The census of massive clusters of galaxies in the local Universe is almost complete, thanks to their prominent observational signatures at X-ray, optical, and sub-mm wavelengths. Nevertheless, a number of such systems are likely to be missing and hidden behind the plane of our Galaxy, where high interstellar absorption as well as strong contamination by foreground stellar and diffuse sources prevent detection of even the brightest and the most massive ones. Here we report the discovery and multiwavelength characterization of such a cluster in the zone of avoidance (ZoA) SRGe CL0512.7+3712 in the data of SRG/eROSITA all-sky survey. Combining the data of radio, optical, and infrared surveys, we identify overdensity of possible red sequence galaxies, as well as the candidate brightest cluster galaxy. Follow-up optical and X-ray observations confirm that the newly found object is a massive (M_500c=(4-5)· 10^14M_⊙, kT≈ 5 keV) galaxy cluster at redshift z=0.0745 with possible indications of unrelaxed dynamical scale. Location and elongation of this cluster is consistent with an expectation from the large-scale structure at this redshift, and it might be a part of an extended overdensity of such objects in the Galactic Anticenter direction. Examination of X-ray, radio, and infrared data in the locations of ZoA, where similar objects are expected to be found based on the large-scale structure properties, might reveal another ∼10 clusters at this redshift in future.

We report results of optical identification and multi-wavelength study of a new polar-type magnetic cataclysmic variable (MCV), SRGA J213151.5+491400, discovered by Spectrum Roentgen-Gamma (SRG) observatory in the course of the all-sky survey. We present optical data from telescopes in Turkey (RTT-150 and T100 at the TÜBITAK National Observatory), and in Russia (6-m and 1-m at SAO RAS), together with the X-ray data obtained with ART-XC and eROSITA telescopes aboard SRG and the NICER observatory. We detect SRGA J213151.5+491400 in a high state in 2020 (17.9 mag) that decreases about 3 mag into a low state (21 mag) in 2021. We find only one significant period using optical photometric time series analysis which reveals the white dwarf spin/orbital period to be 0.059710(1) days (85.982 min). The long slit spectroscopy in the high state yields a power law continuum increasing towards the blue with a prominent He II line along with the Balmer line emissions with no cyclotron humps; consistent with MCV nature. Doppler Tomography confirms the polar nature revealing ballistic stream accretion along with magnetic stream during the high state. These characteristics show that the new source is a polar-type MCV. SRG ART-XC detections yield an X-ray flux of (4.0-7.0)×10^-12 erg cm^2 s^-1 in the high state. eROSITA detects a dominating hot plasma component (kT_max > 21 keV in the high state) declining to (4.0-6.0)×10^-13 erg cm^2 s^-1 in 2021 (low state). The NICER data obtained in the low state reveal a two-pole accretor showing a soft X-ray component at (6-7)σ significance with a blackbody temperature of 15-18 eV. A soft X-ray component has never been detected for a polar in the low state before.

Context. The paper is comprised of optical identification and multiwavelength studies of a new X-ray source discovered by the Spectrum Roentgen-Gamma (SRG) observatory during the ART-XC survey and its follow-up optical and X-ray observations. Aims. We aim to identify SRGA J213151.5+491400 in the optical wavelengths. We determine spectra and light curves in the optical high and low states to find periodicities in the light curves and resolve emission lines in the system using optical ground-based data. We intend to study the spectral and temporal X-ray characteristics of the new source using the SRG surveys in the high and low states and NICER data in the low state. Methods. We present optical data from telescopes in Türkiye (RTT-150 and T100 at the TÜBİTAK National Observatory) and in Russia (6-m and 1-m at SAO RAS), together with the X-ray data obtained with ART-XC and eROSITA telescopes aboard SRG and the NICER observatory. Using the optical data, we performed astrometry, photometry, spectroscopy, and power spectral analysis of the optical time series. We present optical Doppler tomography along with X-ray data analysis producing light curves and spectra. Results. We detected SRGA J213151.5+491400 in a high state in 2020 (17.9 mag) that decreased by about 3 mag into a low state (21 mag) in 2021. We find only one significant period using optical photometric time series analysis, which reveals the white dwarf spin (orbital) period to be 0.059710(1) days (85.982 min). The long slit spectroscopy in the high state yields a power-law continuum increasing towards the blue with a prominent He II line along with the Balmer line emissions with no cyclotron humps, which is consistent with a magnetic cataclysmic variable (MCV) nature. Doppler Tomography confirms the polar nature revealing ballistic stream accretion along with magnetic stream during the high state. These characteristics show that the new source is a polar-type MCV source. ART-XC detections yield an X-ray flux of (4.0−7.0) × 10−12 erg s−1 cm−2 in the high state. eROSITA detects a dominating hot plasma component (kTmax > 21 keV in the high state) declining to (4.0−6.0) × 10−13 erg s−1 cm−2 in 2021 (low state). The NICER data obtained in the low state reveal a two-pole accretor showing a soft X-ray component at (6−7)σ significance with a blackbody temperature of 15−18 eV. A soft X-ray component has never been detected for a polar in the low state before.

Context. The paper is comprised of optical identification and multiwavelength studies of a new X-ray source discovered by the Spectrum Roentgen-Gamma (SRG) observatory during the ART-XC survey and its follow-up optical and X-ray observations. Aims. We aim to identify SRGA J213151.5+491400 in the optical wavelengths. We determine spectra and light curves in the optical high and low states to find periodicities in the light curves and resolve emission lines in the system using optical ground-based data. We intend to study the spectral and temporal X-ray characteristics of the new source using the SRG surveys in the high and low states and NICER data in the low state. Methods. We present optical data from telescopes in T & uuml;rkiye (RTT-150 and T100 at the T & Uuml;B & Idot;TAK National Observatory) and in Russia (6-m and 1-m at SAO RAS), together with the X-ray data obtained with ART-XC and eROSITA telescopes aboard SRG and the NICER observatory. Using the optical data, we performed astrometry, photometry, spectroscopy, and power spectral analysis of the optical time series. We present optical Doppler tomography along with X-ray data analysis producing light curves and spectra. Results. We detected SRGA J213151.5+491400 in a high state in 2020 (17.9 mag) that decreased by about 3 mag into a low state (21 mag) in 2021. We find only one significant period using optical photometric time series analysis, which reveals the white dwarf spin (orbital) period to be 0.059710(1) days (85.982 min). The long slit spectroscopy in the high state yields a power-law continuum increasing towards the blue with a prominent He II line along with the Balmer line emissions with no cyclotron humps, which is consistent with a magnetic cataclysmic variable (MCV) nature. Doppler Tomography confirms the polar nature revealing ballistic stream accretion along with magnetic stream during the high state. These characteristics show that the new source is a polar-type MCV source. ART-XC detections yield an X-ray flux of (4.0-7.0) x 10-12 erg s-1 cm-2 in the high state. eROSITA detects a dominating hot plasma component (kTmax > 21 keV in the high state) declining to (4.0-6.0) x 10-13 erg s-1 cm-2 in 2021 (low state). The NICER data obtained in the low state reveal a two-pole accretor showing a soft X-ray component at (6-7)sigma significance with a blackbody temperature of 15-18 eV. A soft X-ray component has never been detected for a polar in the low state before.

Cataclysmic variables (CVs) that have evolved past the period minimum during their lifetimes are predicted to be systems with a brown dwarf donor. While population synthesis models predict that around $\approx 40-70\%$ of the Galactic CVs are post-period minimum systems referred to as "period bouncers", only a few dozen confirmed systems are known. We report the study and characterisation of a new eclipsing CV, SRGeJ041130.3+685350 (SRGeJ0411), discovered from a joint SRG/eROSITA and ZTF program. The optical spectrum of SRGeJ0411 shows prominent hydrogen and helium emission lines, typical for CVs. We obtained optical high-speed photometry to confirm the eclipse of SRGeJ0411 and determine the orbital period to be $P_\textrm{orb} \approx 97.530$ minutes. The spectral energy distribution suggests that the donor has an effective temperature of $\lesssim 1,800$ K. We constrain the donor mass with the period--density relationship for Roche-lobe-filling stars and find that $M_\textrm{donor} \lesssim 0.04\ M_\odot$. The binary parameters are consistent with evolutionary models for post-period minimum CVs, suggesting that SRGeJ0411 is a new period bouncer. The optical emission lines of SRGeJ0411 are single-peaked despite the system being eclipsing, which is typically only seen due to stream-fed accretion in polars. X-ray spectroscopy hints that the white dwarf in SRGeJ0411 could be magnetic, but verifying the magnetic nature of SRGeJ0411 requires further investigation. The lack of optical outbursts has made SRGeJ0411 elusive in previous surveys, and joint X-ray and optical surveys highlight the potential for discovering similar systems in the near future.

In this study, we carried out photometric, spectroscopic, and for the first time, polarimetric observations of the Amor-type near-Earth asteroid (2059) Baboquivari. Our findings represent the first reliable determination of Baboquivari's physical properties. We used data from a 1m-class telescope (T100) along with ALCDEF data for photometric analyses and a 1.5-m-class telescope (RTT150) for polarimetric, spectroscopic, and additional photometric observations. We obtained the synodic rotation period of Baboquivari as 129.93 +/- 2.31 h and the standard phase function parameters H and G as 16.05 +/- 0.05, 0.22 +/- 0.02, respectively. Our colour index (V-R) measurement of 0.45 +/- 0.02 is consistent with spectroscopic observations, indicating an S (or sub-S) spectral type. Using the polarimetric and spectroscopic data, we found that the geometric albedo is 0.15 +/- 0.03, and the spectral type is Sq. Based on the estimated albedo and absolute magnitude, Baboquivari has an effective diameter of 2.12 +/- 0.21 km. Due to the scattered data in the light curve, its slow rotation and location among the NEAs suggest that Baboquivari may be a non-principal axis (NPA) rotator.

AM CVn systems are ultra-compact binaries where a white dwarf accretes from a helium-rich degenerate or semi-degenerate donor. Some AM CVn systems will be among the loudest sources of gravitational waves for the upcoming Laser Interferometer Space Antenna (LISA), yet the formation channel of AM CVns remains uncertain. We report the study and characterisation of a new eclipsing AM CVn, SRGeJ045359.9+622444 (hereafter SRGeJ0453), discovered from a joint SRG/eROSITA and ZTF program to identify cataclysmic variables (CVs). We obtained optical photometry to confirm the eclipse of SRGeJ0453 and determine the orbital period to be $P_\textrm{orb} = 55.0802 \pm 0.0003$ min. We constrain the binary parameters by modeling the high-speed photometry and radial velocity curves and find $M_\textrm{donor} = 0.044 \pm0.024 M_{\odot}$ and $R_\textrm{donor}=0.078 \pm 0.012 R_{\odot}$. The X-ray spectrum is approximated by a power-law model with an unusually flat photon index of $\Gamma\sim 1$ previously seen in magnetic CVs with SRG/eROSITA, but verifying the magnetic nature of SRGeJ0453 requires further investigation. Optical spectroscopy suggests that the donor star of SRGeJ0453 could have initially been a He star or a He white dwarf. SRGeJ0453 is the ninth eclipsing AM CVn system published to date, and its lack of optical outbursts have made it elusive in previous surveys. The discovery of SRGeJ0453 using joint X-ray and optical surveys highlights the potential for discovering similar systems in the near future.

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.

We present the results of modelling the 35-day superorbital changes in the B and V lightcurves and X-ray flux of HZ Her/Her X-1. The model is implemented in a computer program written in the C programming language, with a module for parameter optimisation written in Python. The model includes a tilted precessing and warped accretion disc around a freely precessing neutron star. The disc is warped near its inner edge due to interaction with the rotating neutron star magnetosphere. The magnetic torque depends on the precessional phase of the neutron star. The neutron star X-ray emission flux also depends on the free precession phase, which modulates the X-ray illumination of the optical star's atmosphere and the intensity of gas streams. We demonstrate that this model is able to reproduce both the optical observations of HZ Her and the behaviour of the system's 35-day X-ray cycle.

ABSTRACT We report the discovery of X-ray emission from CFHQS J142952+544717, the most distant known radio-loud quasar at z = 6.18, on 2019 December 10–11 with the eROSITA telescope on board the SRG satellite during its ongoing all-sky survey. The object was identified by cross-matching an intermediate SRG/eROSITA source catalogue with the Pan-STARRS1 distant quasar sample at 5.6 < z < 6.7. The measured flux ∼8 × 10−14 erg cm−2 s−1 in the 0.3–2 keV energy band corresponds to an X-ray luminosity of $2.6^{+1.7}_{-1.0}\times 10^{46}$ erg s−1 in the 2–10 keV rest-frame energy band, which renders CFHQS J142952+544717 the most X-ray luminous quasar ever observed at z > 6. Combining our X-ray measurements with archival and new photometric measurements in other wavebands (radio to optical), we estimate the bolometric luminosity of this quasar at ∼(2–3) × 1047 erg s−1. Assuming Eddington limited accretion and isotropic emission, we infer a lower limit on the mass of the supermassive black hole of ∼2 × 109 M⊙. The most salient feature of CFHQS J142952+544717 is its X-ray brightness relative to the optical/UV emission. We argue that it may be linked to its radio-loudness (although the object is not a blazar according to its radio properties), specifically to a contribution of inverse Compton scattering of cosmic microwave background photons off relativistic electrons in the jets. If so, CFHQS J142952+544717 might be the tip of the iceberg of high-z quasars with enhanced X-ray emission, and SRG/eROSITA may find many more such objects during its 4-yr all-sky survey.

ABSTRACT At the age of about 1 yr, the spectra of most Type Ia supernovae (SNe Ia) are dominated by strong forbidden nebular emission lines of Fe ii and Fe iii. Later observations (at about 2 yr) of the nearby SN 2011fe showed an unexpected shift of ionization to Fe i and Fe ii. Spectra of the very nearby SN Ia 2014J at an intermediate phase (1–1.5 yr) that are presented here show a progressive decline of Fe iii emission, while Fe i is not yet strong. The decrease in ionization can be explained if the degree of clumping in the ejecta increases significantly at ∼1.5 yr, at least in the Fe-dominated zone. Models suggest that clumps remain coherent after about one year, behaving like shrapnel. The high density in the clumps, combined with the decreasing heating rate, would then cause recombination. These data may witness the phase of transition from relatively smooth ejecta to the very clumpy morphology that is typical of SN remnants. The origin of the increased clumping may be the development of local magnetic fields.

In this work are presented the results of modelling of 35 d superorbital changes of B and V lightcurves and X-ray flux of HZ Her/Her X-1. The model implemented in the new code written in C programming language, with module for parameter optimisation written in Python. The model includes a tilted precessing and warped accretion disc around a freely precessing neutron star. The disc is warped near its inner edge due to interaction with the rotating neutron star magnetosphere. The magnetic torque depends on the precessional phase of the neutron star. The X-ray emission flux from the neutron star also depends on the free precession phase which modulates the X-ray illumination of the optical star atmosphere and the intensity of gas streams. We demonstrate that this model is able to well reproduce both optical observations of HZ Her and the behaviour of the 35-day X-ray cycle.

Nikolay Shakura∗1,2, Dmitry Kolesnikov1, Konstantin Postnov1,2, Igor Volkov1,3, Ilfan Bikmaev2, Tatiana Irsmambetova1, Rüdiger Staubert4, Joern Wilms5, Eldar Irtuganov2, Pavel Shurygin2, Polina Golysheva1, Sergey Shugarov1,6, Igor Nikolenko3, Eugene Trunkovsky1, Gabriele Schönherr8, Axel Schwope8, Dmitry Klochkov4 1Sternberg Astronomical Institute, Moscow State University, 119234, Moscow, Russia 2Kazan Federal University, Kazan, Russia 3Institute of Astronomy RAS, Moscow, Russia 4Institute for Astronomy and Astrophysics, Tubingen, Germany, 5Astronomical Institute of the University of Erlangen-Nuremberg, Bamberg, Germany, 6Astronomical Institute of the Slovak Academy of Scienses, Tatranska Lomnica, Slovakia 7Crimean Astrophysical Observatory, Nauchny, Russia 8Leibniz Institute for Astrophysics, Potsdam, Germany E-mail: nikolai.shakura@gmail.com, kolesnikovkda@gmail.com

Accretion onto magnetized neutron stars is considered using as a case study long-term X-ray and optical observations of HZ Her/Her X-1, an X-ray binary system with a 1.7-day orbital period where disc accretion occurs from the optical donor star (HZ Her) onto a neutron star (Her X-1). On top of orbital variability and pulsating X-ray emission from the neutron star rotating with a period of about one second, a 35-day X-ray modulation of emission is observed. The 35-day variability is due to a tilted precessing accretion disc that periodically screens X-ray emission from the neutron star. The disc precession that occurs in the direction opposite to the orbital motion is determined by the joint action of the tidal torque from the donor and dynamical torque from the gas streams. Several dozen thousand broadband UBV photometric observations of HZ Her have been obtained since 1972. The shape of the orbital light curves of HZ Her also changes with the 35-day cycle phase. The orbital variability can be reproduced in a model that includes a precessing tilted and warped accretion disc around a freely precessing neutron star. The disc is warped near its inner edge due to interaction with the rotating neutron star magnetosphere. The magnetic torque depends on the precessional phase of the neutron star. The X-ray emission flux from the neutron star also depends on the free precession phase that modulates the heating of the optical-star atmosphere and the intensity of gas streams. We show that this model reproduces well both optical observations of HZ Her and the behavior of the 35-day X-ray cycle.

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Abstract. The black hole X-ray binary V404Cyg was studied during of the 2015 outburst. Optical photometry and spectroscopy were performed by using 1.5-meter Russian-Turkish telescope (RTT-150) facilities at the TUBITAK National Observatory (Antalya, Turkey). From June 22 to June 28, 2015, shell expansion velocity decreased from 650 to 400 km s–1 as measured by Hα and Hβ lines and from 450 to 330 km s–1 as measured by HeI and HeII lines. Thus, the shell expansion occurred with deceleration, where the hydrogen and helium line formation regions are at different radial distances from the center of the star. The correlation of flow variability in the optical and X-ray ranges is caused by fluctuations in the rate of accretion near a compact source where X-ray photons are generated.

The X-ray binary Her X-1 consists of an accreting neutron star and the optical component HZ Her. The 35-day X-ray 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 the warped accretion disk tilted to the orbital plane. We argue that the observed features of the 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 precession. The model parameters include a) the intensity (power) of the stream of matter flowing out of the optical star; b) the X-ray luminosity of the neutron star; c) the optical flux of the accretion disk; d) the X-ray irradiation pattern on the donor star; e) the tilt of the inner and outer edge 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.