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A conducting cylinder with a central source of electrons, in a uniform magnetic field along its axis, and radial temperature gradient, is considered at the stationary state. Interaction of heat flux, magnetic field and charge distribution is discussed. Four different models are considered, regarding of the electronic supply and possibility of electrons to leave the cylinder.

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%, Comment: 15 pages, 9 figures

Interaction of plasma flow with a magnetic obstacle is a frequent process in many laser-plasma experiments in the laboratory, and is an important event in many astrophysical objects such as X-ray pulsars, AGN, GRB etc. As a result of plasma penetration through the magnetic wall we could expect a formation of magnetohydrodynamic (MHD) shock waves, as well as of electromagnetic (EM) ones. To study these processes we need equations following from hydrodynamic and Maxwell equations, which in the limiting situations describe MHD and EM waves, and are valid for the general case, when both phenomena are present. Here we derive a set of equations following from hydrodynamic and Maxwell equations, without neglecting a displacement current, needed for a formation of EM waves. We find a dispersion equation describing a propagation of a weak linear wave in a magnetized plasma along the $x$ axis, perpendicular to the magnetic field $H_y(x)$, which contains MHD, hydrodynamic 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, but with a finite electric conductivity with a scalar coefficient., Comment: 8 papers, 8 figures, 1 table, to be submitted in PRE

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 ($\Lambda$) and the other, smaller variable component DEV ($\Lambda_V$), 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 $\rho_{DM}=\alpha\;\rho_{DEV}$, is considered. Variants with a dependence of the coefficient $\alpha(z)$ 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 $\alpha(z)$ 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.

A conducting cylinder, with a uniform magnetic field along its axis, and radial temperature gradient, is considered. 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., Comment: 15 pages, 13 figures

The T2K experiment widely uses plastic scintillator as a target for neutrino interactions and an active medium for the measurement of charged particles produced in neutrino interactions at its near detector complex. Over 10 years of operation the measured light yield recorded by the scintillator based subsystems has been observed to degrade by 0.9--2.2\% per year. Extrapolation of the degradation rate through to 2040 indicates the recorded light yield should remain above the lower threshold used by the current reconstruction algorithms for all subsystems. This will allow the near detectors to continue contributing to important physics measurements during the T2K-II and Hyper-Kamiokande eras. Additionally, work to disentangle the degradation of the plastic scintillator and wavelength shifting fibres shows that the reduction in light yield can be attributed to the ageing of the plastic scintillator., Comment: 29 pages, 18 figures. Prepared for submission to JINST

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., Comment: 6 pages

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., Comment: 8 pages, 4 figures

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., Comment: 17 pages, 18 figures

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., Comment: 8 pages, 1 figure

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 $\gamma$. In the interval $1<\gamma<\gamma_{cr} \sim 1.16$ analytic and numeric solutions fill all the space without any voids and they are rather close to each other. At larger $\gamma>\gamma_{cr}$ 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 $\gamma$. 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 $\gamma \approx 1.4$ in both solutions., Comment: Keywords: cosmology, strong shock wave, self-similar solution. arXiv admin note: text overlap with arXiv:1808.02112

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 $\Lambda$, and another, smaller component DEV ($\Lambda_V$) 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 $\rho_{DM}=\alpha \rho_{DEV}$. This part of the dark energy in not connected with the cosmological constant $\Lambda$, 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 $\alpha$ 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., Comment: 17 pages

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., Comment: JETP 2019, V.129, p. 503. Special issue in honor of I.M. Khalatnikov 100th anniversary

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., Comment: Proceedings paper from "High Energy Phenomena in Relativistic Outflows VII" (HEPRO7, 9-12 July 2019, Facultat de Fisica, Universitat de Barcelona, Spain), accepted by Proceedings of Science (PoS)

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., Comment: 14 pages, 16 figures; based on talk at the conference 'Multifrequency Behaviour of High Energy Cosmic Sources - XIII - MULTIF2019'; see also our previous papers arXiv:1804.04898, arXiv:1805.03311

We consider the Kepler two-body problem in presence of the cosmological constant $\Lambda$. Contrary to the classical case, where finite solutions exist for any angular momentum of the system $L$, in presence of $\Lambda$ finite solutions exist only in the interval $0

The exact time-dependent solution is obtained for a magnetic field growth during a spherically symmetric accretion into a black hole (BH) with a Schwarzschild metric. Magnetic field is increasing with time, changing from the initially uniform into a quasi-radial field. Equipartition between magnetic and kinetic energies in the falling gas is established in the developed stages of the flow. Estimates of the synchrotron radiation intensity are presented for the stationary flow. The main part of the radiation is formed in the region $r \leq 7 r_g$, here $r_g$ is a BH gravitational radius. The two-dimensional stationary self-similar magnetohydrodynamic solution is obtained for the matter accretion into BH, in a presence of a large-scale magnetic field, when the magnetic field far from the BH is homogeneous and does not influence the flow. At the symmetry plane perpendicular to the direction of the distant magnetic field, the quasi-stationary disk is formed around BH, which structure is determined by dissipation processes. Parameters of the shock forming due to matter infall onto the disk are obtained. The radiation spectrum of the disk and the shock are obtained for the $10\,\, M_\odot$ BH. The luminosity of such object is about the solar one, for a characteristic galactic gas density, with possibility of observation at distances less than 1 kpc. The spectra of a laminar and a turbulent disk structure around BH are very different. The turbulent disk emits a large part of its flux in the infrared. It may occur that some of the galactic infrared star-like sources are a single BH in the turbulent accretion state. The radiative efficiency of the magnetized disk is very high, reaching $\sim 0.5\,\dot M\,c^2$ so it was called recently as a magnetically arrested disk (MAD). Numerical simulations of MAD, and its appearance during accretion into neutron stars are considered and discussed., Comment: 26 pages, 6 figures, accepted for publication in the "Universe" journal

Jets are observed in young stellar objects, X-ray sources, active galactic nuclei (AGN). The mechanisms of jet formation may be divided in regular, acting continuously for a long time, and explosive ones. Continuous mechanisms are related with electrodynamics and radiation pressure acceleration, hydrodynamical acceleration in the nozzle inside a thick disk, acceleration by relativistic beam of particles. Explosive jet formation is connected with supernovae, gamma ray bursts and explosive events in galactic nuclei. Mechanisms of jet collimation may be connected with magnetic confinement, or a pressure of external gas. Explosive formation of jets in the laboratory is modeled in the experiments with powerful laser beam, and plasma focus., Comment: 18 pages, 17 figures, PoS (FRAPWS2018) 061

The idea of the magnetorotational explosion mechanism is that the energy of rotation of the neutron star formed in the course of a collapse is transformed into the energy of an expanding shock wave by means of a magnetic field. In the two-dimensional case, the time of this transformation depends weakly on the initial strength of the poloidal magnetic field because of the development of a magnetorotational instability. Differential rotation leads to the twisting and growth of the toroidal magnetic-field component, which becomes much stronger than the poloidal component. In the case where the initial configuration of the magnetic field is close to a dipole configuration, the ejection of matter has a jet character, whereas, in the case of a quadrupole configuration, there arises an equatorial ejection. In either case, the energy release is sufficient for explaining the observed average energy of supernova explosion. Neutrinos are emitted as the collapse and the formation of a rapidly rotating neutron star proceeds. In addition, neutrino radiation arises in the process of magnetorotational explosion owing to additional rotational-energy losses. In order to explain an interval of 4.5 hours between the two observed neutrino signals from SN 1987A, it is necessary to assume a weakening of the magnetorotional instability and a small initial magnetic field (10(9)-10(10)G) in the newly formed rotating neutron star. The existence of a black hole in the SN 1987A remnant could explain the absence of any visible pointlike source at the center of the explosion., Comment: 14 pages, 15 figures

The results of MHD numerical simulations of the formation and development of magnetized jets are presented. Similarity criteria for comparisons of the results of laboratory laser experiments and numerical simulations of astrophysical jets are discussed. The results of laboratory simulations of jets generated in experiments at the Neodim laser installation are presented., Comment: 21 pages, 19 figures, published in Astronomy reports 62, 162-188, 2018

A solution to the Boltzmann equation is obtained for a magnetized plasma with strongly degenerate nonrelativistic electrons and nondegenerate nuclei. The components of the diffusion, thermal diffusion and diffusion thermoeffect tensors in a nonquantizing magnetic field are calculated in the Lorentz approximation without allowance for electron-electron collisions, which is asymptotically accurate for plasma with strongly degenerate electrons. Asymptotically accurate analytical expressions for the electron diffusion, thermal diffusion and diffusion thermoeffect tensors in the presence of a magnetic field are obtained for the first time. The expressions reveal a considerably more complicated dependence on magnetic field than analogous dependences derived in the previous publications on this subject., Comment: 12 pages, 4 figures

If the large scale structure of the Universe was created, even partially, via Zeldovich pancakes, than the fluctuations of the CMB radiation should be formed due to bulk comptonization of black body spectrum on the contracting pancake. Approximate formulas for the CMB energy spectrum after bulk comptonization are obtained. The difference between comptonized energy spectra of the CMB due to thermal and bulk comptonization may be estimated by comparison of the plots for the spectra in these two cases., Comment: 13 pages, 4 figures

A mechanism of formation of gravitational waves in the Universe is considered for a nonspherical collapse of matter. Nonspherical collapse results are presented for a uniform spheroid of dust and a finite-entropy spheroid. Numerical simulation results on core-collapse supernova explosions are presented for the neutrino and magnetorotational models. These results are used to estimate the dimensionless amplitude of the gravitational wave with a frequency \nu ~1300 Hz, radiated during the collapse of the rotating core of a pre-supernova with a mass of 1:2M(sun) (calculated by the authors in 2D). This estimate agrees well with many other calculations (presented in this paper) that have been done in 2D and 3D settings and which rely on more exact and sophisticated calculations of the gravitational wave amplitude. The formation of the large-scale structure of the Universe in the Zel'dovich pancake model involves the emission of very long-wavelength gravitational waves. The average amplitude of these waves is calculated from the simulation, in the uniform spheroid approximation, of the nonspherical collapse of noncollisional dust matter, which imitates dark matter. It is noted that a gravitational wave radiated during a core-collapse supernova explosion in our Galaxy has a sufficient amplitude to be detected by existing gravitational wave telescopes.

In the framework of the Theory of General Relativity, models of stars with an unusual equation of state $\rho c^2<0$, $P>0$ where $\rho$ is the mass density and $P$ is the pressure, are constructed. These objects create outside themselves the forces of gravitational repulsion. The equilibrium of such stars is ensured by a non-standard balance of forces. Negative mass density, acting gravitationally on itself, creates an acceleration of the negative mass, directed from the center. Therefore in the absence of pressure such an object tends to expand. At the same time, the positive pressure, which falls just like in ordinary stars from the center to the surface, creates a force directed from the center. This force acts on the negative mass density, which causes acceleration directed the opposite of the acting force, that is to the center of the star. This acceleration balances the gravitational repulsion produced by the negative mass. Thus, in our models gravity and pressure change roles: the negative mass tends to create a gravitational repulsion, while the gradient of the pressure acting on the negative mass tends to compress the star. In this paper, we construct several models of such a star with various equations of state., Comment: 6 pages, 4 figures

It is shown that investigation of ionization zones left after supernova explosions can yield information on the mechanism active in those explosions., Comment: 3 pages. In connection with: arXiv:1709.09190v1 [astro-ph.SR] 26 Sep 2017

The observational properties of Soft Gamma Repeaters and Ano\-malous X-ray Pulsars (SGR/AXP) indicate to necessity of the energy source different from a rotational energy of a neutron star. The model, where the source of the energy is connected with a magnetic field dissipation in a highly magnetized neutron star (magnetar) is analyzed. Some observational inconsistencies are indicated for this interpretation. The alternative energy source, connected with the nuclear energy of superheavy nuclei stored in the nonequilibrium layer of low mass neutron star is discussed., Comment: 29 pages, 13 figures, Springer International Publishing Switzerland 2016 A.W. Alsabti, P. Murdin (eds.), Handbook of Supernovae

Electron-positron annihilation line from a Galactic center direction was discovered by the balloon-borne germanium gamma-ray telescope, and confirmed by OSSE experiment of CGRO mission. Extensive observations by INTEGRAL observatory permit to determine properties of the annihilation line from Galactic bulge. Possible sources of $e^{+}$ already discussed are supernovae explosions, microquasars, gamma-ray bursts, tidal disruption events, activity near black hole of Sgr A*, $e^{+}$ generation by subrelativistic cosmic rays, or even dark matter. One remarkable feature of the line emission is an absence of resolved point like sources. Any model should take into account this feature. We consider flares of low-mass stars as a possible cumulative $e^{+}$ source of the observed annihilation line from the bulge. Our estimations show that $e^{+}$ production by numerous flaring stars in Galaxy bulge can be significant, or even the main source of $e^{+}$ responsible for the annihilation line observed from the central part of our Galaxy.

Strong discontinuities in solutions of the gas dynamic equations under isentropic conditions, i.e., with continuity of entropy at the discontinuity, are examined. Solutions for a standard shock wave with continuity of energy at the discontinuity are compared with those for an isentropic 'shock wave'. It is shown that numerical simulation of astrophysical problems in which high-amplitude shock waves are encountered (supernova explosions, modelling of jets) with conservation of entropy, rather than of energy, leads to large errors in the shock calculations. The isentropic equations of gas dynamics can be used only when there are no strong discontinuities in the solution or when the intensity of the shocks is not high and they do not significantly affect the flow., Comment: 10 pages, 5 figures

The solution of Boltzmann equation for plasma in magnetic field, with arbitrarily degenerate electrons and non-degenerate nuclei, is obtained by Chapman-Enskog method. Functions, generalizing Sonin polynomials are used for obtaining an approximate solution. Fully ionized plasma is considered. The tensor of the heat conductivity coefficients in non-quantized magnetic field is calculated. For non-degenerate and strongly degenerate plasma the asymptotic analytic formulas are obtained, which are compared with results of previous authors. The Lorentz approximation, with neglecting of electron-electron encounters, is asymptotically exact for strongly degenerate plasma. We obtain, for the first time, in three polynomial approximation, with account of electron-electron collisions, analytical expressions for the heat conductivity tensor for non-degenerate electrons, in presence of a magnetic field. Account of the third polynomial improved substantially the precision of results. In two polynomial approximation our solution coincides with the published results. For strongly degenerate electrons we obtain, for the first time, an asymptotically exact analytical solution for the heat conductivity tensor in presence of a magnetic field. This solution has considerably more complicated dependence on the magnetic field than those in previous publications, and gives several times smaller relative value of a thermal conductivity across the magnetic field at $\omega\tau\gtrsim 0.8$., Comment: 23 pages, 3 figures. arXiv admin note: text overlap with arXiv:0710.0854 by other authors

We develop models for time lag between maxima of the source brightness in different wavelengths during a transient flash of luminosity connected with a short period of increase of the mass flux onto the central compact object. We derive a simple formula for finding the time delay among events in different wavelengths, valid in general for all disk accreting cosmic sources, and discuss quantitatively a model for time lag formation in AGNs. In close binaries with accretion disks the time lag is connected with effects of viscosity defining a radial motion of matter in the accretion disk. In AGN flashes, the falling matter has a low angular momentum, and the time lag is defined by the free fall time to the gravitating center. We show the validity of these models by means of several examples of galactic and extragalactic accreting sources., Comment: 7 pages

The motion of classical spinning test particles in the equatorial plane of a Kerr black hole is considered for the case where the particle spin is perpendicular to the equatorial plane. We review some results of our recent research of the innermost stable circular orbits (ISCO) [P.I. Jefremov, O.Yu. Tsupko and G.S. Bisnovatyi-Kogan, Phys.Rev. D 91 124030 (2015)] and present some new calculations. The ISCO radius, total angular momentum, energy, and orbital angular frequency are considered. We calculate the ISCO parameters numerically for different values of the Kerr parameter $a$ and investigate their dependence on both black hole and test particle spins. Then we describe in details how to calculate analytically small-spin corrections to the ISCO parameters for an arbitrary values of $a$. The cases of Schwarzschild, slowly rotating Kerr and extreme Kerr black hole are considered. The use of the orbital angular momentum is discussed. We also consider the ISCO binding energy. It is shown that the efficiency of accretion onto an extreme Kerr black hole can be larger than the maximum known efficiency (42 %) if the test body has a spin., Comment: Paper is written for special issue of the journal dedicated to the centenary of Prof. K.P. Staniukovich. Some results of our previous paper arXiv:1503.07060 are used

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. Radiopulsars in close binaries and millisecond pulsars, which have passed the stage of disk accretion (recycled radiopulsars), have magnetic fields 2-4 orders of magnitude smaller than ordinary single pulsars. Most probably, the magnetic field of the neutron star was screened by the infalling material. Several screening models are considered. Formation of single recycled pulsars loosing its companion is discussed. Magnetic fields of some 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 repeaters and their interpretation as magnetars are critically analyzed., Comment: 29 pages, 5 figures, Chapter from "Neutron Stars, Supernovae and Supernovae Remnants", Authors: A. Ankay, G. S. Bisnovatyi-Kogan, O. H. Guseinov, D. M. Sedrakian, K. M. Shahabasyan, S. O. Tagieva, Dejan Urosevic, E. Yazgan. : Nova Sci. Publ. 2007. arXiv admin note: text overlap with arXiv:astro-ph/0311031, arXiv:astro-ph/0401086 by other authors

We compare polarization properties of the cyclotron, and relativistic dipole radiation of electrons moving in the magnetic field on a helix with ultra-relativistic longitudinal and non-relativistic transverse velocity components. The applicability of these models in the case of accretion onto a neutron star is discussed. The test, based on polarization observations is suggested, to distinguish between the cyclotron, and relativistic dipole origin of features, observed in X-ray spectra of some X-ray sources, among which the Her X-1 is the most famous., Comment: 8 pages, 13 figures

The influence of plasma on different effects of gravitational lensing is reviewed. Using the Hamiltonian approach for geometrical optics in a medium in the presence of gravity, an exact formula for the photon deflection angle by a black hole (or another body with a Schwarzschild metric) embedded in plasma with a spherically symmetric density distribution is derived. The deflection angle in this case is determined by the mutual combination of different factors: gravity, dispersion, and refraction. While the effects of deflection by the gravity in vacuum and the refractive deflection in a nonhomogeneous medium are well known, the new effect is that, in the case of a homogeneous plasma, in the absence of refractive deflection, the gravitational deflection differs from the vacuum deflection and depends on the photon frequency. In the presence of a plasma nonhomogeneity, the chromatic refractive deflection also occurs, so the presence of plasma always makes gravitational lensing chromatic. In particular, the presence of plasma leads to different angular positions of the same image if it is observed at different wavelengths. It is discussed in detail how to apply the presented formulas for the calculation of the deflection angle in different situations. Gravitational lensing in plasma beyond the weak deflection approximation is also considered., Comment: review; see also our previous papers: arXiv:0803.2468, arXiv:0809.1021, arXiv:1006.2321, arXiv:1305.7032, arXiv:1505.06481

In this paper we review a recently developed approximate method for investigation of dynamics of compressible ellipsoidal figures. Collapse and subsequent behaviour are described by a system of ordinary differential equations for time evolution of semi-axes of a uniformly rotating, three-axis, uniform-density ellipsoid. First, we apply this approach to investigate dynamic stability of non-spherical bodies. We solve the equations that describe, in a simplified way, the Newtonian dynamics of a self-gravitating non-rotating spheroidal body. We find that, after loss of stability, a contraction to a singularity occurs only in a pure spherical collapse, and deviations from spherical symmetry prevent the contraction to the singularity through a stabilizing action of nonlinear non-spherical oscillations. The development of instability leads to the formation of a regularly or chaotically oscillating body, in which dynamical motion prevents the formation of the singularity. We find regions of chaotic and regular pulsations by constructing a Poincare diagram. A real collapse occurs after damping of the oscillations because of energy losses, shock wave formation or viscosity. We use our approach to investigate approximately the first stages of collapse during the large scale structure formation. The theory of this process started from ideas of Ya. B. Zeldovich, concerning the formation of strongly non-spherical structures during nonlinear stages of the development of gravitational instability, known as 'Zeldovich's pancakes'. In this paper the collapse of non-collisional dark matter and the formation of pancake structures are investigated approximately. We estimate an emission of very long gravitational waves during the collapse, and discuss the possibility of gravitational lensing and polarization of the cosmic microwave background by these waves., Comment: Review for special issue: 'Macroscopic randomness in astrophysical plasmas: The legacy and vision of Ya. B. Zeldovich'; see also our previous papers: arXiv:astro-ph/0212268, arXiv:astro-ph/0410573, arXiv:astro-ph/0512309, arXiv:0801.2538, arXiv:0809.1007

The optical behaviour of the Be star in the high-mass X-ray transient A0535+26/HDE245770 shows that at periastron the luminosity is typically enhanced by 0.02 to a few tenths magnitude, and the X-ray outburst occurs eight days after the periastron. Indeed, at the periastron an increase of the mass flux occurs. This sort of flush reaches the external part of the temporary accretion disk around the neutron star and moves to the hot central parts of the accretion disk and the neutron star surface. The time necessary for this way is dependent on the turbulent viscosity in the accretion disk, as discussed by Giovannelli, Bisnovatyi-Kogan, and Klepnev (2013) (GBK13). In this paper we will show the behaviour of this system in optical band around the predicted periastron passage on 21st February 2014, by using the GBK13 ephemeris that we used to schedule our spectroscopic and photometric optical observations. Spectroscopic unusual activity detected in the Balmer lines and the enhancement in the emission in B, V, and R bands around the periastron passage, and the subsequent X-ray event definitively demonstrate the existence of about 8 day delay between optical and X-ray flares.

Core-collapse supernovae are accompanied by formation of neutron stars. The gravitation energy is transformed into the energy of the explosion, observed as SN II, SN Ib,c type supernovae. We present results of 2-D MHD simulations, where the source of energy is rotation, and magnetic field serves as a "transition belt" for the transformation of the rotation energy into the energy of the explosion. The toroidal part of the magnetic energy initially grows linearly with time due to differential rotation. When the twisted toroidal component strongly exceeds the poloidal field, magneto-rotational instability develops, leading to a drastic acceleration in the growth of magnetic energy. Finally, a fast MHD shock is formed, producing a supernova explosion. Mildly collimated jet is produced for dipole-like type of the initial field. At very high initial magnetic field no MRI development was found., Comment: 8 pages, 13 figures; Proc. Frascati Workshop 2013, Acta Polytechnica Supplement (accepted)

The optical behaviour of the Be star in the high mass X-ray transient A0535+26/HDE245770 shows that at the periastron typically there is an enhancement in the luminosity of order 0.05 to few tenths mag, and the X- ray outburst happens about 8 days after the periastron. We construct a quantitative model of this event, basing on a nonstationary accretion disk behavior, connected with a high ellipticity of the orbital motion. We explain the observed time delay between the peaks of the optical and X-ray outbursts in this system by the time of radial motion of a matter in the accretion disk, after increase of the mass flux in the vicinity of a periastral point in the binary. This time is determined by the turbulent viscosity, with the parameter $ \alpha=0.1-0.3$, estimated from the comparison of the model with observational data, Comment: 6 pages, 3 figures. arXiv admin note: substantial text overlap with arXiv:1305.5149

Propagation of the strong shock in the flat expanding Friedman universe is investigated using methods of dimension and similarity. Exact analytic solution of self-similar equations is obtained, determining dependences of the radius and velocity of the shock wave on time and radius. It is obtained, that in the expanding medium the velocity of shock decreases as $\sim t^{-1/5}$, what is slower than the shock velocity in the static uniform medium $\sim t^{-3/5}$. The radius of the shock wave in the expanding self-gravitating medium increases $\sim t^{4/5}$, more rapidly than the shock wave radius in the uniform non-gravitating medium $\sim t^{2/5}$. So, the shock propagates in the direction of decreasing density with larger speed, that in the static medium, due to accelerating action of the decreasing density, even in the presence of a self-gravitation., Comment: 16 pages. Shorter version is published in Gravitation and Cosmology, 2015, vol.21, pp. 236-240

In this paper we address the question of why the observed periods of the Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are clustered in the range 2-12 s. We explore a possibility to answer this question assuming that AXPs and SGRs are the descendants of High Mass X-ray Binaries (HMXBs) which have been disintegrated in the core-collapse supernova explosion. The spin period of neutron stars in HMXBs evolves towards the equilibrium period, averaging around a few seconds. After the explosion of its massive companion, the neutron star turns out to be embedded into a dense gaseous envelope, the accretion from which leads to the formation of a residual magnetically-levitating (ML) disk. We show that the expected mass of a disk in this case is 10^-7 - 10^-8 M_sun which is sufficient to maintain the process of accretion at the rate 10^14 - 10^15 g/s over a time span of a few thousand years. During this period the star manifests itself as an isolated X-ray pulsar with a number of parameters resembling those of AXPs and SGRs. Period clustering of such pulsars can be provided if the lifetime of the residual disk does not exceed the spin-down timescale of the neutron star., Comment: accepted for publication in Astronomy Reports v. 59, No. 5 (2015)

We explore a possibility to explain the phenomenon of the Anomalous X-ray Pulsars (AXP) and Soft Gamma-ray Repeaters (SGR) within the scenario of fall-back magnetic accretion onto a young isolated neutron star. The X-ray emission of the pulsar in this case is originated due to accretion of matter onto the surface of the neutron star from the magnetic slab surrounding its magnetosphere. The expected spin-down rate of the neutron star within this approach is close to the observed value. We show that these neutron stars are relatively young and are going through a transition from the propeller state to the accretor state. The pulsars activity in the gamma-rays is connected with their relative youth and is provided by the energy stored in the non-equilibrium layer located in the crust of low-mass neutron stars. This energy can be released due to mixing of matter in the neutron star crust with super heavy nuclei approaching its surface and getting unstable. The nuclei fission in the low-density region initiates chain reactions leading to the nuclear explosion. The outbursts are likely to be triggered by an instability developing in the region where the matter accreted by the neutron star is accumulated at the magnetic pole regions., Comment: 15 pages, Published in Astronomy Reports, Vol. 58, Nr. 4, p. 217 (2014)