The dynamical instability of relativistic polytropic spheres, embedded in a spacetime with a repulsive cosmological constant, is studied in the framework of general relativity. We apply the methods used in our preceding paper to study the trapping polytropic spheres with Λ=0, namely, the critical point method and the infinitesimal and adiabatic radial perturbations method developed by...

Supermassive black holes (SMBHs) are among the most powerful energy sources in the Universe due to enormous energy stored in their spins. However, efficient extraction of this mechanical energy requires interactions of SMBHs with magnetic fields of external origin. In this talk I will discuss the role of black hole spin and magnetic field on the acceleration of charged matter for chosen 25...

The observations in the near-infrared domain revealed that old bright late-type stars have a flat to a decreasing surface-density profile, which is in contrast to the cusp-like distribution of young OB/Wolf-Rayet stars. The core-like distribution of bright red giants is apparent from the decrease in the strength of the CO bandhead as well as from the stellar number counts. More recently, it...

The origin of Ultra High Energy Cosmic Rays is still unknown, and Active Galactic Nuclei (AGN) have been proposed as candidates to accelerate these particles. The aim of our work is to test the model of the particle acceleration in the hotspots of the AGNs. Using the multi-wavelength data of radiogalaxy 3C 105 S we calculate the maximum achievable energy of the cosmic rays through via the...

Different types of astrophysical black hole sources show variable accretion flows. They are found in gamma ray bursts, active galactic nuclei and radio loud objects such as blazars. These objects often have relativistic jets pointing at a small angle from our line of sight. Observational studies have found correlations between the minimum variability time-scale and Lorentz factor for these...

We investigate the accretion induced spin up of the black hole via numerical simulations. Our method is based on general-relativistic hydrodynamics of the slowly-rotating flows in the Kerr metric. We account for the changing black hole mass and spin during accretion. We study non-magnetized flows with shocks, and finally, we also include magnetic field endowed in the gas. The aim...

Accretion disks in High mass X-ray binaries (HMXB's) are mostly fed by the stellar wind from their companion star. These winds also affect the observed X-ray spectra arising from the hot coronal flow.

Cygnus X-1 and its companion star, HDE-226868 is one of such HMXBs. It is one of the brightest X-ray sources observed and shows the X-ray intensity variations in both the soft and hard X-rays....

I shortly describe the concept of the effective potential in Newton's theory and claim that even if one does not know Einstein's general relativity, one may correctly guess the topology of the potential in Einstein's theory from Newton. This allows one to discover, without Einstein's equations, all "relativistic effects" in particles' and photons' motion around compact objects in the strong...

In the era of massive all-sky surveys, calculating spectroscopic redshifts (Spec-Z) is time-consuming and costly. Instead, broadband photometry may be used as a proxy for spectrographic measurements. We train an off-the-shelf classifier to estimate photometric redshifts (Photo-Z) from SDSS Quasar imaging data. We outline the technique of cross-validation to reduce bias and variance in the...

Linear time series analysis, mainly the Fourier transform based methods,

has been quite successful in extracting information contained in the ever-modulating light curves (Lcs) of active galactic nuclei, and thereby contribute in characterizing the general features of supermassive black hole systems. In particular, the statistical properties of $\gamma$-ray variability of blazars are found to...

The standard scenario of a geometrically thin, planar accretion disk can be violated by a number of effects that must operate in astrophysically more realistic schemes. Even within a highly simplified framework of an axially symmetric (2D), steady, Keplerian accretion, the radial structure can be different from the predictions of the classical Shakura-Sunyaev theory. For example, stars and...

Black holes attract gaseous material from the surrounding environment. Cosmic plasma is largely ionized magnetized because of electric currents flowing in the highly conductive environment near black holes; the process of accretion then carries the magnetic flux onto the event horizon, $r\simeq R_+$. On the other hand, magnetic pressure acts against accretion. It can not only arrest the inflow...

The close neighbourhood of a supermassive black hole contains not only the accreting gas and dust but also stellar-sized objects like stars, stellar-mass black holes, neutron stars, dust-enshrouded objects, etc. These entities interact with the accreting medium and perturb the quasi-stationary configuration of the accretion flow. We investigate how the passage of a star can influence the black...

Acceleration of the high energy cosmic rays protons in the active galactic nuclei is considered.

The major acceleration stage is the centrifugal acceleration in a magnetosphere of the central machine light cylinder surface. In the during to calculations, the received dependence of the maximum energy on the parameter of the magnetization and parameter of relation toroidal and poloidal...

Synchrotron radio emission from non-relativistic jets powered by massive protostars has been reported, indicating the presence of relativistic electrons and magnetic fields of about 1 mG. We study diffusive shock acceleration and magnetic field amplification in protostellar jets with speeds between 300 and 1000 km/s. In this talk I will show that the magnetic field in the synchrotron emitter...

The motion and acceleration of an electrically charged and magnetized particle around a cylindrical black hole in the presence of an external asymptotically uniform magnetic field parallel to the z axis is investigated. We look at circular orbits around a central object and study the dependence of the most internal stable circular orbits (ISCO) on the so-called magnetic coupling parameters,...

We will present results for oscillation modes of thick accretion tori in Kerr spacetime using finite-element numerical method. We will derive relativistic version of the Papaloizou-Pringle equation for disks with general angular momentum distribution. Then we focus on the case of constant angular momentum tori and compare our results with those previously found by analytic perturbation methods.

We examine the influence of the quadrupole moment of slowly rotating neutron stars on the oscillations of non-slender accretion tori. We assume a perfect fluid, polytropic, constant speciﬁc angular momentum, non-selfgravitating torus and analytically calculate formulas for the oscillation frequencies. So far, these have only been studied in the Kerr geometry. We apply known methods for...

We explore the influence of non-geodesic pressure forces that are present in an accretion disk on the frequencies of its axisymmetric and non-axisymmetric epicyclic oscillation modes. We discuss its implications for models of high-frequency quasi-periodic oscillations (QPOs) that have been observed in the X-ray flux of accreting black holes (BHs) and neutron stars (NSs). In particular, we...

The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the...

The effect of strong gravitational lensing in the vicinity of Kehagias-Sfetsos compact object surrounded by plasma is studied. We examined the effect of plasma as well as spacetime parameter on deflection of photon and magnification of image.

We consider a simple model of a black hole surrounded by a toroidal structure and experiencing deformation due to magnetic field and the tidal force. For this system we construct the shadows of Preston-Poisson black hole, apply it to supermassive black hole M87∗ and compare it with the shadows of corresponding Schwarzschild and Kerr black holes. We find that for large deviation parameters...

We show that in some ranges of the charge parameters charged stringy black hole metric can provide the same ISCO as in the case of the well known Kerr metric for the test particles. Based on the idea that charge parameters of the stringy black hole metric can provide the same ISCO for the test particles it would be difficult to distinguish these two metrics if one relies on the observations...

The regularized four-dimensional Einstein-Gauss-Bonnet model has been recently proposed in [D. Glavan and C. Lin, Phys. Rev. Lett. \textbf{124}, 081301 (2020)] whose formulation is different of the Einstein theory, allowing us to bypass the Lovelock theorem. The action is formulated in higher dimensions (D>4) by adding the Gauss-Bonnet correction to the conventional Einstein-Hilbert action...

A finite-size quasi-two-dimensional Bose-Einstein condensate collapses if the attraction between atoms is sufficiently strong. Here we present a theory of collapse for condensates with the interatomic attraction and spin-orbit coupling. We consider two realizations of spin-orbit coupling: the axial Rashba coupling and the balanced, effectively one-dimensional Rashba-Dresselhaus one. In both...

We perform global 2D axisymmetric general relativistic radiation magnetohydrodynamic simulations of super-critical disk accretion onto a neutron star with a modest dipolar magnetic field strength of 20 GigaGauss as a model of a ULX. We study the effect of the boundary condition on the structure of the accretion column, outflow, and radiative output. In addition to fully absorbing and...

In our 3D numerical simulations of accretion disk around a supermassive black hole with the GRMHD code Athena++, we follow the time evolution of magnetic field. In the case of SANE configuration, with multiple loops of oppositely directed initial poloidal magnetic field in the torus around a black hole, we follow the forming of flux ropes atop the disk, and their release into corona.

Long term observations of black-hole X-ray binaries show that these systems exhibit extreme, aperiodic variability on time scales of few milliseconds to seconds. The observed light-curves display various characteristic features like log-normal distribution and linear rms-flux relation, which indicates that the underlying variability process is stochastic in nature and is thought to be...

We perform resistive MHD simulations of accretion disk with alpha-viscosity, accreting onto a rotating star endowed with a magnetic dipole. We find backflow in the presence of strong magnetic field and large resistivity, and probe for the dependence on Prandtl number. We find that in the magnetic case the distance from the star at which backflow begins, the stagnation radius, is different than...

In this talk we will introduce a new formalism to calculate nonlinear oscillations of fluids based on non-self-adjoint operators. The formalism is very intuitive, based on Eulerian description of perturbations and can be seen as an alternative to the standard Lagrangian approach used in the theory of nonlinear stellar pulsations. First we will review basic properties of nonlinear oscillations...

For a wide class of spherically symmetric naked singularities there is a sphere within which gravity is effectively repulsive. In such space-times accreting matter cannot reach the singularity and will instead form a levitating atmosphere, which is kept suspended by gravity alone. The density of the atmosphere has a maximum at a definite radius. In its properties the atmosphere is analogous to...

I will present results inferred from numerical study of geometrically thin accretion disk. The radiation-pressure dominated thin accretion disks undergo thermal instability which has been infamous for decades. We confirmed this instability in a 3D global radiative GRMHD simulation. In order to further understand thermal instability, we simulated GRHD models with \alpha-viscosity and concluded...

Strange quark stars (SQS) are a possible type of compact objects, which remain after the end of the life of supermassive stars. After the first explosion of a massive star, if the density of matter in the core of the star increases to values above the nuclear saturation density, the quarks deconfine and a huge value of energy (1047 J) is released, leading to the second explosion that is super...

In this talk I will review and address the long standing problem of determination of the state of matter in the cores of neutron stars. Our understanding of the cold, dense nuclear matter in neutron star interiors has suffered a dramatic revolution during the recent years. On the one hand, laboratory experiments have been able to probe higher and higher densities that comprise the equation of...

It is well-known fact that a light ray does not follow the null geodesics of spacetime in nonlinear electrodynamics, instead it follows the null geodesics of so-called effective spacetime. Moreover, in general relativity coupled to the nonlinear electrodynamics the spacetime cannot possess both electric and magnetic charges at the same time. By combining these two phenomena, we aim to discover...

Observational properties and energetics of oscillating and rotating magnetized neutron stars will be discussed. A qualitative model for the explanation of the phenomenology of intermittent pulsars in terms of stellar oscillations that are periodically excited by star glitches has been proposed. The conditions for radio emission in rotating and oscillating magnetars, by focusing on the main...

In this work we perform dynamical analysis of a broad classes of barotropic fluids and a non- minimally coupled real scalar fields in the Friedmann-Robertson-Walker (FRW) spacetime frame- work. The first part of our study concerns the dynamics of a fluid with an unspecified barotropic equation of state (EoS) having as the only assumption the non-negativity of the fluid’s energy den- sity. The...

The talk will discuss thermal solutions of a thin accretion disk model with magnetic fields and a radial gradient of entropy.