In this work, we studied X-ray source SRGe J021932.4-040154, that we associated with a single X-ray active star of spectral class G2V-G4V and the rotational period $P_{\rm rot} = 3.2 \pm 0.5$ days. The SRGe J021932 was observed with the SRG/eRROSITA during eUDS survey in 2019 in much dimmer state compared to the XMM-Newton catalogue 4XMM-DR12. Detailed analysis revealed that the archival...
The Nuclear Stellar Disk (NSD), along with the Nuclear Stellar Cluster (NSC) and the supermassive black hole Sgr A*, forms the central region of the Milky Way. Observations have well established that the Galaxy's X-ray emission is associated with the stellar population and predominantly arises from the integrated emission of accreting white dwarfs. This study focuses on the NSD emission in the...
Gamma-ray bursts (GRBs) are among the most explosive and brightest transient phenomena in the universe, occurring at cosmological distances. After decades of investigation, the origin of the jet composition, the radiation, and energy dissipation mechanisms of GRBs are among the most important open questions regarding the nature of the GRB central engine.
Here, we consider the evolution of a...
The origins of supermassive black holes (SMBHs) at the centers of massive galaxies are a topic of intense investigation. One way to address this subject is to identify the seeds of SMBH as intermediate-mass black holes (IMBHs; $100M_\odot < M_{BH} < 10^6M_\odot$). Given the observed relationship between the mass of a black hole and the mass of its host galaxy, IMBHs are expected to be found at...
We present a study of the radio properties of distant quasars at z$>$3. The complete sample consists of 101 objects with a flux density level greater than 100 mJy at 1.4 GHz selected in a declination range from -35° to +49°. The study based on simultaneous RATAN-600 observations at frequencies of 1.2, 2.3, 4.7, 8.2, 11.2, and 22.3 GHz in 2017-2020. The flux density measurements uncertainties...
Constraining the mass and spin of black holes is among the holy grails of contemporary research into accreting sources. Any image of a sufficiently optically thin source near a black hole exhibits contributions of extremely lensed light rays, which orbit the black hole due to its inexorable pull before travelling to the observer. In interferometric images like those produced by the Event...
Temperature of the hot gas in galaxy clusters is known to be a reliable proxy for the their total gravitating mass, allowing one to use spectroscopic X-ray observations for halo mass function measurements. Data of shallow wide area surveys, however, often precludes direct fitting of the X-ray spectra, given possible biases arising due to unresolved (multitemperature) inner structure of the...
In this talk we discuss constraints on the parameters of decaying sterile neutrinos as candidates for dark matter particles from the data of telescopes ART-XC and NuSTAR. We also discuss constraints that can be obtained from the data of these telescopes for annihilating dark matter.
The GRAVITY instrument made a remarkable observation during the Near-Infrared flares of 2018, detecting a fast-moving hot spot in what seemed to be a circular orbit around SgrA, the supermassive black hole in our Galactic Center. The Gravity Collaboration attempted to fit the observed flaring behavior with a circular Keplerian orbit, a few gravitational radii from the supermassive black hole....
We investigate accretion flows around rotating black holes (BHs) and obtain self-consistent transonic solutions in full general relativistic prescription. The flow is assumed to be viscous and radiative. Viscosity helps in the removal of angular momentum outwards, allowing matter to get accreted inwards. In addition, viscous heat dissipated makes the matter hotter. On the other hand, radiation...
The new generation of galaxy surveys will provide unprecedented data allowing us to test gravity at cosmological scales. A robust cosmological analysis of the large-scale structure demands exploiting the nonlinear information encoded in the cosmic web. This study delves into the meticulous task of mapping modified gravity (MG) signatures within cosmic volumes by employing a large...
