arXiv subjects

G. S. Bisnovatyi-Kogan

Publications and source records attributed to G. S. Bisnovatyi-Kogan.

At least 109 records · Page 6Linked to original sources

arXiv2004

Very high frequency gravitational wave background in the universe

Astrophysical sources of high frequency gravitational radiation are considered in association with a new interest to very sensitive HFGW receivers required for the laboratory GW Hertz experiment. A special attention is paid to the phenomenon of primordial black holes evaporation. They act like black body to all kinds of radiation, including gravitons, and, therefore, emit an equilibrium spectrum of gravitons during its evaporation. Limit on the density of high frequency gravitons in the Universe is obtained, and possibilities of their detection are briefly discussed.

gr-qc↗

arXiv2004

Accretion disks around black holes with account of magnetic fields

Accretion disks are observed in young stars, cataclysmic variables, binary X-ray sources et al. Accretion disk theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disk. Important part of this theory was the phenomenological treatment of the turbulent viscosity, known the `` alpha'' prescription, where the $(r ϕ)$ component of the stress tensor was connected with the pressure as $αP$. Sources of turbulence in the accretion disk are discussed, including hydrodynamic turbulence, convection and magnetic field role. Optically thin solution and advective disks are considered. Related problems of mass ejection from magnetized accretion disks and jet formation are discussed.

astro-ph↗

arXiv2004

Dynamic stability of compact stars

After some historical remarks we discuss different criteria of dynamical stability of stars, and properties of the critical states where dynamical stability is lost, leading to collapse with formation of the neutron star or a black hole. At the end some observational and theoretical problems related to quark stars are discussed.

astro-ph↗

arXiv2004

Physical Limits of Different Models of Cosmic Gamma-Ray Bursts

The present common view about GRB origin is related to cosmology, what is based on statistical analysis, and on measurements of the redshifts in the GRB optical afterglows of long GRB. No correlation is found between redshifts, GRB spectrum, and total GRB fluence. Comparison of KONUS and BATSE data about statistics and hard X-ray lines is done, and some differences are noted. Hard gamma-ray afterglows, prompt optical spectra, hard X-ray lines measurements could be important for farther insight into GRB origin. Possible possible connection of short GRB with soft gamma repeaters is discussed.

astro-ph↗

arXiv2003

Physical restrictions on the models of gamma-ray bursts

The present common view about GRB origin is related to cosmology. There are two evidences in favor of this interpretation. The first is connected with statistics, the second is based on measurements of the redshifts in the GRB optical afterglows. Redshifts in optical afterglows had been observed only in long GRB. Comparison of KONUS and BATSE data about statistics and hard X-ray lines is done, and some differences are noted. Hard gamma-ray afterglows, prompt optical spectra and polarization measurements could be very important for farther insight into GRB origin. Possibility of galactic origin of short GRB is discussed as well as their possible connection with soft gamma repeaters.

astro-ph↗

arXiv2003

Magnetorotational mechanism of supernova type II exlosion

Results of 2D simulations of the magnetorotational mechanism of supernova type II are presented. Amplification of toroidal magnetic field of the star due to differential rotation of the star leads to the transformation of the rotational (gravitational) energy to the energy of the supernova explosion. In our simulation the energy of the explosion is $1.12 \cdot 10^{51}erg$. The explosion ejects about $0.11 M_\odot$.

astro-ph↗

arXiv2003

Stochastic appearance in stars and high energy sources

Many phenomena such as stellar variability, stellar explosions, behavior of different kind of X-ray and gamma-ray sources, processes in active galactic nuclei and other astrophysical objects show stochastic features. Brief description of these phenomena is given in this review.

astro-ph↗

arXiv2002

A simplified model of the formation of structures in the dark matter, and a background of very long gravitational waves

Collapse of the rotating spheroid is approximated by a system of ordinary differential equations describing its dynamics. The gravitational potential is approximated by the one of the iniform Maclaurin spheroid. Developement of gravitational instability and collapse in the dark matter medium do not lead to any shock formation or radiation, but is characterized by non-collisional relaxation, which is accompanied by the mass and angular momentum losses. Phenomenological account of these processes is done in this model. Formation of the equilibrium configuration dynamics of collapse is investigated for several parameters, characterizing the configuration. A very long gravitational wave emission during the collapse is estimated, and their possible connection with the observed gravitational lenses is discussed.

astro-ph↗

arXiv2002

Possible mechanism of electrical field origin around celestial bodies

Slow magnetic field variations in stars and planets create a quasistationary electrical field which may be observed. It is supposed that the electrical field near the Earth surface may be partially connected with variation of the Earth magnetic field. Two examples of the electrical field distribution around the infinite cylinder, and the circular loop with a lineary growing with time electrical currents are given.

astro-ph↗

arXiv2002

Advective Accretion Disks and Related Problems Including Magnetic Fields

Accretion disk theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disk. One of the most important new invention of this theory was the phenomenological treatment of the turbulent viscosity, known the `` alpha'' prescription, where the (r phi) component of the stress tensor was approximated by (alpha p) with a unknown constant alpha. This prescription played the role in the accretion disk theory as well important as the mixing-length theory of convection for stellar evolution. Sources of turbulence in the accretion disk are discussed, including hydrodynamical turbulence, convection and magnetic field role. In parallel to the optically thick geometrically thin accretion disk models, a new branch of the optically thin accretion disk models was discovered, with a larger thickness for the same total luminosity. The choice between these solutions should be done of the base of a stability analysis. The ideas underlying the necessity to include advection into the accretion disk theory are presented and first models with advection are reviewed. The present status of the solution for a low-luminous optically thin accretion disk model with advection is discussed and the limits for an advection dominated accretion flows (ADAF) imposed by the presence of magnetic field are analysed. Related problems of mass ejection from accretion disks and jet formation are discussed.

astro-ph↗

arXiv2002

A Cosmic Battery Reconsidered

We revisit the problem of magnetic field generation in accretion flows onto black holes owing to the excess radiation force on electrons. This excess force may arise from the Poynting-Robertson effect. Instead of a recent claim of the generation of dynamically important magnetic fields, we establish the validity of earlier results from 1977 which show only small magnetic fields are generated. The radiative force causes the magnetic field to initially grow linearly with time. However, this linear growth holds for only a {\it restricted} time interval which is of the order of the accretion time of the matter. The large magnetic fields recently found result from the fact that the linear growth is unrestricted. A model of the Poynting-Robertson magnetic field generation close to the horizon of a Schwarzschild black hole is solved exactly using General Relativity, and the field is also found to be dynamically insignificant. These weak magnetic fields may however be important as seed fields for dynamos.

astro-ph↗

arXiv2001

Jet Formation from Rotating Magnetized Objects

Jet formation is connected most probably with matter acceleration from the vicinity of rotating magnetized bodies. It is usually related to the mass outflows and ejection from accretion disks around black holes. Problem of jet collimation is discussed. Collapse of a rotating magnetized body during star formation or supernovae explosion may lead to a jet-like mass ejection for certain angular velocity and magnetic field distributions at the beginning of the collapse. Jet formation during magnetorotational explosion is discussed basing on the numerical simulation of collapse of magnetized bodied with quasi-dipole field.

astro-ph↗

arXiv2001

Gamma Ray Bursts statistical properties and limitations on the physical model

The present common view about GRB origin is related to cosmology. There are two evidences in favour of this interpretation. The first is connected with statistics, the second is based on measurements of the redshifts in the GRB optical afterglows. Red shifts in optical afterglows had been observed only in long GRB. Statistical errors, and possibility of galactic origin of short GRB is discussed; their connection with Soft Gamma Repeaters (SGR) is analyzed.

astro-ph↗

arXiv2000

Oscillations and convective motion in stars with URCA shells

It is shown that in presence of URCA shell pulsational energy losses due to neutrino emission and nonequilibrium beta heating are much less than energy losses by excitation of short-wavelength acoustic waves. Convective motion in presence of URCA shell is considered, and equations generalizing the mean free path model of convection are derived.

astro-ph↗

arXiv2000

Old models for Cygnus X-1 and AGN

Recently, there appeared many papers devoted to the modeling of X-ray properties of Cygnus X-1 and other black hole accretion disk candidates: e.g. J.Poutanen, J.Krolik, F.Ryde, MNRAS 292 (1997) L21; E.Agol, J.Krolik, ApJ 507 (1998) 304; A.Beloborodov, in ``High Energy Processes in Accreting Black Holes'', ASP Conf. Series, 161, (1999), p.295. The goal of this electronic publication is to draw attention to our old papers where many ideas of recent discussions were anticipated (hot coronae, Comptonization, photon damping of waves, particle acceleration and matter ejection from accretion disks with large scale poloidal magnetic fields, etc.)

astro-ph↗

arXiv2000

Magnetosphere of Oscillating Neutron Star. Nonvacuum Treatment

We generalize a formula for the Goldreich-Julian charge density (ρ), originally derived for rotating neutron star, for arbitrary oscillations of a neutron star with arbitrary magnetic field configuration under assumption of low current density in the inner parts of the magnetosphere. As an application we consider toroidal oscillation of a neutron star with dipole magnetic field and calculate energy losses. For some oscillation modes the longitudinal electric field can not be canceled by putting charged particles in the magnetosphere without a presence of strong electric current j = (c/(ωr))ρc. It is shown that the energy losses are strongly affected by plasma in the magnetosphere, and cannot be described by vacuum formulas.

astro-ph↗

arXiv1999

Nonstationary magnetorotational processes in a rotating magnetized cloud

We perform 2D numerical simulations of a magnetorotational explosion of a rotating magnetized gas cloud. We found that amplification of a toroidal magnetic field due to the differential rotation leads to a transformation of the part of the rotational energy of the cloud to the radial kinetic energy. Simulations have been made for 3 initial values of $ξ$ (the relation of magnetic energy to the gravitational energy of the cloud): $ξ=10^{-2},10^{-4},10^{-6}$. Part of the matter - $\sim 7%$ of the mass of the cloud ($\sim 3.3%$ of the final gravitational energy of the cloud) - gets radial kinetic energy which is larger than its potential energy and can be thrown away to the infinity. It carries about 30% of the initial angular momentum of the cloud. This effect is important for angular momentum loss in the processes of stellar formation, and for the magnetorotational mechanism of explosion suggested for supernovae. Simulations have been made on the basis of the Lagrangian 2D numerical implicit scheme on a triangular grid with grid reconstruction.

astro-ph↗

arXiv1999

Magnetic fields of neutron stars: very low and very high

Estimations of magnetic fields of neutron stars, observed as radio and X-ray pulsars, are discussed. It is shown, that theoretical and observational values for different types of radiopulsars are in good correspondence. Magnetic fields of X-ray pulsars are estimated from the cyclotron line energy. In the case of Her X-1 this estimation exceeds considerably the value of its magnetic field obtained from long term observational data related to the beam structure evolution. Another interpretation of the cyclotron feature, based on the relativistic dipole radiation mechanism, could remove this discrepancy. Observational data about soft gamma repeators and their interpretation as magnetars are analyzed.

astro-ph↗