arXiv subjects

M. V. Barkov

Publications and source records attributed to M. V. Barkov.

At least 19 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↗

arXiv2026

Modeling of the magnetic stellar wind braking of the ssrAp 33 Lib (HD137949)

Using the ssrAp star 33 Lib (HD137949) as an example, we show that magnetic braking caused by the interaction of a strong magnetic field with a stellar wind can play a key role in slowing the rotation of ssrAp stars. Numerical modeling of stellar rotation spin-down in the MESA package, taking into account the evolution of magnetic fields and the stellar wind, shows that stars with rotation periods of up to 80 years and longer can form. Moreover, braking by a magnetized wind makes it possible to estimate the mass-loss rate for stars of moderate mass ($1.25 M_\odot < M < 2 M_\odot$), which is difficult to do by other methods. We introduce the dimensionless parameter $Ξ$, which reflects the spin-down time and the stellar lifetime. Thus, when $Ξ\gg 1$, braking is important, whereas when $Ξ\ll 1$, it is negligible.

astro-ph.SR↗

arXiv2025

Magnetar-like flares behind the high-energy emission in LS 5039

LS 5039 hosts a high-mass star, and a compact object that might be a strongly magnetized neutron star (NS). This scenario requires a mechanism to power its persistent and strong nonthermal emission. We investigate a mechanism in which the nonsteady interaction structure of the stellar and the NS winds can regularly excite NS magnetospheric activity, releasing extra energy and fueling the source nonthermal emission. The NS wind shocked by the stellar wind can recurrently touch the NS magnetosphere, triggering magnetic instabilities whose growth can release extra energy into the NS wind in a cyclic manner. To illustrate and study the impact of these cycles on the two-wind interaction structure on different scales, we performed relativistic hydrodynamics simulations in 2D and 3D with periods of an enhanced power in the NS wind along the orbit. We also used analytical tools to characterize processes near the NS relevant for the nonthermal emission. As the NS wind termination shock touches the magnetosphere energy dissipation occurs, but the whole shocked two-wind structure is eventually driven away halting the extra energy injection. However, due to the corresponding drop in the NS wind ram pressure, the termination shock propagates back toward the magnetosphere, resuming the process. These activity cycles excite strong waves in the shocked flows, intensifying their mixing and the disruption of their spiral-like structure produced by orbital motion. Further downstream, the shocked winds can become a quasi-stable, relatively smooth flow. The recurrent interaction between the NS magnetosphere and shocked wind can fuel a relativistic outflow powerful enough to explain the nonthermal emission of LS 5039. A magnetospheric multipolar magnetic field much stronger than the dipolar one may provide the required energetics, and help to explain the lack of evidence of a recent supernova remnant.

astro-ph.HE↗

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↗

arXiv2025

Modelling of the atomic lines' emission of fast moving pulsar nebulae

Bow shocks generated by pulsars moving through weakly ionized interstellar medium (ISM) produce emission dominated by non-equilibrium atomic transitions. These bow shocks are primarily observed as H$_α$ nebulae. We developed a package, named Shu, that calculates non-LTE intensity maps in more than 150 spectral lines, taking into account geometrical properties of the pulsars' motion and lines of sight. We argue here that atomic (CI, NI, OI) and ionic (SII, NII, OIII, NeIV) transitions can be used as complementary and sensitive probes of ISM. We perform self-consistent 2D relativistic hydrodynamic calculations of the bow shock structure and generate non-LTE emissivity maps, combining global dynamics of relativistic flows, and detailed calculations of the non-equilibrium ionization states. We find that though typically H$_α$ emission is dominant, spectral fluxes in OIII, SII and NII may become comparable for relatively slowly moving pulsars. Overall, morphology of non-LTE emission, especially of the ionic species, is a sensitive probe of the density structures of the ISM.

astro-ph.HE↗

arXiv2024

Pushchino multibeam pulsar search -- V. The bright FRB 20190203 detected at 111 MHz

We report the discovery of a bright pulse having a dispersion measure (DM) equal to 134.4 \pm 2 pc cm^{-3}, a peak flux density (S_p) equal to 20 \pm 4 Jy and a half-width (W_e) equal to 211 \pm 6 ms. The excessive DM of the pulse, after taking into account the Milky Way contribution, is 114 pc cm^{-3} that indicates its extragalactic origin. Such value of DM corresponds to the luminosity distance 713 Mpc. The above parameters make the pulse to be a reliable candidate to the fast radio burst (FRB) event, and then it is the second FRB detected at such a large λ\sim 2.7 m wavelength and the first one among non-repeating FRBs. The normalized luminosity L_νof the event, which we have designated as FRB 20190203, estimated under assumption that the whole excessive DM is determined by the intergalactic environment toward the host galaxy, is equal to \simeq 10^{34} erg s^{-1} Hz{-1}. In addition to the study of radio data we analyzed data from the quasi-simultaneous observations of the sky in the high energy (\ge 80 keV) band by the omnidirectional detector SPI/ACS aboard the INTEGRAL orbital observatory (in order to look for a possible gamma-ray counterpart of FRB 20190203). We did not detect any transient events exceeding the background at a statistically significant level. In the INTEGRAL archive, the FRB 20190203 localization region has been observed many times with a total exposure of \sim 73.2 days. We have analyzed the data but were unable to find any reliable short gamma-ray bursts from the FRB 20190203 position. Finally we note that the observed properties of FRB 20190203 can be reproduced well in the framework of a maser synchrotron model operating in the far reverse shock (at a distance of \sim 10^{15} cm) of a magnetar. However, triggering the burst requires a high conversion efficiency (at the level of 1%) of the shock wave energy into the radio emission.

astro-ph.HE↗

arXiv2017

Scenarios for ultrafast gamma-ray variability in AGN

We analyze three scenarios to address the challenge of ultrafast gamma-ray variability reported from active galactic nuclei. We focus on the energy requirements imposed by these scenarios: (i) external cloud in the jet, (ii) relativistic blob propagating through the jet material, and (iii) production of high-energy gamma rays in the magnetosphere gaps. We show that while the first two scenarios are not constrained by the flare luminosity, there is a robust upper limit on the luminosity of flares generated in the black hole magnetosphere. This limit depends weakly on the mass of the central black hole and is determined by the accretion disk magnetization, viewing angle, and the pair multiplicity. For the most favorable values of these parameters, the luminosity for 5-minute flares is limited by $2\times10^{43}\rm\,erg\,s^{-1}$, which excludes a black hole magnetosphere origin of the flare detected from IC310. In the scopes of scenarios (i) and (ii), the jet power, which is required to explain the IC310 flare, exceeds the jet power estimated based on the radio data. To resolve this discrepancy in the framework of the scenario (ii), it is sufficient to assume that the relativistic blobs are not distributed isotropically in the jet reference frame. A realization of scenario (i) demands that the jet power during the flare exceeds by a factor $10^2$ the power of the radio jet relevant to a timescale of $10^8$ years.

astro-ph.HE↗

arXiv2016

The effects of the stellar wind and orbital motion on the jets of high-mass microquasars

High-mass microquasar jets propagate under the effect of the wind from the companion star, and the orbital motion of the binary system. The stellar wind and the orbit may be dominant factors determining the jet properties beyond the binary scales.} An analytical study is performed to characterize the effects of the stellar wind and the orbital motion on the jet properties. Accounting for the wind thrust transferred to the jet, we derive analytical estimates to characterize the jet evolution under the impact of the stellar wind. We include the Coriolis force effect, induced by orbital motion and enhanced by the wind presence. Large-scale evolution of the jet is sketched accounting for wind-to-jet thrust transfer, total energy conservation, and wind-jet flow mixing. If the angle of the wind-induced jet bending is larger than its half-opening angle, the following is expected: (i) a strong recollimation shock; (ii) bending against orbital motion, caused by Coriolis forces and enhanced by the wind presence; and (iii) non-ballistic helical propagation further away. Even if disrupted, the jet can reaccelerate due to ambient pressure gradients, but wind entrainment can weaken this acceleration. On large scales, the opening angle of the helical structure is determined by the wind-jet thrust relation, and the wind-loaded jet flow can be rather slow. The impact of stellar winds on high-mass microquasar jets can yield non-ballistic helical jet trajectories, jet partial disruption and wind mixing, shocks, and possibly non-thermal emission. Among several observational diagnostics at different bands, the radio morphology on milliarcsecond scales can be particularly insightful regarding the wind-jet interaction.

astro-ph.HE↗

arXiv2015

Orbital evolution of colliding star and pulsar winds in 2D and 3D; effects of: dimensionality, EoS, resolution, and grid size

(abridged)The structure formed by the shocked winds of a massive star and a non-accreting pulsar in a binary suffers periodic and random variations of orbital and non-linear dynamical origin. For the 1st time, we simulate in 3 D the interaction of isotropic stellar and relativistic pulsar winds along 1 full orbit, on scales well beyond the binary size. We also investigate the impact of grid resolution and size, and of different EoOs: a gamma-constant ideal gas, and an ideal gas with gamma dependent on temperature. We carry out, with the code PLUTO, relativistic HD simulations in 2 and 3 D of the interaction of a slow wind and a relativistic wind with Gamma=2 along 1 full orbit up to ~100 x the binary size. The different 2-D simulations are carried out with equal and larger grid resolution and size, and 1 of them is done with a more realistic equation of state, than in 3 D. The simulations in 3 D confirm previous results in 2 D. The shocked flows are subject to a faster instabilities growth in 3 D, which enhances the presence of shocks, mixing, and large-scale disruption. In 2 D, higher resolution simulations confirm lower resolution results, simulations with larger grid sizes strengthen the case for the loss of global coherence of the shocked-wind structure, and simulations with 2 different EoOs yield very similar results. In addition to the KHI, we find that the Richtmyer-Meshkov and the RTI are likely acting together in the shocked flow evolution. Simulations in 3 D confirm that the interaction of stellar and pulsar winds yields structures that evolve non-linearly and get strongly entangled. The evolution is accompanied by strong kinetic energy dissipation, rapid changes in flow orientation and speed, and turbulent motion. The results strengthen the case for the loss of global coherence of the shocked structure on large scales, although higher pulsar wind speed simulations are needed.

astro-ph.HE↗

arXiv2015

Star-Jet Interactions and Gamma-Ray Outbursts from 3c454.3

We propose a model to explain the ultra-bright GeV gamma-ray flares observed from the blazar \c454. The model is based on the concept of a relativistic jet interacting with compact gas condensations produced when a star (red giant) crosses the jet close to the central black hole. The study includes an analytical treatment of the evolution of the envelop lost by the star within the jet, and calculations of the related high-energy radiation. The model readily explains the day-long, variable on timescales of hours, GeV gamma-ray flare from \c454, observed during November 2010 on top of a weeks-long plateau. In the proposed scenario, the plateau state is caused by a strong wind generated by the heating of the star atmosphere by nonthermal particles accelerated at the jet-star interaction region. The flare itself could be produced by a few clouds of matter lost by the red giant after the initial impact of the jet. In the framework of the proposed scenario, the observations constrain the key model parameters of the source, including the mass of the central black hole: $M_{\rm BH}\simeq 10^9 M_{\odot}$, the total jet power: $L_{\rm j}\simeq 10^{48}\,\rm erg\,s^{-1}$, and the Doppler factor of the gamma-ray emitting clouds, $δ\simeq 20$. Whereas we do not specify the particle acceleration mechanisms, the potential gamma-ray production processes are discussed and compared in the context of the proposed model. We argue that synchrotron radiation of protons has certain advantages compared to other radiation channels of directly accelerated electrons.

astro-ph.HE↗

arXiv2013

Star-Jet Interactions and Gamma-Ray Outbursts from 3C454.3

We propose a model to explain the ultra-bright GeV gamma-ray flares observed from the blazar 3C454.3. The model is based on the concept of a relativistic jet interacting with compact gas condensations produced when a star (red giant) crosses the jet close to the central black hole. The study includes an analytical treatment of the evolution of the envelop lost by the star within the jet, and calculations of the related high-energy radiation. The model readily explains the day-long, variable on timescales of hours, GeV gamma-ray flare from 3C454.3, observed during November 2010 on top of a weeks-long plateau. In the proposed scenario, the plateau state is caused by a strong wind generated by the heating of the star atmosphere by nonthermal particles accelerated at the jet-star interaction region. The flare itself could be produced by a few clouds of matter lost by the red giant after the initial impact of the jet. In the framework of the proposed scenario, the observations constrain the key model parameters of the source, including the mass of the central black hole: $M_{\rm BH}\simeq 10^9 M_{\odot}$, the total jet power: $L_{\rm j}\simeq 10^{48}\,\rm erg\,s^{-1}$, and the Doppler factor of the gamma-ray emitting clouds, $δ\simeq 20$. Whereas we do not specify the particle acceleration mechanisms, the potential gamma-ray production processes are discussed and compared in the context of the proposed model. We argue that synchrotron radiation of protons has certain advantages compared to other radiation channels of directly accelerated electrons.

astro-ph.HE↗

arXiv2012

Jets and gamma-ray emission from isolated accreting black holes

The large number of isolated black holes (IBHs) in the Galaxy, estimated to be 10^8, implies a very high density of 10^-4 pc^-3 and an average distance between IBHs of 10 pc. Our study shows that the magnetic flux, accumulated on the horizon of an IBH because of accretion of interstellar matter, allows the Blandford-Znajeck mechanism to be activated. Thus, electron-positron jets can be launched. We have performed 2D numerical modelling which allowed the jet power to be estimated. Their inferred properties make such jets a feasible electron accelerator which, in molecular clouds, allows electron energy to be boosted up to 1 PeV. For the conditions expected in molecular clouds the radiative cooling time should be comparable to the escape time. Thus these sources can contribute both to the population of unidentified point-like sources and to the local cosmic ray (CR) electron spectrum. The impact of the generated electron CRs depends on the diffusion rate inside molecular clouds (MCs). If the diffusion regime in a MC is similar to Galactic diffusion, the produced electrons should rapidly escape the cloud and contribute to the Galactic CR population at very high energies >100 TeV. However, due to the modest jet luminosity (at the level of 10^35 erg s^-1) and low filling factor of MC, these sources cannot make a significant contribution to the spectrum of cosmic ray electrons at lower energies. On the other hand, if the diffusion within MCs operates at a rate close to the Bohm limit, the CR electrons escaping from the source should be confined in the cloud, significantly contributing to the local density of CRs. The IC emission of these locally-generated CRs may explain the variety of gamma ray spectra detected from nearby MCs.

astro-ph.HE↗

arXiv2012

Simulations of stellar/pulsar wind interaction along one full orbit

The winds from a non-accreting pulsar and a massive star in a binary system collide forming a bow-shaped shock structure. The Coriolis force induced by orbital motion deflects the shocked flows, strongly affecting their dynamics. We study the evolution of the shocked stellar and pulsar winds on scales in which the orbital motion is important. Potential sites of non-thermal activity are investigated. Relativistic hydrodynamical simulations in two dimensions, performed with the code PLUTO and using the adaptive mesh refinement technique, are used to model interacting stellar and pulsar winds on scales ~80 times the distance between the stars. The hydrodynamical results suggest the suitable locations of sites for particle acceleration and non-thermal emission. In addition to the shock formed towards the star, the shocked and unshocked components of the pulsar wind flowing away from the star terminate by means of additional strong shocks produced by the orbital motion. Strong instabilities lead to the development of turbulence and an effective two-wind mixing in both the leading and trailing sides of the interaction structure, which starts to merge with itself after one orbit. The adopted moderate pulsar-wind Lorentz factor already provides a good qualitative description of the phenomena involved in high-mass binaries with pulsars, and can capture important physical effects that would not appear in non-relativistic treatments. Simulations show that shocks, instabilities, and mass-loading yield efficient mass, momentum, and energy exchanges between the pulsar and the stellar winds. This renders a rapid increase in the entropy of the shocked structure, which will likely be disrupted on scales beyond the simulated ones. Several sites of particle acceleration and low- and high-energy emission can be identified. Doppler boosting will have significant and complex effects on radiation.

astro-ph.HE↗

arXiv2011

Large-scale flow dynamics and radiation in pulsar gamma-ray binaries

Several gamma-ray binaries show extended X-ray emission that may be associated to interactions of an outflow with the medium. Some of these systems are, or may be, high-mass binaries harboring young nonaccreting pulsars, in which the stellar and the pulsar winds collide, generating a powerful outflow that should terminate at some point in the ambient medium. This work studies the evolution and termination, as well as the related radiation, of the shocked-wind flow generated in high-mass binaries hosting powerful pulsars. A characterization, based on previous numerical work, is given for the stellar/pulsar wind interaction. Then, an analytical study of the further evolution of the shocked flow and its dynamical impact on the surrounding medium is carried out. Finally, the expected nonthermal emission from the flow termination shock, likely the dominant emitting region, is calculated. The shocked wind structure, initially strongly asymmetric, becomes a quasi-spherical, supersonically expanding bubble, with its energy coming from the pulsar and mass from the stellar wind. This bubble eventually interacts with the environment on ~pc scales, producing a reverse and, sometimes, a forward shock. Nonthermal leptonic radiation can be efficient in the reverse shock. Radio emission is expected to be faint, whereas X-rays can easily reach detectable fluxes. Under very low magnetic fields and large nonthermal luminosities, gamma rays may also be significant. We conclude that the complexity of the stellar/pulsar wind interaction is likely to be smoothed out outside the binary system, where the wind-mixed flow accelerates and eventually terminates in a strong reverse shock. This shock may be behind the extended X-rays observed in some binary systems. For very powerful pulsars, part of the unshocked pulsar wind may directly interact with the large-scale environment.

astro-ph.HE↗

arXiv2009

Supercollapsars and their X-ray Bursts

The very first stars in the Universe can be very massive, up to $10^3M_\odot$. If born in large numbers, such massive stars can have strong impact on the subsequent star formation, producing strong ionising radiation and contaminating the primordial gas with heavy elements. They would leave behind massive black holes that could act as seeds for growing supermassive black holes of active galactic nuclei. Given the anticipated fast rotation, such stars would end their live as supermassive collapsars and drive powerful magnetically-dominated jets. In this letter we investigate the possibility of observing the bursts of high-energy emission similar to the Long Gamma Ray Bursts associated with normal collapsars. We show that during the collapse of supercollapsars, the Blandford-Znajek mechanism can produce jets as powerful as few$\times10^{52}$erg/s and release up to $10^{54}$erg of the black hole rotational energy. Due to the higher intrinsic time scale and higher redshift the initial bright phase of the burst can last for about $10^4$ seconds, whereas the central engine would remain active for about one day. Due to the high redshift the burst spectrum is expected to be soft, with the spectral energy distribution peaking at around 20-30keV. The peak total flux density is relatively low, $10^{-7}{erg} {cm}^{-2} {s}^{-1}$, but not prohibitive. If one supercollapsar is produced per every mini-halo of dark matter arising from the 3-$σ$ cosmological fluctuations then the whole sky frequency of such bursts could reach several tens per year.

astro-ph.HE↗

arXiv2009

Close Binary Progenitors of Long Gamma Ray Bursts

The strong dependence of the neutrino annihilation mechanism on the mass accretion rate makes it difficult to explain the LGRBs with duration in excess of 100 seconds as well as the precursors separated from the main gamma-ray pulse by few hundreds of seconds. Even more difficult is to explain the Swift observations of the shallow decay phase and X-ray flares, if they indeed indicate activity of the central engine for as long as 10,000 seconds. These data suggest that some other, most likely magnetic mechanisms have to be considered. The magnetic models do not require the development of accretion disk within the first few seconds of the stellar collapse and hence do not require very rapidly rotating stellar cores at the pre-supernova state. This widens the range of potential LGRB progenitors. In this paper, we re-examine the close binary scenario allowing for the possibility of late development of accretion disks in the collapsar model and investigate the available range of mass accretion rates, black hole masses, and spins. A particularly interesting version of the binary progenitor involves merger of a WR star with an ultra-compact companion, neutron star or black hole. In this case we expect the formation of very long-lived accretion disks, that may explain the phase of shallow decay and X-ray flares observed by Swift. Similarly long-lived magnetic central engines are expected in the current single star models of LGRB progenitors due to their assumed exceptionally fast rotation.

astro-ph.HE↗

arXiv2009

Activation of the Blandford-Znajek mechanism in collapsing stars

Collapse of massive stars may result in formation of accreting black holes in their interior. The accreting stellar matter may advect substantial magnetic flux onto the black hole and promote release of its rotational energy via magnetic stresses (the Blandford-Znajek mechanism). In this paper we explore whether this process can explain the stellar explosions and relativistic jets associated with long Gamma-ray-bursts. In particularly, we show that the Blandford-Znajek mechanism is activated when the rest mass-energy density of matter drops below the energy density of magnetic field in the very vicinity of the black hole (within its ergosphere). We also discuss whether such a strong magnetic field is in conflict with the rapid rotation of stellar core required in the collapsar model and suggest that the conflict can be avoided if the progenitor star is a component of close binary. In this case the stellar rotation can be sustained via spin-orbital interaction. In an alternative scenario the magnetic field is generated in the accretion disk but in this case the magnetic flux through the black hole ergosphere is not expected to be sufficiently high to explain the energetics of hypernovae by the BZ mechanism alone. However, this energy deficit can be recovered via additional power provided by the disk.

astro-ph.HE↗

arXiv2008

Hyper-accreting tori of Gamma Ray Bursters

We present numerical simulations of axisymmetric magnetised massive tori around rotating black holes taking into account the energy losses due to emission of neutrinos. A realistic equation of state is used which takes into account the energy losses due to dissociation of nuclei. The heating due to neutrino-antineutrino annihilation is not included. We study the cases of optically thick, semi-transparent, and optically thin to neutrino discs. We show that neutrino cooling does not change significantly the structure of accretion flow and the total energy release. The time scale of accretion is set by the torus angular momentum. Due to the lack of magnetic dynamo in our calculations, it is the initial strength of magnetic field and its topology that determine the process of jet formation and its energetics. Extrapolation of our results gives the total energy released in the jet $\sim 10^{52}$ erg. This is sufficient to explain the hypernovae explosions associated with GRB 980425 and GRB 030329.

astro-ph↗