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G. S. Bisnovatyi-Kogan

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

At least 127 records · Page 7Linked to original sources
arXiv1999

Accretion Discs Around Black Holes: Developement of Theory

Standard accretion disk theory is formulated which is based on the local heat balance. The energy produced by a turbulent viscous heating is supposed to be emitted to the sides of the disc. Sources of turbulence in the accretion disc are connected with nonlinear hydrodynamic instability, convection, and magnetic field. In standard theory there are two branches of solution, optically thick, and optically thin. Advection in accretion disks is described by the differential equations what makes the theory nonlocal. Low-luminous optically thin accretion disc model with advection at some suggestions may become advectively dominated, carrying almost all the energy inside the black hole. The proper account of magnetic filed in the process of accretion limits the energy advected into a black hole, efficiency of accretion should exceed $\sim 1/4$ of the standard accretion disk model efficiency.

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arXiv1999

The cyclotron emission of anisotropic electrons in the X-ray pulsars

The spectrum of cyclotron radiation is calculated for anisotropically distributed relativistic electrons with a nonrelativistic velocity scattering across the magnetic field. It is shown that if such electrons are responsible for a formation of the "cyclotron" line in the spectrum of Her X-1, then the value of its magnetic field $(3-6)\cdot 10^{10}$ Gs resulting from this interpretation is in a good agreement with some other observations and theoretical estimations. Observations of a time dependence of the energy of this "cyclotron" line in the spectra of several X-ray pulsars is explained by a variability of the average longitude energy of the electrons, decreasing with increasing liminocity due to radiational braking of the accretion flow.

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arXiv1999

Optical counterparts of cosmological GRBs due to heating of ISM in the parent galaxy

We investigated influence of cosmological GRB on the surrounding interstellar medium. It was shown that γ-radiation from the burst heats interstellar gas to the temperatures > 10^4 K up to the distance \sim 10 pc. For high density ISM optical and UV radiation of the heated gas can be observed on the Eath several years as a GRB`s counterpart

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arXiv1999

Origin of GRB Afterglows in the Model of Galactic Neutron Stars

The launch of the Beppo-Sax satellite gave a unique opportunity to investigate gamma ray bursts (GRB) in different spectral regions. The large diversity of the afterglow behavior creates additional problems for the cosmological model with a fireball. Formation of the afterglow giving the observed diversity of properties is suggested in the Galactic neutron star model of GRBs. It is based on the transient accretion disc formation around the neutron star with a low-mass brown companion irradiated by the neutron star.

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arXiv1999

Magnetic Field Limitations on Advection Dominated Flows

Recent papers discussing advection dominated accretion flows (ADAF) as a solution for astrophysical accretion problems should be treated with some caution because of their uncertain physical basis. The suggestions underlying ADAF involve ignoring the magnetic field reconnection in heating of the plasma flow, assuming electron heating due only to binary Coulomb collisions with ions. Here, we analyze the physical processes in optically thin accretion flows at low accretion rates including the influence of an equipartition turbulent magnetic field. For these conditions there is continuous destruction of magnetic flux by reconnection. The reconnection is expected to significantly heat the electrons which can efficiently emit magnetobremstrahlung radiation. Because of this electron emission, the radiative efficiency of the ADAF is not small. We suggest that the small luminosities of nearby galactic black holes is due to outflows rather than ADAF accretion.

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arXiv1998

Magnetic propeller outflows

A model is developed for magnetic `propeller'-driven outflows which cause a rapidly rotating magnetized star accreting from a disk to spin-down. Energy and angular momentum lost by the star goes into expelling most of the accreting disk matter. The theory gives an expression for the effective Alfven radius $R_A$ (where the inflowing matter is effectively stopped) which depends on the mass accretion rate, the star's mass and magnetic moment, and the star's rotation rate. The model points to a mechanism for `jumps' between spin-down and spin-up evolution and for the reverse transition, which are changes between two possible equilibrium configurations of the system. In for example the transistion from spin-down to spin-up states the Alfven radius $R_A$ decreases from a value larger than the corotation radius to one which is smaller. In this transistion the `propeller' goes from being ``on'' to being ``off.'' The ratio of the spin-down to spin-up torque (or the ratio for the reverse change) in a jump is shown to be of order unity.

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arXiv1998

Gas dynamic investigation of rotating gas accretion

We study axisymmetric accretion of slowly rotating matter onto a gravitation center. The process of the flow restructuring is presented as the angular velocity of accreting gas approaches the Keplerian velocity. For spherically-symmetric accretion we found conditions of existence of steady-state supersonic solutions for various external boundary conditions. These conditions are used for choosing proper initial and boundary conditions for accretion of the rotating matter. The study is performed on the basis of the numerical solution of the Euler gas dynamic equations using the second order extension of the Lax-Friedrichs scheme with the implicit approximation of the source term.

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arXiv1998

Accretion Disc Theory: From the Standard Model Until Advection

Accretion disc 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 disc. One of the most important new invention of this theory was a phenomenological treatment of the turbulent viscosity, known as ''alpha'' prescription, when the (r$ϕ$) component of the stress tensor was approximated by ($α$ P) with a unknown constant $α$. This prescription played the role in the accretion disc theory as well important as the mixing-length theory of convection for stellar evolution. Sources of turbulence in the accretion disc are discussed, including nonlinear hydrodynamical turbulence, convection and magnetic field role. In parallel to the optically thick geometrically thin accretion disc models, a new branch of the optically thin accretion disc 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 disc theory are presented and first models with advection are reviewed. The present status of the solution for a low-luminous optically thin accretion disc model with advection is discussed and the limits for an advection dominated accretion flows (ADAF) imposed by the presence of magnetic field are analysed.

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arXiv1997

New approach to modeling of outflowing envelopes at arbitrary optical depths

We have considered a stationary outflowing envelope from the star accelerated by the radiative force in arbitrary optical depth case. Introduced approximations provide satisfactory description of the behavior of the matter flux with partially separated radiation at arbitrary optical depths. The obtained system of differential equations provides a continuous transition of the solution between optically thin and optically thick regions. We analytically derived approximate representation of the solution at the vicinity of the sonic point. Using this representation we numerically integrate the system of equations from the critical point to the infinity. Matching the boundary conditions we obtain the solution of the describing the problem system of differential equations. We have discuss in details the problem of the outer boundary condition. The situation of the photosphere was clarified for the obtained expanding solution. Theoretical approach submitted in this work could be useful for self - consistent simulations of the massive stars evolution with mass loss.

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arXiv1997

Magnetorotational Supernova Explosion - 2D Numerical Simulation

Results of 2D numerical simulation of magnetorotational mechanism of supernova explosion are presented. It is shown that due to the differential rotation of the star toroidal component of magnetic field appears and grows with time. Angular momentum transfers outwards by the toroidal magnetic field. With the evolution of the process part of the envelope of the star is throwing away. The amount of thrown away mass and energy are estimated. The results of the simulation are qualitatively correspond to supernova explosion picture.

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arXiv1997

Timing of gamma-ray pulsars: search in seven - parametric space

Timing of Geminga gamma-ray pulsar is done using data of COS B and EGRET. It is shown, that errors in angular coordinates of sources similar to Geminga strongly influence a determination of $\ddot ν$, so that at angular precision less then $10^{-3}$ arc sec determination of the value of $\ddot ν$ by means of criterion gives error more then $100 %$. Attempt have been done to improve coordinates of a gamma-ray pulsar using timing analysis. In addition to search of $ν$, $\dotν$ and $\ddotν$, a technique is first developed permitting search of two angular coordinates, absolute speed value and direction of a proper motion. In that way timing of gamma-ray pulsars gives amount of information compatible with radiopulsars, but using of real data gives much poorer precision. In gamma-ray sources with rare pulses the periodicity criteria are quite different from the ones in radio region. Data on the coordinates and proper motion of Geminga, obtained from timing studies, do not contradict inside the errors to its identification with $G''$ star and its proper motion. These errors are larger then ones in optical measurements, but are smaller then corresponding errors in X-ray and $γ$-ray data. Estimations of the gamma-ray pulsar coordinates and its proper motion could be obtained independently on its optical or radio component, and are available in their absence.

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arXiv1997

Gamma Ray Burst Source Statistics in the Presence of Stochastic Errors

Selection effects, connected with stochastic errors in source flux and threshold value determination are analyzed. Normal and normal logarithmic distributions of stochastic deviations are considered. These two kind of distributions produce different effects on the source statistics. Applications to Gamma Ray Burst statistics are discussed. A physical test for checking a close neutron star model of GRBs is suggested.

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arXiv1997

Thin Layer Approximation in 3-D

Equations are derived which describe a propagation of strong shocks in the interstellar matter, without any demands for a symmetry, in a thin layer approximation (2.5 dimensions). Using these equations permits to calculate a propagation of shock waves from nonsymmetric supernovae explosions in the medium with arbitrary density distribuion, formation of superbubbles in galaxies.

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arXiv1997

High Velocity Neutron Stars as a Result of Asymmetric Neutrino Emission

Formation of a neutron star is accompanied by neutrino emission carring about 10% of the rest energy of the star. Toroidal field produced by twisting of a dipole field in differentially rotating star is antisymmetric. Its summation with antisymmetric toroidal field results in braking of mirror symmetry of the magnetic field . For large magnetic field the neutron decay rate depends on its strength. Neutrino is emitted more in one direction leading to flux asymmetry and recoil of the neutron star. Estimations show that the neutron star can reach velocities $\sim 1000$ km/s for 3% asymmetry of the neutrino flux.

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arXiv1997

Asymmetric neutrino emision and formation of rapidly moving pulsars

The neutron star formation during the collapse with the strong magnetic field may lead to a mirror symmetry violation and formation of an asymmetric magnetic field. Dependence of the week interaction cross-section on the magnetic field strength lead to the asymmetric neutrino flux and formation of rapidly mooving pulsars due to the recoil action as well as rapidly moving black holes.

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arXiv1997

Influence of Ohmic Heating on Advection-Dominated Accretion Flows

Advection-dominated, high-temperature, quasi-spherical accretion flow onto a compact object, recently considered by a number of authors, assume that the dissipation of turbulent energy of the flow heats the ions and that the dissipated energy is advected inward. It is suggested that the efficiency of conversion of accretion energy to radiation can be very much smaller than unity. However, it is likely that the flows have an equipartition magnetic field with the result that dissipation of magnetic energy at a rate comparable to that for the turbulence must occur by Ohmic heating. We argue that this heating occurs as a result of plasma instabilities and that the relevant instabilities are current driven in response to the strong electric fields parallel to the magnetic field. We argue further that these instabilities heat predominantly the electrons. We conclude that the efficiency of conversion of accretion energy to radiation can be much smaller than unity only for the unlikely condition that the Ohmic heating of the electrons is negligible.

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