma-ray source 4FGL J1824.2+1231 belongs to the tMSP family. To find the counterpart to 4FGL J1824.2+1231, we used data from SRG/eROSITA and Swift X-ray catalogues, and from different optical catalogues. We also performed time-series photometric optical observations of the source with the 2.1-m telescope of the Observatorio Astronomico Nacional San Pedro Martir, the 1.5-m telescope of the Maidanak Astronomical Observatory and the 1.5-m Russian-Turkish telescope. In addition, we carried out optical spectroscopic observations with the Russian-Turkish telescope and used archival spectroscopic data obtained with the Gemini-North telescope. Within the position error ellipse of 4FGL J1824.2+1231, we found only one X-ray source which coincides with an optical object. We consider it as a likely multiwavelength counterpart to 4FGL J1824.2+1231. The source shows strong optical variability and significant proper motion. The latter strongly implies that this is a Galactic source. Double-peaked H and He emission lines are detected in its spectrum with a flat continuum, as often observed in accretion disks of compact binary systems. The X-ray spectrum is well fitted by a power law with the photon index 1.7. The derived intrinsic X-ray-to-gamma-ray flux ratio is about 0.2. If the X-ray/optical source is the true counterpart to 4FGL J1824.2+1231, then all its properties suggest that it is a tMSP in the subluminous disk state.

astro-ph.HE↗

arXiv2025

850 SRG/eROSITA X-ray sources associated with Pleiades stars

Using data from the SRG/eROSITA all-sky X-ray survey and the GAIA-based catalog of 2,209 members of the Pleiades open star cluster, we found 850 X-ray sources associated with the cluster stars. Over 650 of them were detected in X-rays for the first time. At the distance of the Pleiades, the nominal sensitivity of eROSITA corresponds to a luminosity of $L_X \sim 1.6 \cdot 10^{28}$ erg/s in the 0.3-2.3 keV band. The eROSITA sources associated with Pleiades stars have a total luminosity of $L_{X,tot} \sim 1.3 \cdot 10^{32}$ erg/s , a million times greater than the X-ray luminosity of the quiet Sun. Strong X-ray variability, more than 10 times, was recorded for 27 sources. Most of them are known as eruptive optical variables of the dM class. The value of $R_X=log(L_X/L_{bol})$ increases with decreasing effective temperature of the star from $R_X\approx -5$ to $R_X\approx -2$. The distribution of stars over $R_X$ is bimodal, with the left peak at $R_X\sim-4.3$ being formed by stars of FGK classes, and the right peak at $R_X\sim-3.1$ being mainly populated by M-stars. The relation between $R_X$ and the Rossby number $Ro$ depends on the spectral class. For K- and M- stars, at low Rossby numbers $R_X\sim -3$ and depends weakly on $Ro$. At $Ro \gt 0.25$, a rapid drop in $R_X$ is observed for K stars, while in our sample there are no M stars with large Rossby number. Most of F- and G- stars appear to have smaller $R_X\sim -4.5$, however, our sample size is insufficient for a more detailed characterization of their $R_X-Ro$ dependence.

astro-ph.HE↗

arXiv2024

SRGe J194401.8+284452 -- an X-ray Cataclysmic Variable in the Field of the Gamma-Ray Source 4FGL J1943.9+2841

SRGe J194401.8+284452 is the brightest point-like X-ray object within the position uncertainty ellipse of an unidentified $γ$-ray source 4FGL J1943.9+2841. We performed multi-wavelength spectral and photometric studies to determine its nature and possible association with the $γ$-ray source. We firmly established its optical counterpart with the Gaia based distance of about 415 pc. Our data show that the object is a cataclysmic variable with an orbital period of about 1.5 hours. SRGe J194401.8+284452 exhibits fast spontaneous transitions between the high and low luminosity states simultaneously in the optical and X-rays, remaining relatively stable between the transitions on scales of several months/years. This can be caused by an order of magnitude changes in the accretion rate. The brightness of the source is about 17 mag and 20 mag in the optical range and $5\times 10^{-12}$ and $5\times 10^{-13}$ erg/cm$^2$/s in the $0.3- 10$ keV range in the high and low states, respectively. We constrained the mass of the white dwarf ($0.3 - 0.9$ $M_\odot$) and its temperature in the low state (14750 $\pm$ 1250 K), the mass of the donor star ($\leq$ 0.08 $\pm$ 0.01 $M_ \odot$). In the low state, we detected regular optical pulsations with an amplitude of 0.2 mag and a period of 8 min. They are likely associated with the spin of the white dwarf, rather than with its non-radial pulsations. In the high state, the object demonstrates only stochastic optical brightness variations on time scales of $1 - 15$~minutes with amplitudes of $0.2 - 0.6$~mag. We conclude, that SRGe J194401.8+284452 can be classified as an intermediate polar, and its association with the $γ$-ray source is very unlikely.

astro-ph.HE↗

arXiv2024

Merger of massive galaxy cluster CL0238.3+2005 at z~0.4: just after pericenter passage?

Massive clusters of galaxies are very rare in the observable Universe. Even rarer are mergers of such clusters observed close to pericenter passage. Here, we report on one such case: a massive (~ $10^{15}\,M_\odot$) and hot (kT ~ 10 keV) cluster CL0238.3+2005 at $z\approx 0.42$. For this cluster, we combine X-ray data from SRG/eROSITA and Chandra, optical images from DESI, and spectroscopy from BTA and RTT-150 telescopes. The X-ray and optical morphologies suggest an ongoing merger with the projected separation of subhalos of $\sim 200$ kpc. The line-of-sight velocity of galaxies tentatively associated with the two merging halos differs by 2000-3000 km/s. We conclude that, most plausibly, the merger axis is neither close to the line of sight nor to the sky plane. We compare CL0238 with two well-known clusters MACS0416 and Bullet, and conclude that CL0238 corresponds to an intermediate phase between the pre-merging MACS0416 cluster and the post-merger Bullet cluster. Namely, this cluster has recently (only $\lesssim 0.1$ Gyr ago) experienced an almost head-on merger. We argue that this "just after" system is a very rare case and an excellent target for lensing, Sunyaev-Zeldovich effect, and X-ray studies that can constrain properties ranging from dynamics of mergers to self-interacting dark matter, and plasma effects in intracluster medium that are associated with shock waves, e.g., electron-ion equilibration efficiency and relativistic particle acceleration.

astro-ph.CO↗

arXiv2024

Optical Identification and Spectroscopic Redshift Measurements of 216 Galaxy Clusters from the SRG/eROSITA All-Sky Survey

We present the results of the optical identification and spectroscopic redshift measurements of 216 galaxy clusters detected in the SRG/eROSITA all-sky X-ray survey. The spectroscopic observations were performed in 2020-2023 with the 6-m BTA telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences, the 2.5-m telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University, the 1.6-m AZT-33IK telescope at the Sayan Solar Observatory of the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences, and the 1.5-m Russian-Turkish telescope (RTT-150) at the TÜBİTAK Observatory. For all of the galaxy clusters presented here the spectroscopic redshift measurements have been obtained for the first time. Of these, 139 galaxy clusters have been detected for the first time in the SRG/eROSITA survey and 22 galaxy clusters are at redshifts $z_{spec} \gtrsim 0.7$, including three at $z_{spec} \gtrsim 1$. Deep direct images with the rizJK filters have also been obtained for four distant galaxy clusters at $z_{spec} > 0.7$. For these observations the most massive clusters are selected. Therefore, most of the galaxy clusters presented here most likely will be included in the cosmological samples of galaxy clusters from the SRG/eROSITA survey.

astro-ph.GA↗

arXiv2024

X-Ray Variability of SDSS Quasars Based on the SRG/eROSITA All-Sky Survey

We examine the long-term (rest-frame time scales from a few months to $\sim 20$ years) X-ray variability of a sample of 2344 X-ray bright quasars from the SDSS DR14Q Catalogue, based on the data of the SRG/eROSITA All-Sky Survey complemented for $\sim 7$% of the sample by archival data from the XMM-Newton Serendipitous Source Catalogue. We characterise variability by a structure function, $SF^2(Δt)$. We confirm the previously known anti-correlation of the X-ray variability amplitude with luminosity. We also study the dependence of X-ray variability on black hole mass, $M_{\rm BH}$, and on an X-ray based proxy of the Eddington ratio, $λ_{\rm X}$. Less massive black holes prove to be more variable for given Eddington ratio and time scale. X-ray variability also grows with decreasing Eddington ratio and becomes particularly strong at $λ_{\rm X}$ of less than a few per cent. We confirm that the X-ray variability amplitude increases with increasing time scale. The $SF^2(Δt)$ dependence can be satisfactorily described by a power law, with the slope ranging from $\sim 0$ to $\sim 0.4$ for different ($M_{\rm BH}$, $λ_{\rm X}$) subsamples (except for the subsample with the lowest black hole mass and lowest Eddington ratio, where it is equal to $1.1\pm 0.4$)

astro-ph.HE↗

arXiv2023

Transient events in the near-nuclear regions of AGNs and quasars as the sources of the proper motion imitations

We present a sample of SRG/eROSITA X-ray sources located in the eastern Galactic hemisphere (0<l<180 deg), with significant proper motions according to GAIA eDR3 measurements and whose extragalactic nature has been confirmed. The catalog consists of 248 extragalactic sources with spectroscopically measured redshifts. It includes all objects available in the Simbad database and matched to the identified optical component within a radius of 0.5 arcsec. Additionally, the catalog includes 18 sources with the spectral redshift measurements based on observations at the Russian-Turkish 1.5-m telescope RTT-150. The sources of 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 (previously known in astrometry as the VIM effect) by the presence of transient events on the line of sight in the field of view of AGN nuclei and quasars (within the GAIA resolution element). Such astrophysical phenomena may be the supernovae outbursts, tidal destruction events in AGNs with double nuclei, variability of large-mass supergiants, the presence of O-B associations in field of view of variable brightness AGN, etc. A model of flares with a fast rise and exponential decay profile allows to describe the variable positional parameters of most similar sources observed in GAIA. This cross-matching approach of the X-ray source catalogs of the SRG/eROSITA observatory and the optical catalog of the GAIA observatory can be used as an independent technique for detecting transient events in the neighborhood of AGN core (on scales of several hundred parsecs in the picture plane).

astro-ph.HE↗

arXiv2023

Optical study of the polar BM CrB in low accretion state

This paper presents a spectral and photometric study of the poorly studied polar BM CrB. Three states of the polar brightness and signs of transition from one-pole to two-pole accretion mode were found by an analysis of ZTF data. It is shown that the transition from the low state to the high state changes the longitude of the main accretion spot (by $\approx 17^{\circ}$) and increases its elongation (by $\approx 10^{\circ}$). The spectra contain Zeeman absorptions of the H$α$ line which are formed at a magnetic field strength of $15.5\pm1$ MG. These absorptions are likely produced by a cold halo extending from the accretion spot at $\approx {^1/_4}$ of the white dwarf radius. Modeling of the behavior of the H$α$ emission line shows that the main source of emission is the part of the accretion stream near the Lagrangian point L$_1$, which is periodically eclipsed by the donor star. The spectra exhibit a cyclotron component formed in the accretion spot. Its modeling by a simple accretion spot model gives constraints on the magnetic field strength $B=15-40$ MG and the temperature $T_e\gtrsim15$ keV.

astro-ph.HE↗

arXiv2023

Detection of AGNs and quasars having significant proper motions according to Gaia data within SRG/eRosita X-Ray sources catalog

Based on a comparison of the SRG/eROSITA catalog of X-ray active stars and the Gaia catalog, a sample of 502 peculiar objects was obtained for which Gaia, on one hand, detects statistically significant values of parallax or proper motion and, on the other hand, registers signs of the non zero source extent in the optical band. In the log ($F_X/F_{\rm opt}$) - (G-RP) color diagram these objects are separated from the balk of X-ray active stars and are located in the region typical for the galaxies with active nuclei. According to the SIMBAD database, about $\sim 50$% of them are confirmed AGNs and galaxies with spectroscopically measured redshifts, and only $\sim$1.4% are confirmed Galactic objects. Spectroscopic observations of 19 unidentified objects on the RTT-150 telescope demonstrated, that 18 of them are AGNs at redshifts $\sim$0.01-0.3, and one object is a M star in our Galaxy. We discuss various scenarios explaining the nature of such peculiar objects.

astro-ph.HE↗

arXiv2022

SRGe2149+6736 -- the new candidate to AM~Her type variables discovered by eROSITA telescope on "Spectrum--Roentgen--Gamma" orbital observatory

We present the results of the optical identification, classification, as well as analysis of photometric and spectral observations of the X-ray transient SRGe2149+6736 detected by the eROSITA telescope during SRG all-sky X-ray survey. Photometric observations of the optical companion of SRGe2149+6736 were carried out on 6m telescope BTA SAO RAS, 1.5m Russian-Turkish telescope RTT-150 and 2.5m telescope CMO of Moscow State University. Together with ZTF data they showed that the source is a cataclysmic variable with an orbital period $P=85\pm0.4$~min which demonstrates long-term brightness variability from $23.5$~mag (low state) to $20$~mag (high state). The high-state light curves are consistent with a model of accreting magnetic white dwarf and suggest that SRGe2149+6736 belongs to AM~Her type variables. The optical spectra obtained in the low state are consistent with a spectral energy distribution of a white dwarf with a temperature of ~24000 K.

astro-ph.HE↗

arXiv2022

Mass estimation of the very massive galaxy cluster SRGe CL2305.2$-$2248 from strong lensing

The galaxy cluster SRGe CL2305.2$-$2248 (SPT-CL J2305$-$2248, ACT-CL J2305.1$-$2248) is one of the most massive clusters at high redshifts ($z \simeq 0.76$) and is of great interest for cosmology. For an optical identification of this cluster, deep images were obtained with the 1.5-m Russian-Turkish telescope RTT-150. Together with the open archival data of the Hubble Space Telescope, it became possible to identify candidates for gravitationally lensed images of distant blue galaxies in the form of arcs and arclets. The observed giant arc near the brightest cluster galaxies allowed us to estimate the radius of the Einstein ring, which is $ 9.8 \pm 1.3 $ arcseconds. The photometric redshift of the lensed source was obtained ($ z_s = 2.44 \pm 0.07 $). Its use in combination with the Einstein radius estimate made it possible to independently estimate the \cl2305 mass. It was done by extrapolating the strong lensing results to large radii and using the model density distribution profiles in relaxed clusters. This extrapolation leads to mass estimates $ \sim 1.5-3 $ times smaller than those obtained from X-ray and microwave observations. A probable cause for this discrepancy may be the process of cluster merging, which is also confirmed by SRGe CL2305.2-2248 morphology in the optical range.

astro-ph.CO↗

arXiv2021

First tidal disruption events discovered by SRG/eROSITA: X-ray/optical properties and X-ray luminosity function at z<0.6

We present the first sample of tidal disruption events (TDEs) discovered during the SRG all-sky survey. These 13 events were selected among X-ray transients detected in the 0<l<180 deg hemisphere by eROSITA during its second sky survey (10 June - 14 December 2020) and confirmed by optical follow-up observations. The most distant event occurred at z=0.581. One TDE continued to brighten at least 6 months. The X-ray spectra are consistent with nearly critical accretion onto black holes of a few 10^3 to 10^8 M_Sun, although supercritical accretion is possibly taking place. In two TDEs, a spectral hardening is observed 6 months after the discovery. Four TDEs showed an optical brightening apart from the X-ray outburst. The other 9 TDEs demonstrate no optical activity. All 13 TDEs are optically faint, with Lopt/Lx<0.3 (Lopt and Lx being the g-band and 0.2-6 keV luminosity, respectively). We have constructed a TDE X-ray luminosity function, which can be fit by a power law with a slope of -0.6+/-0.2, similar to the trend observed for optically selected TDEs. The total rate is estimated at (1.1+/-0.5)10^-5 TDEs per galaxy per year, an order of magnitude lower than inferred from optical studies. This suggests that X-ray bright events constitute a minority of TDEs, consistent with models predicting that X-rays can only be observed from directions close to the axis of a thick accretion disk formed from the stellar debris. Our TDE detection threshold can be lowered by a factor of ~2, which should allow a detection of ~700 TDEs by the end of the SRG survey.

astro-ph.HE↗

arXiv2021

Observation of a very massive galaxy cluster at z=0.76 in SRG/eROSITA all-sky survey

The results of multiwavelength observations of the very massive galaxy cluster SRGe CL2305.2-2248 detected in X-rays during the first SRG/eROSITA all-sky survey are discussed. This galaxy cluster was also detected earlier in microwave band through the observations of Sunyaev-Zeldovich effect in South Pole Telescope (SPT-CL J2305-2248), and in Atacama Cosmological Telescope (ACT-CL J2305.1-2248) surveys. Spectroscopic redshift measurement, $z=0.7573$, was measured at the Russian 6-m BTA telescope of SAO RAS, in good agreement with its photometric estimates, including a very accurate one obtained using machine learning methods. In addition, deep photometric measurements were made at the Russian-Turkish 1.5-m telescope (RTT150), which allows to study cluster galaxies red sequence and projected galaxies distribution. Joint analysis of the data from X-ray and microwave observations show that this cluster can be identified as a very massive and distant one using the measurements of its X-ray flux and integral comptonization parameter only. The mass of the cluster estimated according to the eROSITA data is $M_{500}=(9.0\pm2.6)\cdot10^{14}\, M_\odot$. We show that this cluster is found among of only several dozen of the most massive clusters in the observable Universe and among of only a few the most massive clusters of galaxies at $z>0.6$.

astro-ph.CO↗

arXiv2021

SRG/ART-XC discovery of SRGA J204318.2+443815: towards the complete population of faint X-ray pulsars

We report a discovery of a new long-period X-ray pulsar SRGA J204318.2+443815/SRGe J204319.0+443820 in the Be binary system. The source was found in the second all-sky survey by the Mikhail Pavlinsky telescope on board the SRG mission. The follow-up observations with XMM-Newton, NICER and NuSTAR observatories allowed us to discover a strong coherent signal in the source light curve with the period of $\sim742$ s. The pulsed fraction was found to depend on the energy increasing from $\sim20$% in soft X-rays to $>50$% at high energies, as it is typical for X-ray pulsars. The source demonstrate a quite hard spectrum with an exponential cutoff at high energies and bolometric luminosity of $L_X \simeq 4\times10^{35}$ erg/s. Dedicated optical and infrared observations with the RTT-150, NOT, Keck and Palomar telescopes revealed a number of emission lines (H$_α$, HeI, Pashen and Braket series) with the strongly absorbed continuum. All of above suggests that SRGAJ204318.2+443815/ SRGeJ204319.0+443820 is a new persistent low luminosity X-ray pulsar in a distant binary system with a Be-star of the B0-B2e class. Thus the SRG observatory allow us to unveil the hidden population of faint persistent objects including the population of slowly rotating X-ray pulsars in Be systems.

astro-ph.HE↗

arXiv2021

Optical identification of active galactic nucleus candidates detected by the Mikhail Pavlinsky ART-XC telescope aboard the SRG observatory during the all-sky X-ray survey

We present the results of our identification of eight objects from a preliminary catalogue of X-ray sources detected in the 4-12 keV energy band by the Mikhail Pavlinsky ART-XC telescope aboard the SRG observatory during its first all-sky survey. Three of them (SRGAJ005751.0+210846,SRGAJ014157.0-032915, SRGAJ232446.8+440756) have been discovered by ART-XC, while five were already known previously as X-ray sources, but their nature remained unknown. The last five sources have also been detected in soft X-rays by the eROSITA telescope of the SRG observatory. Our optical observations were carried out at the 1.6-m AZT-33IK telescope of the Sayan Observatory and the 1.5-m Russian-Turkish telescope (RTT-150). All of the investigated objects have turned out to be active galactic nuclei (AGNs) at redshifts from 0.019 to 0.283. Six of them are Seyfert 2 galaxies(including one Seyfert 1.9 galaxy), one (SRGAJ005751.0+210846) is a "hidden" AGN (in an edge-on galaxy), and one (SRGAJ224125.9+760343) is a narrow-line Seyfert 1 galaxy. The latter object is characterized by a high X-ray luminosity (~(2-13)*10^44 erg/s in the 4-12 keV band) and,according to our black hole mass estimate (~2*10^7 M_sun), an accretion rate close to the Eddington limit. All three AGNs discovered by the ART-XC telescope (which are not detected by the eROSITA telescope) are characterized by a high absorption column density (N_H > 3*10^23 cm^-2). The results obtained confirm the expectations that the ART-XC telescope is an efficient instrument for searches of heavily obscured and other interesting AGNs in the nearby (z<0.3) Universe. The SRG all-sky survey will last for more than 3 years more, which will allow many such objects to be discovered.

astro-ph.GA↗

arXiv2020

Modeling of 35-day superorbital cycle of $B$ and $V$ light curves of IMXB HZ Her/Her X-1

The X-ray binary Her X-1 consists of an accreting neutron star and the optical component HZ Her. The 35-day X-ray superorbital variability of this system is known since its discovery in 1972 by the Uhuru satellite and is believed to be caused by forced precession of a warped accretion disk tilted to the orbital plane. We argue that the observed features of the 35-day optical variability of HZ Her can be explained by free precession of the neutron star with a period close to that of the forced disk. The model parameters include a) the X-ray luminosity of the neutron star; b) the optical flux from the accretion disk; c) the tilt of the inner and outer edges of the accretion disk. A possible synchronization mechanism based on the coupling between the neutron star free precession and the dynamical action of non-stationary gas streams is discussed.

astro-ph.HE↗