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G. S. Bisnovatyi-Kogan

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

At least 19 recordsLinked to original sources
arXiv2026

Uniform models of neutron and quark (strange) stars in General Relativity

Models of neutron and strange stars are considered in the approximation of a uniform density distribution. A universal algebraic equation, valid for any equation of state, is used to find the approximate mass of a star of a given density without resorting to the integration of differential equations. Equations of state for neutron stars had been taken for degenerate neutron gas and for more realistic ones, used by Bethe, Malone, Johnson (1975). Models of homogeneous strange stars for the equation of state in the "quark bag model" have a simple analytical solution. The solutions presented in the paper for various equations of state differ from the exact solutions obtained by the numerical integration of differential equations by at most $ \sim 20 \%$. The formation of strange stars is examined as a function of the deconfinement boundary (DB), at which quarks become deconfined. Existing experimental data indicate that matter reaches very high densities in the vicinity of the DB. This imposes strong constraints on the maximum mass of strange stars and prohibits their formation at the final stages of stellar evolution, because the limiting mass of neutron stars is substantially higher and corresponds to considerably lower matter densities.

astro-ph.HE↗
arXiv2026

Magnetic field dynamics in presence of Hall conductivity and thermal diffusion

Anisotropy of kinetic coefficients in presence of a magnetic field is represented by Hall currents, which appear in a collisional medium due to action of the Lorentz force on the charged particles between collisions. We derive equations, describing dynamics of the magnetic field in presence of thermal diffusion with Hall currents, using a standard electrodynamic consideration. The influence of the Hall currents, at presence of thermal diffusion, on the magnetic field structure is considered in simple models. The equation is derived, which includes additional term for a seed magnetic field creation by the mechanism known as "Biermann battery" in the unmagnetized plasma.

physics.plasm-ph↗
arXiv2025

Nonlinear Hall effect in the stationary cylinder with a radial heat flux

A conducting cylinder with a uniform magnetic field along its axis and radial temperature gradient is considered at the stationary state. At large temperature gradients the azimuthal Hall electrical current creates the axial magnetic field which strength may be comparable with the original one. It is shown, that the magnetic field, generated by the azimuthal Hall current, leads to the decrease of magnetic field originated by external sources, and this suppression increases with increase of the electromotive force, connected with a thermodiffusion. Obtained results can help to investigate influence of the Hall current on the coupled magneto-thermal evolution of magnetic and electric fields in neutron stars, white dwarfs, and, possibly, in a laboratory facilities.

astro-ph.HE↗
arXiv2025

Stellar critical parameters in the uniform density approximation

Stellar models are calculated in the approximation of a uniform density distribution. Variational method was used for determination of the boundary of a stability loss, for stellar masses in the range from 2 up to $10^5$ $M_{\odot}$. The effects of the general relativity had been taken into account. The equation of state in the temperature and density ranges $10^9 < T < 10^{10} K$, $10^5 < ρ< 10^{10}\,\frac{g}{cm^3}$ had been taken from the work of Imshennik and Nadyozhin (1965). The critical parameters for the values of entropy and stellar masses differ from more accurate values, obtained using a more complicated variant of accepted density distribution, not more than 12$\%$.

astro-ph.HE↗
arXiv2024

Thermodiffusion unipolar electric generator

A model of a conducting cylinder with a radial temperature gradient which creates an electric field that increases with time in the surrounding vacuum is examined. The conditions under which this model functions are pointed out. An electric field is also generated when a magnetic field exists along the axis of the cylinder. This article discusses the interactions of the thermal flux, magnetic field, and charge distribution. Four models are considered with different conditions for the supply of electrons from a central source and the possibility either of capturing electrons inside the cylinder or their freely leaving it through the outer boundary.

astro-ph.HE↗
arXiv2024

Wave propagation and transformation in the frame of magnetohydrodynamics with a vortex electric field

Bright transient objects in different wave bands have been discovered in recent years. To explain these short (from ms to s), and very powerful events different models, galactic and extragalactic, have been considered. One of popular model is based on the suggestion of transformation of the magnetized plasma blob, presumably a MHD shock wave, moving with relativistic speed, into electromagnetic pulse after collision with some obstacle, or at entering in media with different properties. Such transformation cannot be described in the frame of classical MHD equations, which don't contain the displacement current, and don't include such transformation. Updating of the classic MHD equations by using of full set of Maxwell equation with displacement current, made recently by several authors, gave a possibility to describe hydrodynamic phenomena, together with electromagnetic ones by the same set of equations. Here we apply these equations for study of propagation of different waves with account of displacement current, and of transformation between different kind of waves in the nonuniform media. We derive a dispersion equation describing a propagation of a weak linear wave in a uniform magnetized plasma, and consider in details the cases with perpendicular and parallel motion to the magnetic field. It contains MHD, HD, Alfven and EM waves in the limiting cases, and some new types of behaviour in a general situation. We consider a plasma with zero viscosity and heat conductivity, with a finite scalar electric conductivity. We have shown, that transformation of MHD into EM wave may happen only in a highly magnetized plasma, and have found the region of parameters, where no waves exist, and only damping static perturbations may be present.

physics.plasm-ph↗
arXiv2023

Neutron stars in a uniform density approximation

Models of neutron stars are considered in the case of a uniform density distribution. An algebraic equation, valid for any equation of state, is obtained. This equation allows one to find the approximate mass of a star of a given density without resorting to the integration of differential equations. The solutions presented in the paper for various equations of state, including more realistic ones, differ from the exact solutions obtained by the numerical integration of differential equations by at most ~20%

astro-ph.HE↗
arXiv2023

Eliminating the Hubble Tension in the Presence of the Interconnection between Dark Energy and Matter in the Modern Universe

It is accepted in modern cosmology that the scalar field responsible for the inflationary stage of the early Universe is completely transformed into matter. It is assumed that the accelerated expansion is currently driven by dark energy (DE), which is likely determined by Einstein's cosmological constant. We consider a cosmological model where DE can have two components, one of which is Einstein's constant ($Λ$) and the other, smaller variable component DEV ($Λ_V$), is associated with the remnant of the scalar field that caused inflation after the main part of the scalar field has turned into matter. It is assumed that such a transformation continues at the present time and is accompanied by the reverse process of the DM transformation into a scalar field. The interconnection between DM and DEV, which leads to a linear relationship between the energy densities of these components after recombination $ρ_{DM}=α\;ρ_{DEV}$, is considered. Variants with a dependence of the coefficient $α(z)$ on the redshift are also considered. One of the problems that have arisen in modern cosmology, called Hubble Tension (HT), is the discrepancy between the present values of the Hubble constant measured from observations at small redshifts $z\lesssim1$ and the values found from fluctuations of the cosmic microwave background at large redshifts $z\approx1100$. In the considered model, this discrepancy can be explained by the deviation of the real cosmological model from the conventional cold dark matter (CDM) model of the Universe by action of the additional DE component at the stages after recombination. Within this extended model, we consider various $α(z)$ functions that can eliminate the HT. To maintain the ratio of DEV and DM energy densities close to constant over the interval $0\le z\le1100$, we assume the existence of a wide spectrum of DM particle masses.

astro-ph.CO↗
arXiv2022

About the observational check of the mechanism of gamma radiation in Soft Gamma Repeaters (SGR)

Soft gamma repeaters (SGR) are identified as single neutron stars (NS) inside the Galaxy, or nearby galaxies, with sporadic transient gamma radiation. A total number of discovered SGR, including relative Anomalous X-ray pulsars (AXP), is few tens of objects. Many of them show periodic radiation, connected with NS rotation, with periods 2-12 s. The slow rotation is accompanied by small rate of loss of rotational energy, which is considerably smaller than the observed sporadic gamma ray luminosity, and is many orders less that the luminosity during giant bursts, observed in 4 SGR. Therefore the energy source is usually connected with annihilation of very strong NS magnetic field. Another model is based on release of a nuclear energy stored in the NS nonequilibrium layer. We suggest here an observational test with could distinguish between these two models.

astro-ph.HE↗
arXiv2022

White dwarfs in a uniform sphere approximation, with general relativity effects taken into account

The limiting mass of cold white dwarfs was first calculated by E. Stoner in an approximate model of a uniform star and was soon reduced by ~20% in papers by S. Chandrasekhar and L. D. Landau based on an exact solution of the equations for the stellar equilibrium. Here we examine uniform models of white dwarfs taking general relativity effects and the influence of finite temperature into account. Solutions are obtained in the form of finite analytic formulas and, for masses differing by no more than ~20% from the exact solutions, found by numerical integration of the differential equations for the stellar equilibrium.

astro-ph.SR↗
arXiv2021

Dynamic model of a non-equilibrium chemical composition formation in the shell of single neutron stars

The process of a non-equilibrium chemical composition formation in the crust of a new born neutron star is considered, during cooling due to neutrino energy losses. A model is constructed for explaining accumulation of a large quantity of nuclear energy, which can maintain the X-ray luminosity of such compact objects for a long period of time. We studied numerically the dependence of the final chemical composition on various parameters of the model.

astro-ph.HE↗
arXiv2021

Multilayer spherical stellar cluster with uniform density

Various solutions of the kinetic equation for the equilibrium of a gravitating sphere of uniform density with a quadratic gravitational potential and a linear dependence of gravitational force on radius are examined. New analytic solutions are obtained for a uniform sphere with a hollow spherical volume and central mass inside the sphere. Solutions are also obtained for an arbitrary number of spherical layers with the same density, but with different equilibrium distribution functions.

astro-ph.GA↗
arXiv2021

Strong Shock in the Uniformly Expanding Universe with a Spherical Void

Propagation of strong shock wave in the expanding universe is studied using approximate analytic, and exact numerical solution of self-similar equations. Both solutions have similar properties, which change qualitatively, depending on the adiabatic powers $γ$. In the interval $1<γ<γ_{cr} \sim 1.16$ analytic and numeric solutions fill all the space without any voids and they are rather close to each other. At larger $γ>γ_{cr}$ a pressure becomes zero at finite radius, and a spherical void appears around the origin in both solutions. All matter is collected in thin layer behind the shock wave front. The structure of this layer qualitatively depends on $γ$. At the inner edge of the layer the pressure is always zero, but the density on this edge is jumping from zero to infinity at $γ\approx 1.4$ in both solutions.

gr-qc↗
arXiv2021

Phenomenological model explaining Hubble Tension origin

One of the problem revealed recently in cosmology is a so-called Hubble tension (HT), which is the difference between values of the present Hubble constant, measured by observation of the universe at redshift $z \lesssim 1$, and by observations of a distant universe with CMB fluctuations originated at $z \sim 1100$. In this paper we suggest, that this discrepancy may be explained by deviation of the cosmological expansion from a standard Lambda-CDM %simple Friedman model of a flat universe, during the period after recombination at $z \lesssim 1100$, due to action of additional variable component of a dark energy of different origin.. We suppose, that a dark matter (DM) has a common origin with a variable component of a dark energy (DEV). DE presently may have two components, one of which is the Einstein constant $Λ$, and another, smaller component DEV ($Λ_V$) comes from the remnants of a scalar fields responsible for inflation. Due to common origin and interconnections the densities of DEV and DM are supposed to be connected, and remain almost constant during, at least, the time after recombination, when we may approximate $ρ_{DM}=αρ_{DEV}$. This part of the dark energy in not connected with the cosmological constant $Λ$, but is defined by existence of scalar fields with a variable density. Taking into account the influence of DEV on the universe expansion we find the value of $α$ which could remove the HT problem. In order to maintain the almost constant DEV/DM energy density ratio during the time interval at $z<1100$, we suggest an existence of a wide mass DM particle distribution.

astro-ph.CO↗
arXiv2020

Nonequilibrium Layer in the Crust of Neutron Stars and Nonequilibrium Beta-Processes in Astrophysics

The formation of the chemical composition of neutron star envelopes, at high densities is considered. As hot matter is compressed in the process of collapse, which leads to the explosion of a core collapse supernova, the stage of nuclear equilibrium with free neutrino escape, kinetic equilibrium in \b{eta}-processes, and, as a result, the establishment of limited nuclear equilibrium with a fixed number of nuclei takes place. Cold matter is compressed at a fixed number of nuclei whose atomic weight initially does not change and subsequently decreases. A pycnonuclear reaction of the fusion of available nuclei and a decrease in their number begin at the end. The compression of cold matter is accompanied by an increase in the mass fraction of free neutrons. In this case, the chemical composition of the envelope differs significantly from the equilibrium one and contains a considerable store of nuclear energy. Nonequilibrium \b{eta}-reactions proceed at densities exceeding the upper bound for the non-equilibrium layer density, which lead to heating, nuclear energy release, and the possible attainment of a state of complete thermodynamic equilibrium. The thermodynamics of nonequilibrium \b{eta}-processes, which lead to the heating of matter as neutrinos escape freely, is considered.

astro-ph.HE↗
arXiv2019

3D numerical study of an anisotropic heat transfer in outer layers of magnetized neutron stars

Periodic changes in a thermal soft X-ray flux of a rotating neutron star indicate a non-uniform distribution of the surface temperature. A possible cause of this phenomenon is a suppression of the heat flux across the magnetic field lines in a crust and an envelope of magnetized neutron stars. In this paper we study three-dimensional effects, associated with non-axisymmetric magnetic fields in neutron stars. We calculate the surface temperature distribution by solving numerically a three dimensional heat transfer equation in a magnetized neutron star crust. We adopt an anisotropic (tensorial) electron thermal conductivity coefficient, which is derived as an analytical solution of the Boltzmann equation with a Chapman-Enskog method. To calculate the surface temperature distribution, we construct a local one-dimensional plane-parallel model ("Ts-Tb"-relationship) of a magnetized neutron star envelope. We then use it as an outer boundary condition for the three-dimensional problem in the crust to find the self-consistent solution. To study possible observational manifestations from anisotropic temperature distributions we calculate light curves with a composite black-body model. Our calculations show, that a non-axisymmetric magnetic field distribution can lead to the irregular non-sinusoidal shape of a pulse profile as well as in some cases a significant amplification of pulsations of the thermal flux in comparison to the pure-dipolar magnetic field configurations.

astro-ph.HE↗
arXiv2019

Shadow of black holes at local and cosmological distances

A brief illustrative discussion of the shadows of black holes at local and cosmological distances is presented. Starting from definition of the term and discussion of recent observations, we then investigate shadows at large, cosmological distances. On a cosmological scale, the size of shadow observed by comoving observer is expected to be affected by cosmic expansion. Exact analytical solution for the shadow angular size of Schwarzschild black hole in de Sitter universe was found. Additionally, an approximate method was presented, based on using angular size redshift relation. This approach is appropriate for general case of any multicomponent universe (with matter, radiation and dark energy). It was shown, that supermassive black holes at cosmological distances in universe with matter may give the shadow size comparable with the shadow size in M87, and in the center of our Galaxy.

gr-qc↗