arXiv subjects

A. N. Baushev

Publications and source records attributed to A. N. Baushev.

11 recordsLinked to original sources

arXiv2026

A semi-analytical approach to cosmic void evolution

We present a theoretical study of the non-linear evolution of cosmic voids -- underdense regions that occupy a large fraction of the observable Universe. We model a void as an isolated homogeneous spheroidal (axisymmetric) ellipsoid embedded in a homogeneous $Λ$CDM universe. Starting from a small initial density contrast at redshift $z=500$, we numerically integrate the equations of motion for the ellipsoid semi-axes and follow their evolution to the present epoch. We examine the anisotropic expansion of the void and the corresponding change in its shape, characterised by the eccentricity $e$. We find that the void non-sphericity always decreases, but rather slowly: the eccentricity drops from $e\approx0.87$ at $z=500$ to $e\approx0.81$ at $z=0$. Thus the void becomes rounder but remains aspherical throughout the evolution. The evolution and final value of the void underdensity are virtually independent of the void's eccentricity. The nonlinearity of void evolution becomes apparent very early: a ten percent deviation from the linear regime occurs already at $z\simeq8$, when $\varepsilon = Δρ/ρ\sim10\%$. Notably, our calculations show that the majority of voids are not strongly underdense and contain a significant amount of matter, $μ>0.5$.

astro-ph.CO↗

arXiv2025

The impact of the eccentricity on the collapse of an ellipsoid into a black hole

We consider the gravitational collapse of a homogeneous pressureless ellipsoid. We have shown that the minimal size $r$ that the ellipsoid can reach during collapse depends on its initial eccentricity $e_0$ as $r\propto e_0^ν$, where $ν\approx 15/8$, and this dependence is very universal. We have estimated the parameters (in particular, the initial eccentricity) of a homogeneous pressureless ellipsoid, whereat it collapses directly into a black hole.

gr-qc↗

arXiv2019

The central cusps in dark matter halos: fact or fiction?

We investigate the reliability of standard N-body simulations by modelling of the well-known Hernquist halo with the help of \texttt{GADGET-2} code (which uses the tree algorithm to calculate the gravitational force) and \texttt{ph4} code (which uses the direct summation). Comparing the results, we find that the core formation in the halo center (which is conventionally considered as the first sign of numerical effects, to be specific, of the collisional relaxation) has nothing to do with the collisional relaxation, being defined by the properties of the tree algorithm. This result casts doubts on the universally adopted criteria of the simulation reliability in the halo center. Though we use a halo model, which is theoretically proved to be stationary and stable, a sort of numerical 'violent relaxation' occurs. Its properties suggest that this effect is highly likely responsible for the central cusp formation in cosmological modelling of the large-scale structure, and then the 'core-cusp problem' is no more than a technical problem of N-body simulations.

astro-ph.GA↗

arXiv2017

Cusps in the center of galaxies: a real conflict with observations or a numerical artefact of cosmological simulations?

Galaxy observations and N-body cosmological simulations produce conflicting dark matter halo density profiles for galaxy central regions. While simulations suggest a cuspy and universal density profile (UDP) of this region, the majority of observations favor variable profiles with a core in the center. In this paper, we investigate the convergency of standard N-body simulations, especially in the cusp region, following the approach proposed by \protect{\citep{13}}. We simulate the well known Hernquist model using the SPH code Gadget-3 and consider the full array of dynamical parameters of the particles. We find that, although the cuspy profile is stable, all integrals of motion characterizing individual particles suffer strong unphysical variations along the whole halo, revealing an effective interaction between the test bodies. This result casts doubts on the reliability of the velocity distribution function obtained in the simulations. Moreover, we find unphysical Fokker-Planck streams of particles in the cusp region. The same streams should appear in cosmological N-body simulations, being strong enough to change the shape of the cusp or even to create it. Our analysis, based on the Hernquist model and the standard SPH code, strongly suggests that the UDPs generally found by the cosmological N-body simulations may be a consequence of numerical effects. A much better understanding of the N-body simulation convergency is necessary before a 'core-cusp problem' can properly be used to question the validity of the CDM model.

astro-ph.GA↗

arXiv2012

Spectral analysis of the gamma-ray background near the dwarf Milky Way satellite Segue 1: Improved limits on the cross section of neutralino dark matter annihilation

The indirect detection of dark matter requires that dark matter annihilation products be discriminated from conventional astrophysical backgrounds. Here, we re-analyze GeV-band gamma-ray observations of the prominent Milky Way dwarf satellite galaxy Segue 1, for which the expected astrophysical background is minimal. We explicitly account for the angular extent of the conservatively expected gamma-ray signal and keep the uncertainty in the dark-matter profile external to the likelihood analysis of the gamma-ray data.

astro-ph.HE↗

arXiv2009

Dark matter annihilation in the gravitational field of a black hole

In this paper we consider dark matter particle annihilation in the gravitational field of black holes. We obtain exact distribution function of the infalling dark matter particles, and compute the resulting flux and spectra of gamma rays coming from the objects. It is shown that the dark matter density significantly increases near a black hole. Particle collision energy becomes very high affecting relative cross-sections of various annihilation channels. We also discuss possible experimental consequences of these effects.

astro-ph↗

arXiv2009

Electric charge estimation of a new-born black hole

Though a black hole can theoretically possess a very big charge ($Q/(\sqrt{G} M) \simeq 1$), the charge of the real astrophysical black holes is usually considered to be negligible. This supposition is based on the fact that an astrophysical black hole is always surrounded by some plasma, which is a very good conductor. However, it disregards that the black holes have usually some angular momentum, which can be interpreted as its rotation of a sort. If in the plasma surrounding the hole there is some magnetic field, it leads to the electric field creation and, consequently, to the charge separation. In this article we estimate the upper limit of the electric charge of stellar mass astrophysical black holes. We have considered a new black hole formation process and shown that the charge of a new-born black hole can be significant ($\sim 10^{13}$ {Coulombs}). Though the obtained charge of an astrophysical black hole is big, the charge to mass ratio is small $Q/(\sqrt{G} M) \sim 10^{-7}$, and it is not enough to affect significantly either the gravitational field of the star or the dynamics of its collapse.

astro-ph.HE↗

arXiv2008

A phase-resolved spectroscopic study of the X-ray binary system 4U0115+63 based on Beppo-Sax observations

We present a phase resolved spectral analysis of the X-ray binary 4U0115+63 based upon Beppo-Sax satellite observations. Three strong absorption lines have been detected at all phases. We interpreted them as a cyclotron resonant scattering. Existence of the fourth cyclotron line has been confirmed at some phases. The cyclotron feature is found to be strongly dependent on the phase of the source, while the continuum part of the spectrum depends on the phase relatively weakly. The cyclotron lines turned out to be very nonequidistant. The second line is almost always deeper then the first one. We discussed physical conditions in the active regions of the source, under which a cyclotron feature similar to the observed one can appear.

astro-ph↗

arXiv2008

Light curve and neutrino spectrum emitted during the collapse of a nonrotating, supermassive star

The formation of a neutrino pulse emitted during the relativistic collapse of a spherical supermassive star is considered. The free collapse of a body with uniform density in the absence of rotation and with the free escape of the emitted neutrinos can be solved analytically by quadrature. The light curve of the collapsing star and the spectrum of the emitted neutrinos at various times are calculated.

astro-ph↗

arXiv2008

The cyclotron spectrum of anisotropic ultrarelativistic electrons: interpretation of X-ray pulsar spectra

The spectrum of cyclotron radiation produced by electrons with a strongly anisotropic velocity distribution is calculated taking into account higher harmonics. The motion of the electrons is assumed to be ultrarelativistic along the magnetic field and nonrelativistic across the field. One characteristic feature of the resulting spectrum is that harmonics of various orders are not equally spaced. The physical properties and observed spectra of four X-ray pulsars displaying higher cyclotron harmonics are analyzed. The cyclotron features in the spectra of all four pulsars can be interpreted only as emission lines. Moreover, the observed harmonics are not equidistant, and display certain other properties characteristic of emission by strongly anisotropic ultrarelativistic electrons. In addition, there are indirect theoretical arguments that the electrons giving rise to cyclotron features in the spectra of X-ray pulsars are ultrarelativistic and characterized by strongly anisotropic distributions. As a result, estimates of the magnetic fields of X-ray pulsars (which are usually derived from the energies of cyclotron lines) and certain other physical parameters require substantial revision.

astro-ph↗

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.

astro-ph↗