arXiv subjects

Yu. N. Eroshenko

Publications and source records attributed to Yu. N. Eroshenko.

At least 37 records · Page 2Linked to original sources

arXiv2017

Gravitational waves from primordial black holes collisions in binary systems

It was shown by (Nakamura et al. 1997), (Ioka et al. 1998), and (Sasaki et al. 2016) that primordial black holes (PBHs) binaries can form effectively at the cosmological stage of radiation dominance, and the merge of the PBHs in pairs can explain the gravitational wave burst GW150914. In this paper, the model is re-examined by considering the effect of inflationary dark matter density perturbations which produce additional tidal forces. As a result, the merge rate of PBHs binaries and the corresponding rate of the gravitational bursts are suppressed by the factor $\sim1.5-2$ in comparison with previous calculations. This rate matches the LIGO data if the PBHs constitute the $f\sim5\times10^{-4}-5\times10^{-3}$ fraction of dark matter.

astro-ph.CO↗

arXiv2017

Recurrent fast radio bursts from collisions of neutron stars in the evolved stellar clusters

We propose the model describing the observed multiple fast radio bursts due to the close encounters and collisions of neutron stars in the central clusters of the evolved galactic nuclei. The subsystem of neutron star cluster may originate in the dense galactic nucleus evolutionary in the combined processes of stellar and dynamical evolution. The neutron stars in the compact cluster can produce the short living binaries with the highly eccentric orbits, and finally collide after several orbital revolutions. Fast radio bursts may be produced during the close periastron approach and at the process of the final binary merging. In the sufficiently dense star cluster the neutron stars collisions can be very frequent. Therefore, this model can explain in principle the observed recurrent (multiple) fast radio bursts, analogous to the observed ones from the source FRB 121102. Among the possible observational signatures of the proposed model may be the registration of the gravitational wave bursts by the laser interferometers LIGO/VIRGO or by the next generation of gravitational wave detectors.

astro-ph.HE↗

arXiv2017

Cosmological Horizons as They Are Looked from a Moving Frame

We consider the cosmological horizons in the expanding universe from the point of view of observer moving with respect to CMB frame. The deformation (non-sphericity) of cosmological horizons is demonstrated. Some principle consequences are discussed.

gr-qc↗

arXiv2017

Dark Matter Density Spikes around Primordial Black Holes

We show that density spikes begin to form from dark matter particles around primordial black holes immediately after their formation at the radiation-dominated cosmological stage. This follows from the fact that in the thermal velocity distribution of particles there are particles with low velocities that remain in finite orbits around black holes and are not involved in the cosmological expansion. The accumulation of such particles near black holes gives rise to density spikes. These spikes are considerably denser than those that are formed later by the mechanism of secondary accretion. The density spikes must be bright gamma-ray sources. Comparison of the calculated signal from particle annihilation with the Fermi-LAT data constrains the present-day cosmological density parameter for primordial black holes with masses $M_{\rm BH}\geq10^{-8}M_\odot$ from above by values from $Ω_{\rm BH}\leq1$ to $Ω_{\rm BH}\leq10^{-8}$, depending on $M_{\rm BH}$. These constraints are several orders of magnitude more stringent than other known constraints.

astro-ph.HE↗

arXiv2016

On maximal analytical extension of the Vaidya metric

The classical Vaidya metric is transformed to the special diagonal coordinates in the case of the linear mass function allowing rather easy treatment. We find the exact analytical expressions for metric functions in these diagonal coordinates. Using these coordinates, we elaborate the maximum analytic extension of the Vaidya metric with a linear growth of the black hole mass and construct the corresponding Carter-Penrose diagrams for different specific cases. The derived global geometry seemingly is valid also for a more general behavior of the black hole mass in the Vaidya metric.

gr-qc↗

arXiv2016

Spherically symmetric conformal gravity and "gravitational bubbles"

The general structure of the spherically symmetric solutions in the Weyl conformal gravity is described. The corresponding Bach equations are derived for the special type of metrics, which can be considered as the representative of the general class. The complete set of the pure vacuum solutions is found. It consists of two classes. The first one contains the solutions with constant two-dimensional curvature scalar of our specific metrics, and the representatives are the famous Robertson--Walker metrics. One of them we called the "gravitational bubbles", which is compact and with zero Weyl tensor. Thus, we obtained the pure vacuum curved space-times (without any material sources, including the cosmological constant) what is absolutely impossible in General Relativity. Such a phenomenon makes it easier to create the universe from "nothing". The second class consists of the solutions with varying curvature scalar. We found its representative as the one-parameter family. It appears that it can be conformally covered by the thee-parameter Mannheim--Kazanas solution. We also investigated the general structure of the energy-momentum tensor in the spherical conformal gravity and constructed the vectorial equation that reveals clearly some features of non-vacuum solutions. Two of them are explicitly written, namely, the metrics à la Vaidya, and the electrovacuum space-time metrics.

gr-qc↗

arXiv2015

Physical laboratory at the center of the Galaxy

We review the physical processes that occur at the center of the Galaxy and that are related to the supermassive black hole Sgr A* residing there. The discovery of high-velocity S0 stars orbiting Sgr A* for the first time allowed measuring the mass of this supermassive black hole, the closest one to us, with a 10\% accuracy, with the result $M_h=(4.1\pm0.4)\times 10^6M_\odot$. Further monitoring can potentially discover the Newtonian precession of the S0 star orbits in the gravitational field of the black hole due to invisible distributed matter. This will yield the "weight" of the elusive dark matter concentrated there and provide new information for the identification of dark matter particles. The weak accretion activity of the "dormant quasar" at the Galactic center occasionally shows up as quasiperiodic X-ray and near-IR oscillations with mean periods of $11$ and $19$ min. These oscillations can possibly be interpreted as related to the rotation frequency of the Sgr A* event horizon and to the latitude oscillations of hot plasma spots in the accretion disk. Both these frequencies depend only on the black hole gravitational field and not on the accretion model. Using this interpretation yields quite the accurate values for both the mass $M_h$ and the spin $a$ (Kerr rotation parameter) of Sgr A*: $M_h=(4.2\pm0.2)\times 10^6M_\odot$ and $a=0.65\pm0.05$.

astro-ph.HE↗

arXiv2015

Conformal gravity and "gravitational bubbles"

We describe the general structure of the spherically symmetric solutions in the Weyl conformal gravity. The corresponding Bach equations are derived for the special type of metrics, which can be considered as the representative of the general class. The complete set of the pure vacuum solutions, consisting of two classes, is found. The first one contains the solutions with constant two-dimensional curvature scalar, and the representatives are the famous Robertson--Walker metrics. We called one of them the "gravitational bubbles", which is compact and with zero Weyl tensor. These "gravitational bubbles" are the pure vacuum curved space-times (without any material sources, including the cosmological constant), which are absolutely impossible in General Relativity. This phenomenon makes it easier to create the universe from "nothing". The second class consists of the solutions with varying curvature scalar. We found its representative as the one-parameter family, which can be conformally covered by the thee-parameter Mannheim--Kazanas solution. We describe the general structure of the energy-momentum tensor in the spherical conformal gravity and construct the vectorial equation that reveals clearly some features of non-vacuum solutions.

gr-qc↗

arXiv2015

Weighing of the Dark Matter at the Center of the Galaxy

A promising method for measuring the total mass of the dark matter near a supermassive black hole at the center of the Galaxy based on observations of nonrelativistic precession of the orbits of fast S0 stars together with constraints on the annihilation signal from the dark matter particles has been discussed. An analytical expression for the precession angle has been obtained under the assumption of a power-law profile of the dark matter density. In the near future, modern telescopes will be able to measure the precession of the orbits of S0 stars or to obtain a strong bound on it. The mass of the dark matter necessary for the explanation of the observed excess of gamma radiation owing to the annihilation of the dark matter particles has been calculated with allowance for the Sommerfeld effect.

astro-ph.HE↗

arXiv2014

Non-orientable wormholes as portals to the mirror world

We examine the compatibility of the mirror matter concept with the non-orientable wormholes. If any particle (or classical object) is traversing through the non-orientable wormhole, it turns into a corresponding mirror particle and vice versa. The critical sonic point in the hydrodynamical flow of the perfect fluid is located exactly at the throat of the wormhole. This means that wormholes are traversable for the fluid. The astrophysical signatures of the presence of non-orientable wormholes in the Universe are discussed. Some non-trivial aspects of the non-orientable wormholes electrodynamics are considered.

gr-qc↗

arXiv2014

Black holes in the presence of dark energy

The new, rapidly developing field of theoretical research --- studies of dark energy interacting with black holes (and, in particular, accreting onto black holes) --- is reviewed. The term `dark energy' is meant to cover a wide range of field theory models, as well as perfect fluids with various equations of state, including cosmological dark energy. Various accretion models are analyzed in terms of the simplest test field approximation or by allowing back reaction on the black-hole metric. The behavior of various types of dark energy in the vicinity of Schwarzschild and electrically charged black holes is examined. Nontrivial effects due to the presence of dark energy in the black hole vicinity are discussed. In particular, a physical explanation is given of why the black hole mass decreases when phantom energy is being accreted, a process in which the basic energy conditions of the famous theorem of nondecreasing horizon area in classical black holes are violated. The theoretical possibility of a signal escaping from beneath the black hole event horizon is discussed for a number of dark energy models. Finally, the violation of the laws of thermodynamics by black holes in the presence of noncanonical fields is considered.

gr-qc↗

arXiv2014

Small-scale clumps of dark matter

Small-scale clumps of dark matter are gravitationally bounded structures that have masses comparable to or lower than stellar masses and consist of noninteracting or weakly interacting dark matter particles. In this paper, the current knowledge about the formation and evolution of such structures is reviewed, various types of spectra of primordial cosmological perturbations are considered, and various dark matter models are discussed. Depending on the particular spectrum type, dark matter clumps may differ considerably in their formation processes and ultimate characteristics. The role of clumps in experiments on indirect detection of dark matter particles via their annihilation products is discussed. A number of astrophysical problems and phenomena that are related to dark matter clumps are examined.

astro-ph.HE↗

arXiv2014

Black hole atom as a dark matter particle candidate

We propose the new dark matter particle candidate - the "black hole atom", which is an atom with the charged black hole as an atomic nucleus and electrons in the bound internal quantum states. As a simplified model we consider the the central Reissner-Nordstrom black hole with the electric charge neutralized by the internal electrons in bound quantum states. For the external observers these objects would look like the electrically neutral Schwarzschild black holes. We suppose the prolific production of black hole atoms under specific conditions in the early universe.

astro-ph.CO↗

arXiv2013

Formation and internal structure of superdense dark matter clumps and ultracompact minihaloes

We discuss the formation mechanisms and structure of the superdense dark matter clumps (SDMC) and ultracompact minihaloes (UCMH), outlining the differences between these types of DM objects. We define as SDMC the gravitationally bounded DM objects which have come into virial equilibrium at the radiation-dominated (RD) stage of the universe evolution. Such objects can be formed from the isocurvature (entropy) density perturbations or from the peaks in the spectrum of curvature (adiabatic) perturbation. The axion miniclusters (Kolb and Tkachev 1994) are the example of the former model. The system of central compact mass (e. g. in the form of SDMC or primordial black hole (PBH)) with the outer DM envelope formed in the process of secondary accretion we refer to as UCMH. Therefore, the SDMC can serve as the seed for the UCMH in some scenarios. Recently, the SDMC and UCMH were considered in the many works, and we try to systematize them here. We consider also the effect of asphericity of the initial density perturbation in the gravitational evolution, which decreases the SDMC amount and, as the result, suppresses the gamma-ray signal from DM annihilation.

astro-ph.CO↗

arXiv2013

Stationary Solutions of the Dirac Equation in the Gravitational Field of a Charged Black Hole

A stationary solution of the Dirac equation in the metric of a Reissner-Nordstrom black hole has been found. Only one stationary regular state outside the black hole event horizon and only one stationary regular state below the Cauchy horizon are shown to exist. The normalization integral of the wave functions diverges on both horizons if the black hole is non-extremal. This means that the solution found can be only the asymptotic limit of a nonstationary solution. In contrast, in the case of an extremal black hole, the normalization integral is finite and the stationary regular solution is physically self-consistent. The existence of quantum levels below the Cauchy horizon can affect the final stage of Hawking black hole evaporation and opens up the fundamental possibility of investigating the internal structure of black holes using quantum tunneling between external and internal states.

gr-qc↗

arXiv2013

Observational signatures of the giant planets collisions

We consider observational signatures of the collisions and partial destructions of giant exoplanets at the chaotic stage of the planetary systems evolution. The rate of these collisions in the Galaxy is estimated to be ~0.01-1 per yr. In the inelastic collision with a small relative velocity and small impact parameter, the planets may sufficiently lose their kinetic energy and merge together. Otherwise, if the planet is experienced a shallow tangential collision, it flews safety away. At the same time, the planets loss some part of their gas envelopes during mutual collisions. Collisions of the giant planets must be accompanied also by the radiation of energy from the radio to optical bands. The optical and near UV flashes result from the collision heating of the planet surface layers. Additionally, the collision compression and collapse of the powerful magnetosphere of giant planets are possible, with a successive generation of the radio bursts. According to our estimations, the corresponding rate the radio bursts is ~0.01-1 bursts per year with the maximum spectral flux ~30mJy at the frequencies ~3GHz and with a duration of the each burst ~1.5 hours. These signals are available for registration by the existing radio telescopes.

astro-ph.EP↗

arXiv2012

Swift J164449.3+573451 event: generation in the collapsing star cluster?

We discuss the multiband energy release in a model of a collapsing galactic nucleus, and we try to interpret the unique super-long cosmic gamma-ray event Swift J164449.3+573451 (GRB 110328A by early classification) in this scenario. Neutron stars and stellar-mass black holes can form evolutionary a compact self-gravitating subsystem in the galactic center. Collisions and merges of these stellar remnants during an avalanche contraction and collapse of the cluster core can produce powerful events in different bands due to several mechanisms. Collisions of neutron stars and stellar-mass black holes can generate gamma-ray bursts (GRBs) similar to the ordinary models of short GRB origin. The bright peaks during the first two days may also be a consequence of multiple matter supply (due to matter release in the collisions) and accretion onto the forming supermassive black hole. Numerous smaller peaks and later quasi-steady radiation can arise from gravitational lensing, late accretion of gas onto the supermassive black hole, and from particle acceleration by shock waves. Even if this model will not reproduce exactly all the Swift J164449.3+573451 properties in future observations, such collapses of galactic nuclei can be available for detection in other events.

astro-ph.HE↗

arXiv2011

Dense DM clumps seeded by cosmic string loops and DM annihilation

We develop a model of production of the very dense clumps of DM in RD epoch due to the accretion of DM on the loops of cosmic strings as the seeds. At some time the loops disappear, for example due to the gravitational radiation, and the remaining dense clumps produce the enhancement of the annihilation signal. We take into account the velocity distribution of the strings, and consider the two extreme regimes of DM annihilation: fast decay and continuous evaporation. The produced annihilation flux of gamma radiation is detectable, and for some parameters of DM particles and the strings can exceed the extragalactic flux of the gamma-radiation observed by Fermi. For the fixed parameters of DM particles (e.g. neutralino with fixed masses and cross-section of annihilation) one can obtain the limits on the basic string parameter, tension μ, which is stronger than (more general) limits obtained from WMAP observations, cosmological nucleosynthesis and gravitational lensing. In particular for the neutralino with 100 GeV mass we exclude the interval 5\times10^{-10}<Gμ/c^2<5.1\times10^{-9}.

astro-ph.HE↗