arXiv subjects

V. I. Zhdanov

Publications and source records attributed to V. I. Zhdanov.

At least 19 recordsLinked to original sources

arXiv2025

Static spherically symmetric solutions of the f(R) gravity

Static spherically symmetric (SSS) solutions of f(R) gravity are studied in the Einstein frame. The solutions involve SSS configuration mass M and scalaron mass $μ$ (in geometrized units); for typical astrophysical masses, the dimensionless parameter $Mμ$ has very large value. We found analytic solutions on a finite interval for $Mμ\to \infty$ in case of a family of scalaron potentials. The asymptotically flat solutions on $(0,\infty)$ have been studied numerically for $Mμ$ up to $10^{20}$ in case of the quadratic f(R) model.

gr-qc↗

arXiv2025

Penrose hypothesis and instability of naked singularities in static spherically symmetric systems with scalar fields

General relativistic static spherically symmetric (SSS) asymptotically flat configurations with scalar fields typically contain naked singularities at the center. We consider minimally coupled scalar fields with power-law potentials leading to the Coulomb asymptotic of the field $ϕ(r)\approx Q/r$ for large values of the radial variable r. The configurations are uniquely defined by total mass and a Q-parameter characterizing the strength of the scalar field at spatial infinity. The focus is on the linear stability against radial (monopole) perturbations of the SSS configurations satisfying conditions of asymptotic flatness. Our numerical investigations show the existence of divergent modes of small perturbations against the static background, at least for sufficiently small values of Q. This means instability of the configurations, confirming the well-known Penrose conjecture about the nonexistence of naked singularities - in this particular case. On the other hand, we have not found divergent modes of linear radial perturbations for sufficiently large Q.

gr-qc↗

arXiv2024

Spherically symmetric configurations in the quadratic $f(R)$ gravity

We study spherically symmetric configurations of the quadratic $f(R)$ gravity in the Einstein frame. In case of a purely gravitational system, we have determined the global qualitative behavior of the metric and the scalaron field for all static solutions satisfying the conditions of asymptotic flatness. These solutions are proved to be regular everywhere except for a naked singularity at the center; they are uniquely determined by the total mass $\mathfrak{M}$ and the "scalar charge" $Q$ characterizing the strength of the scalaron field at spatial infinity. The case $Q=0$ yields the Schwarzschild solution, but an arbitrarily small $Q\ne 0$ leads to the appearance of a central naked singularity having a significant effect on the neighboring region, even when the space-time metric in the outer region is practically insensitive to the scalaron field. Approximation procedures are developed to derive asymptotic relations near the naked singularity and at spatial infinity, and the leading terms of the solutions are presented. We investigate the linear stability of the static solutions with respect to radial perturbations satisfying the null Dirichlet boundary condition at the center and numerically estimate the range of parameters corresponding to stable/unstable configurations. In particular, the configurations with sufficiently small $Q$ turn out to be linearly unstable.

gr-qc↗

arXiv2023

Quasi-normal modes of naked singularities in presence of non-linear scalar fields

We study linear perturbations against static spherically symmetric background configurations of General Relativity with a real scalar field (SF), which is minimally coupled with gravity; it is non-linear due to the presence of the self-action potential. The background solutions have a naked singularity at the center of the configuration. The focus is on the stability of the background and fundamental frequencies of the quasi-normal modes (QNM) of the axial perturbations in the Regge-Wheeler gauge. The problem is reduced to one hyperbolic master equation with an effective potential $W_{\rm eff}$, which turns out to be positive for a general non-negative SF potential; this ensures the linear stability with respect to this kind of perturbations. For numerical simulations, the SF potential was chosen in the power-law form $V(ϕ)\simϕ^{2n}$ with $2<n\le 40$. We extracted the fundamental frequencies of QNM for different $n$ and various sets of the background configuration parameters. The results show that even for a small background SF, there is a significant difference between the fundamental frequencies and ones in case of the Schwarzschild background. The results are also compared with the case of the Fisher-Janis-Newman-Winicour background dealing with a massless linear scalar field.

gr-qc↗

arXiv2023

Pion stars embedded in neutrino clouds

We study self-gravitating multi-pion systems (pion stars) in a state of the Bose condensate. To ensure stability of such stars, it is assumed that they are immersed in the lepton background. Two different phenomenological equations of state (EoS) for the pion matter are used, some of them having the first order phase transition. The model parameters are chosen to reproduce the recent lattice QCD data at zero temperature and large isospin chemical potential. It is shown that the mass-radius diagrams of pion stars obtained with phenomenological EoS are close to ones calculated in the ideal gas model. We analyze properties of neutrino clouds which are necessary for stabilizing the pion stars.

hep-ph↗

arXiv2022

Circular orbits of test particles interacting with massless linear scalar field of the naked singularity

We study effects of the particles coupling with scalar field (SF) on the distribution of stable circular orbits (SCO) around the naked singularity described by the well-known Fisher-Janis-Newman-Winicour solution. The power-law and exponential models of the particle--SF interaction are analyzed. The focus is on the non-connected SCO distributions. We show that coupling between particles and SF can essentially complicate the topology of the SCO distributions. In particular, it can lead to new non-overlapping SCO regions, which are separated by unstable orbits and/or by regions where the circular orbits do not exist.

gr-qc↗

arXiv2021

Singularities in static spherically symmetric configurations of General Relativity with strongly nonlinear scalar fields

There are a number of publications on relativistic objects dealing either with black holes or naked singularities in the center. Here we show that there exist static spherically symmetric solutions of Einstein equations with a strongly nonlinear scalar field, which allow the appearance of singularities of a new type (``spherical singularities'') outside the center of curvature coordinates. As the example, we consider a scalar field potential $\sim$$\sinh(ϕ^{2n}),\,n>2$, which grows rapidly for large field values. The space-time is assumed to be asymptotically flat. We fulfill a numerical investigation of solutions with different $n$ for different parameters, which define asymptotic properties at spatial infinity. Depending on the configuration parameters, we show that the distribution of the stable circular orbits of test bodies around the configuration is either similar to that in the case of the Schwarzschild solution (thus mimicking an ordinary black hole), or it contains additional rings of unstable orbits.

gr-qc↗

arXiv2021

Thin accretion discs around spherically symmetric configurations with nonlinear scalar fields

We study stable circular orbits (SCO) around static spherically symmetric configuration of General Relativity with a non-linear scalar field (SF). The configurations are described by solutions of the Einstein-SF equations with monomial SF potential $V(ϕ)=|ϕ|^{2n}$, $n>2$, under the conditions of the asymptotic flatness and behavior of SF $ϕ\sim 1/r$ at spatial infinity. We proved that under these conditions the solution exists and is uniquely defined by the configuration mass $M>0$ and scalar "charge" $Q$. The solutions and the space-time geodesics have been investigated numerically in the range $n\le40$, $|Q|\le 60$, $M\le60$. We focus on how nonlinearity of the field affects properties of SCO distributions (SCOD), which in turn affect topological form of the thin accretion disk around the configuration. Maps are presented showing the location of possible SCOD types for different $M,Q,n$. We found many differences from the Fisher-Janis-Newman-Winicour metric (FJNW) dealing with the linear SF, though basic qualitative properties of the configurations have much in common with the FJNW case. For some values of $n$, a topologically new SCOD type was discovered that is not available for the FJNW metric. All images of accretion disks have a dark spot in the center (mimicking an ordinary black hole), either because there is no SCO near the center or because of the strong deflection of photon trajectories near the singularity.

gr-qc↗

arXiv2020

Verification of Einstein's formula for gravitational deflection of light using observations of Galactic microlensing

The potential of the gravitational microlensing inside our Galaxy for testing the Einstein formula for the gravitational light deflection is discussed. For this purpose, the lens mapping is modified by introducing parameter eps, which characterizes the deviation from this formula. An example of such deviation described by a simple power law is analyzed. We formed a sample of 100 microlensing light curves using the data of the Optical Gravitational Lensing Experiment (OGLE) for 2018. The resulting eps value does not contradict the General Relativity within 1 percent errors.

gr-qc↗

arXiv2020

Static spherically symmetric configurations with N non-linear scalar fields: global and asymptotic properties

In case of a spherically symmetric non-linear scalar field (SF) in flat space, besides singularity at the center, spherical singularities can occur for non-zero values of radial variable $r>0$. We show that in the General Relativity the gravitational field suppresses the occurrence of the spherical singularities under some generic conditions. Our consideration deals with asymptotically flat space-times around static spherically symmetric configurations in presence of $N$ non-linear SFs, which are minimally coupled to gravity. Constraints are imposed on the SF potentials, which guarantee a monotonicity of the fields as functions of radial variable; also the potentials are assumed to be exponentially bounded. We give direct proof that solutions of the joint system of Einstein -- SF equations satisfying the conditions of asymptotic flatness are regular for all values of $r$, except for naked singularities in the center $r=0$ in the Schwarzschild (curvature) coordinates. Asymptotic relations for SF and metric near the center are derived, which appear to be remarkably similar to the case of the Fisher solution for free SF. These relations determine two main types of the corresponding geodesic structure when photons can be captured by the singularity or not depending on the existence of the photon sphere. To illustrate, the case of one SF with monomial potential is analyzed in detail numerically. We show that the image of the accretion disk around the singularity, observed from infinity, can take the form of a bright ring with a dark spot in the center, like the case of an ordinary black hole.

gr-qc↗

arXiv2018

Scalar field vs hydrodynamic models in the homogeneous isotropic cosmology

We study relations between hydrodynamical (H) and scalar field (SF) models of the dark energy in the early Universe. Main attention is paid to SF described by the canonical Lagrangian within the homogeneous isotropic spatially flat cosmology. We analyze requirements that guarantee the same cosmological history for the SF and H-models at least for solutions with specially chosen initial conditions and we present a differential equation for the SF potential that ensures such a restricted equivalence of the SF and H-models. Also, we derived a condition that guarantees an approximate equivalence when there is a small difference between energy momentum tensors of the models. The "equivalent" scalar field potentials for linear equations of state (EOS) are found in an explicit form, we also present an examples with more complicated EOS.

gr-qc↗

arXiv2017

Spherically symmetric configurations of General Relativity in presence of scalar field: separation of test body circular orbits

We study test-body orbits in the gravitational field of a static spherically symmetric object in presence of a minimally coupled nonlinear scalar field. We generated a two-parametric family of scalar field potentials, which allow finding solutions of Einstein's equations in an analytic form. The results are presented by means of hypergeometric functions; they describe either a naked singularity (NS) or a black hole (BH). Our numerical investigation shows that in both cases the stable circular orbits can form separated (non-connected) regions around the configuration. We found existence conditions for such separated regions and present examples for some family parameters in case of NS and BH. The results may be of interest for testing models of the dynamical dark energy.

gr-qc↗

arXiv2017

Qualitative types of cosmological evolution in hydrodynamic models with barotropic equation of state

We study solutions of the Friedmann equations in case of the homogeneous isotropic Universe filled with a perfect fluid. The main points concern the monotony properties of the solutions, the possibility to extend the solutions on all times and occurrence of singularities. We present a qualitative classification of all possible solutions in case of the general smooth barotropic equation of state of the fluid, provided the speed of sound is finite. The list of possible scenarios includes analogs of the "Big Rip" in the future and/or in the past as well as singularity free solutions and oscillating Universes. Extensions of the results to the multicomponent fluids are discussed.

gr-qc↗

arXiv2016

Gravitational microlensing as a probe for dark matter clumps

Extended dark matter (DM) substructures may play the role of microlenses in the Milky Way and in extragalactic gravitational lens systems (GLSs). We compare microlensing effects caused by point masses (Schwarzschild lenses) and extended clumps of matter using a simple model for the lens mapping. A superposition of the point mass and the extended clump is also considered. For special choices of the parameters, this model may represent a cusped clump of cold DM, a cored clump of self-interacting dark matter (SIDM) or an ultra compact minihalo of DM surrounding a massive point-like object. We built the resulting micro-amplification curves for various parameters of one clump moving with respect to the source in order to estimate differences between the light curves caused by clumps and by point lenses. The results show that it may be difficult to distinguish between these models. However, some region of the clump parameters can be restricted by considering the high amplification events at the present level of photometric accuracy. Then we estimate the statistical properties of the amplification curves in extragalactic GLSs. For this purpose, an ensemble of amplification curves is generated yielding the autocorrelation functions (ACFs) of the curves for different choices of the system parameters. We find that there can be a significant difference between these ACFs if the clump size is comparable with typical Einstein radii; as a rule, the contribution of clumps makes the ACFs less steep.

astro-ph.GA↗

arXiv2015

X-Ray Spectral Parameters for a Sample of 95 Active Galactic Nuclei

We present a broadband X-ray analysis of a new homogeneous sample of 95 active galactic nuclei (AGN) from the 22-month Swift/BAT all-sky survey. For this sample we treated jointly the X-ray spectra observed by XMM-Newton and INTEGRAL missions for the total spectral range of 0.5-250 keV. Photon index Γ, relative reflection R, equivalent width of Fe $K_α$ line (EW Fe $K_α$), hydrogen column density $N_{H}$, exponential cut-off energy $E_{c}$ and intrinsic luminosity $L_{corr}$ are determined for all objects of the sample. We investigated correlations Γ- R, EW Fe $K_α$ - $L_{corr}$, Γ- $E_{c}$, EW Fe $K_α$ - $N_{H}$. Dependence Γ- R for Seyfert 1 and 2 type of galaxies has been investigated separately. We found that the relative reflection parameter at low power-law indexes for Seyfert 2 galaxies is systematically higher than for Seyfert 1 ones. This can be related to an increasing contribution of the reflected radiation from the gas-dust torus. Our data show that there exists some anticorrelation between EW Fe $K_α$ and $L_{corr}$, but it is not strong. We have not found statistically significant deviations from the AGN Unified Model.

astro-ph.HE↗

arXiv2014

Cosmological model with variable vacuum pressure

Scenarios of cosmological evolution having inflationary stages are studied by using equations of state (EoS) inspired by the quark bag model. A crucial point is the existence of states where the specific enthalpy of the cosmological fluid vanishes. A large class of barotropic EoS admits, depending upon initial conditions, for a concrete equation of state, analogs of the "Big Rip" as well as solutions describing an exponential inflation followed by the usual matter dominance. We found scalar field potentials that mimic the behavior of the cosmological scale factor in case of special EoS from the above class. Extensions to more general two-parametric EoS are discussed that describe a pre-inflationary evolution.

astro-ph.CO↗

arXiv2013

Observations of Sy2 galaxy NGC 3281 by XMM-Newton and INTEGRAL satellites

We present here the results of our analysis of X-ray properties of Seyfert 2 galaxy NGC 3281, based on the observational data obtained by XMM-Newton and INTEGRAL within the energy ranges 0.2-12 keV and 20-150 keV, respectively. The XMM-Newton spectrum of this object is presented for the first time. We show that fitting the X-ray spectrum of this galaxy with models based on the reflection from the disc with infinite column density yields non-physical results. More appropriate fit takes into account both transmitted and reflected emission, passed through a gas-dusty torus-like structure. Keeping this in mind, to model the inhomogeneous clumpy torus, we used the MYTorus model. Hence, we propose that the torus of NGC 3281 is not continuous structure, but it consists of separate clouds, which is in a good agreement with the results of near-IR observations. Using this assumption, we found that the torus inclination angle and the hydrogen column density are 66.98^{+2.63}_{-1.34} degrees and 2.08^{+0.35}_{-0.18}x10^{24} cm^{-2}, respectively. Also, the emission of the hot diffuse gas with temperature ~590 eV and warm absorption were detected.

astro-ph.HE↗

arXiv2013

Astrophysical and cosmological problems of invisible mass and dark energy in the Universe

The Workshop on results of the Project Kosmomikrofizyka-2 (Astroparticle Physics) of the National Academy of Sciences (NAS) of Ukraine "Astrophysical and cosmological problems of invisible mass and dark energy in the Universe" was held on November 21-22, 2012 in the Institute for Nuclear Research, Kyiv, Ukraine (http://lpd.kinr.kiev.ua/kmf12). This Project was carried out during three years (2010-2012) by scientists from various universities and institutes of the National Academy of Sciences of Ukraine; it was a logical continuation of the previous scientific program of the NAS of Ukraine "Researches of structure and composition of the Universe, hidden mass and dark energy (Kosmomikrofizyka)" in 2007-2009. These programs were devoted to theoretical and experimental investigations in astronomy, astrophysics, cosmology, physics of atomic nuclei and particle physics, which are related with the problems of dark matter and dark energy in the Universe.

astro-ph.CO↗