arXiv subjects

A. M. Bykov

Publications and source records attributed to A. M. Bykov.

At least 37 records · Page 2Linked to original sources

arXiv2022

Inside the core of a young massive star cluster: 3D MHD simulations

Young massive star clusters inhabit regions of star formation and play an essential role in the galactic evolution. They are sources of both thermal and non-thermal radiation, and they are effective cosmic ray accelerators. We present the 3D magnetohydrodynamic (MHD) modeling of the plasma flows in a young compact cluster at the evolutionary stage comprising multiple interacting supersonic winds of massive OB and WR stars. The modeling allows studying the partitioning of the mechanical energy injected by the winds between the bulk motions, thermal heating and magnetic fields. Cluster-scale magnetic fields reaching the magnitudes of $\sim$ 300 $μ$G show the filamentary structures spreading throughout the cluster core. The filaments with the high magnetic fields are produced by the Axford-Cranfill type effect in the downstream of the wind termination shocks, which is amplified by a compression of the fields with the hot plasma thermal pressure in the central part of the cluster core. The hot ($\sim$ a few keV) plasma is heated at the termination shocks of the stellar winds and compressed in the colliding postshock flows. We also discuss a possible role of the thermal conduction effects on the plasma flow, analyse temperature maps in the cluster core and the diffuse thermal X-ray emission spectra. The presence of high cluster-scale magnetic fields supports the possibility of high-energy cosmic ray acceleration in clusters at the given evolutionary stage.

astro-ph.HE↗

arXiv2022

High-resolution, High-sensitivity, Low-frequency uGMRT View of Coma Cluster of Galaxies

We present high-resolution, high-sensitivity upgraded Giant Metrewave Radio Telescope observations of the Coma cluster (A1656) at 250-500 MHz and 550-850 MHz. At 250-500 MHz, 135 sources have extensions $>$ 0.45 arcmin (with peak-to-local-noise ratio $> 4$). Of these, 24 sources are associated with Coma-member galaxies. In addition, we supplement this sample of 24 galaxies with 20 ram pressure stripped galaxies from Chen et al. (2020, eight are included in the original extended radio source sample) and an additional five are detected and extended. We present radio morphologies, radio spectra, spectral index maps, and equipartition properties for these two samples. In general, we find the equipartition properties lie within a narrow range (e.g., $P_{\rm min}$ = 1-3 dynes cm$^{-2}$). Only NGC 4874, one of the two brightest central Coma cluster galaxies, has a central energy density and pressure about five times higher and a radio source age about 50 % lower than that of the other Coma galaxies. We find a diffuse tail of radio emission trailing the dominant galaxy of the merging NGC 4839 group that coincides with the "slingshot" tail, seen in X-rays. The southwestern radio relic, B1253$+$275, has a large extent $\approx$ 32$^\prime$ $\times$ 10$^\prime$ ($\simeq$ 1.08 $\times$ 0.34 Mpc$^2$). For NGC 4789, whose long radio tails merge into the relic and may be a source of its relativistic seed electrons, and we find a transverse radio spectral gradient, a steepening from southwest to northeast across the width of the radio source. Finally, radio morphologies of the extended and RPS samples suggest that these galaxies are on their first infall into Coma on (predominantly) radial orbits.

astro-ph.GA↗

arXiv2022

Modeling of GeV-TeV gamma-ray emission of Cygnus Cocoon

OB-associations and superbubbles being energetically essential galactic powerhouses are likely to be the important acceleration sites of galactic cosmic rays (CRs). The emission profile of gamma-ray sources related to superbubbles and stellar clusters indicates on continuous particle acceleration by winds of massive stars. One of the most luminous galactic gamma-ray sources is Cygnus Cocoon superbubble, observed by multiple instruments, such as Fermi-LAT, ARGO, and, recently, HAWC. We discuss a model of particle acceleration and transport in a superbubble to explain GeV-TeV gamma-ray spectrum of Cygnus Cocoon, which has a break at the energy of about 1 TeV. It is shown that the gamma rays produced by hadronic interactions of high-energy protons accelerated by an ensemble of shocks from winds of massive stars and supernovae in the Cygnus Cocoon can explain the observations. The proton spectral shape at the highest energies depends on the MHD-fluctuation spectrum in the Cocoon. The viable solutions for Cygnus Cocoon may be applied to some other associations showing similar behaviour. We briefly discuss the similarity and differences of particle acceleration processes in extended superbubbles and compact clusters of young massive stars as represented by Westerlund 1 and 2 gamma-ray sources.

astro-ph.HE↗

arXiv2022

LOFAR detection of faint radio emission from the supernova remnant SRGeJ0023+3625=G116.6-26.1: probing the Milky Way synchrotron halo

A supernova remnant (SNR) candidate SRGe~J0023+3625 = G116.6-26.1 was recently discovered in the \textit{SRG}/eROSITA all-sky X-ray survey. This large ($\sim 4$ deg in diameter) SNR candidate lacks prominent counterparts in other bands. Here we report detection of radio emission from G116.6-26.1 in the LOFAR Two-metre Sky Survey (LoTTS-DR2). Radio images show a shell-like structure coincident with the X-ray boundary of the SNR. The measured surface brightness of radio emission from this SNR is very low. Extrapolation of the observed surface brightness to 1~GHz places G116.6-26.1 well below other objects in the $Σ-D$ diagram. We argue that the detected radio flux might be consistent with the minimal level expected in the van der Laan adiabatic compression model, provided that the volume emissivity of the halo gas in the LOFAR band is $\sim 10^{-42}\,{\rm Wm^{-3}Hz^{-1} sr^{-1}}$. If true, this SNR can be considered as a prototypical example of an evolved SNR in the Milky Way halo. In the X-ray and radio bands, such SNRs can be used as probes of thermal and non-thermal components constituting the Milky Way halo.

astro-ph.HE↗

arXiv2022

X-ray signature of antistars in the Galaxy

The existence of macroscopic objects from antimatter (antistars) is envisaged in some models of baryogenesis. Searches for antistars has been usually carried out in gamma-rays through hadronic annihilation of matter. In astrophysically plausible cases of the interaction of neutral atmospheres or winds from antistars with ionized interstellar gas, the hadronic annihilation will be preceded by the formation of excited $p\bar p$ and He$\bar p$ atoms. These atoms rapidly cascade down to low levels prior to annihilation giving rise to a series of narrow lines which can be associated with the hadronic annihilation gamma-ray emission. The most significant are L (3p-2p) 1.73 keV line (yield more than 90\%) from $p\bar p$ atoms, and M (4-3) 4.86 keV (yield $\sim 60\% $) and L (3-2) 11.13 keV (yield about 25\%) lines from $^4$He$\bar p$ atoms. These lines can be probed in dedicated observations by forthcoming sensitive X-ray spectroscopic missions XRISM, \textit{Athena, \textit{Lynx}and in wide-field X-ray surveys like SRG/\textit{eROSITA} all-sky survey.

astro-ph.HE↗

arXiv2021

PeV photon and neutrino flares from galactic gamma-ray binaries

The high-energy radiation from short period binaries containing a massive star with a compact relativistic companion was detected from radio to TeV gamma rays. We show here that PeV regime protons can be efficiently accelerated in the regions of collision of relativistic outflows of a compact object with stellar winds in these systems. The accelerated proton spectra in the presented Monte Carlo model have an upturn in the PeV regime and can provide very hard spectra of sub-PeV photons and neutrinos by photo-meson processes in the stellar radiation field. Recent report of a possible sub-PeV gamma-ray flare in coincidence with a high-energy neutrino can be understood in the frame of this model. The gamma-ray binaries may contribute substantially to the Galactic component of the detected high-energy neutrino flux.

astro-ph.HE↗

arXiv2021

Spatially resolved X-ray spectra of the Galactic SNR G18.95-1.1: SRG/eROSITA view

Aims. We study the X-ray emission of the galactic supernova remnant (SNR) G18.95-1.1 with the eROSITA telescope on board the Spectrum Rentgen Gamma (SRG) orbital observatory. In addition to the pulsar wind nebula that was previously identified and examined by ASCA and Chandra, we study the X-ray spectra of the bright SNR ridge, which is resolved into a few bright clumps. Methods. The wide field of view and the large collecting area in the 0.2-2.3 keV energy range of SRG/eROSITA allowed us to perform spatially resolved spectroscopy of G18.95-1.1. Results. The X-ray ridge of G18.95-1.1 is asymmetric, indicating either supernova ejecta asymmetry or their interaction with a cloud. The X-ray dim northern regions outside the pulsar wind nebula can be described by a thin thermal plasma emission with a temperature ~0.3 keV and a solar composition. The X-ray spectra of a few bright clumps located along the southern ridge may be satisfactorily approximated by a single thermal component of the Si-rich ejecta at the collisional ionization equilibrium with a temperature of about 0.3 keV. The bright ridge can be alternatively fit with a single component that is not dominated by equilibrium ejecta with T ~ 0.6 keV. The high ratio of the derived Si/O abundances indicates that the ejecta originated in deep layers of the progenitor star. The plasma composition of a southern Si-rich clump and the bright ridge are similar to what was earlier found in the Vela shrapnel A and G.

astro-ph.GA↗

arXiv2021

Phase-resolved hard X-ray emission of the high-mass binary LS 5039: a spectral hardening above 50 keV detected with INTEGRAL

Aims: LS 5039 is an enigmatic high-mass gamma-ray binary which hosts a powerful O6.5V companion, but the nature of the compact object is still to be established using multi-wavelength observations. Methods: We analyzed phase-resolved multi-instrument spectra of nonthermal emission from LS 5039 in order to produce reliable spectral models, which can be further employed to select between various scenarios and theoretical models of the binary. Results: The combined phase-resolved hard X-ray and MeV-range gamma-ray spectra obtained with XMM-Newton, Suzaku, NuSTAR, INTEGRAL, and COMPTEL indicate a meaningful spectral hardening above 50~keV. The spectral break observed in both major phases of the binary may indicate the presence of a hardening in the spectrum of accelerated leptons which could originate from the interaction of wind from the O6.5V companion star with the relativistic outflow from a yet unidentified compact object.

astro-ph.HE↗

arXiv2021

SRG/eROSITA discovery of a large circular SNR candidate G116.6-26.1: SN Ia explosion probing the gas of the Milky Way halo?

We report a discovery of a new X-ray-selected supernova remnant (SNR) candidate SRGe~J0023+3625 = G116.6-26.1 found in the SRG/eROSITA all-sky survey. The source features a large angular extent ($\sim 4$ deg in diameter), nearly circular shape and X-ray spectrum dominated by emission lines of helium- and hydrogen-like oxygen. It lacks bright counterparts of similar extent at other wavelengths which could be unequivocally associated with it. Given the relatively high Galactic latitude of the source, $b\approx-26$ deg, we interpret these observational properties as an indication of the off-disk location of this SNR candidate. Namely, we propose that this object originated from a Type Ia supernova which exploded some 40 000 yr ago in the low density ($\sim 10^{-3}\,{\rm cm^{-3}}$) and hot ($\sim (1-2)\times10^6\,{\rm K}$) gas of the Milky Way halo at a distance of $\sim 3\,{\rm kpc}$ from the Sun. The low density of the halo gas implies that the cooling and collisional ionization equilibrium (CEI) timescales downstream of the forward shock are much longer than the age of the SNR. This results in a relatively soft spectrum, reflecting pre-shock ionization state of the gas, and strong boost in the plasma emissivity (compared to CEI) due to enhanced collisional excitation through the increased electron temperature. If confirmed, such a rare object would provide us with a unique "in situ" probe of physical conditions (density, temperature and metallicity) near the interface between the Milky Way's disk and the halo.

astro-ph.GA↗

arXiv2021

Tempestuous life beyond R_500: X-ray view on the Coma cluster with SRG/eROSITA. I. X-ray morphology, recent merger, and radio halo connection

This is the first paper in a series of studies of the Coma cluster using the SRG/eROSITA X-ray data obtained in course of the Calibration and Performance Verification observations. The data cover $\sim3^\circ\times 3^\circ$ area around the cluster with a typical exposure time of more than 20 ks. The stability of the instrumental background and operation of the SRG Observatory in the scanning mode provided us with an excellent data set for studies of the diffuse emission up to a distance of $\sim 1.5R_{200}$ from the Coma center. In this study, we discuss the rich morphology revealed by the X-ray observations (also in combination with the SZ data) and argue that the most salient features can be naturally explained by a recent (on-going) merger with the NGC 4839 group. In particular, we identify a faint X-ray bridge connecting the group with the cluster, which is convincing proof that NGC 4839 has already crossed the main cluster. The gas in the Coma core went through two shocks, first through the shock driven by NGC 4839 during its first passage through the cluster some Gyr ago, and, more recently, through the "mini-accretion shock" associated with the gas settling back to quasi-hydrostatic equilibrium in the core. After passing through the primary shock, the gas should spend much of the time in a rarefaction region, where radiative losses of electrons are small, until the gas is compressed again by the mini-accretion shock. Unlike "runway" merger shocks, the mini-accretion shock does not feature a rarefaction region downstream and, therefore, the radio emission can survive longer. Such a two-stage process might explain the formation of the radio halo in the Coma cluster.

astro-ph.HE↗

arXiv2020

Gamma-ray flux depressions of the Crab nebula in the high-energy range

The giant gamma-ray flares of the Crab nebula discovered by AGILE and Fermi observatories came as a surprise and have challenged the existing models of pulsar wind nebulae. We have carried out an analysis of 10.5 years of Fermi-LAT observations (Aug 2008 -- Feb 2019) and investigated variability of the Crab nebula in the 100-300 MeV range. Besides the flares, we found several month long depressions of the gamma-ray flux and identified several cases of sharp flux drops, where during one week the flux decreased by an order of magnitude with respect to its average value. No statistically significant variations of the nebula flux in the $E>$10 GeV range were found in the data. We discuss possible implications of the observed gamma-ray flux depressions on the model of synchrotron emission of the Crab nebula.

astro-ph.HE↗

arXiv2020

High-energy particles and radiation in star-forming regions

Non-thermal particles and high-energy radiation can play a role in the dynamical processes in star-forming regions and provide an important piece of the multiwavelength observational picture of their structure and components. Powerful stellar winds and supernovae in compact clusters of massive stars and OB associations are known to be favourable sites of high-energy particle acceleration and sources of non-thermal radiation and neutrinos. Namely, young massive stellar clusters are likely sources of the PeV (petaelectronvolt) regime cosmic rays (CRs). They can also be responsible for the cosmic ray composition, e.g., 22Ne/20Ne anomalous isotopic ratio in CRs. Efficient particle acceleration can be accompanied by super-adiabatic amplification of the fluctuating magnetic fields in the systems converting a part of kinetic power of the winds and supernovae into the magnetic energy through the CR-driven instabilities. The escape and CR propagation in the vicinity of the sources are affected by the non-linear CR feedback. These effects are expected to be important in starburst galaxies, which produce high-energy neutrinos and gamma-rays. We give a brief review of the theoretical models and observational data on high-energy particle acceleration and their radiation in star-forming regions with young stellar population.

astro-ph.HE↗

arXiv2019

Simulation of collisionless shocks in plasmas with high metallicity

The results of hybrid simulation of low-beta supercritical quasi-parallel shocks in high metallicity plasma are presented. The structure of upstream and downstream turbulence is addressed and velocities of the corresponding scattering centers are derived. It is shown that independently of their chemical composition the shocks experience self-reformation process. However, the period of self-reformation as well as the wave spectrum is greatly affected by the presence of substantial admixture of weakly charged heavy ions. Also the downstream magnetic field amplification is stronger for the high metallicity case.

astro-ph.HE↗

arXiv2019

High-energy cosmic rays from compact galactic star clusters: particle fluxes and anisotropy

It has been shown that supernova blast waves interacting with winds from massive stars in compact star clusters may be capable of producing cosmic-ray (CR) protons to above $10^{17}$ eV. We give a brief description of the colliding-shock-flows mechanism and look at generalizations of the diffusion of ~ 100 PeV CRs in the turbulent galactic magnetic field present in the galactic disk. We calculate the temporal evolution of the CR anisotropy from a possible distribution of young compact massive star clusters assuming the sources are intermittent on time scales of a few million years, i.e., comparable to their residence time in the Milky Way. Within the confines of our model, we determine the galactic/extra-galactic fraction of high-energy CRs resulting in anisotropies consistent with observed values. We find that galactic star clusters may contribute a substantial fraction of ~ 100 PeV CRs without producing anisotropies above observed limits.

astro-ph.HE↗

arXiv2019

GeV-TeV Cosmic Ray Leptons in the Solar System from the Bow Shock Wind Nebula of the Nearest Millisecond Pulsar J0437-4715

We consider acceleration of leptons up to GeV-TeV energies in the bow shock wind nebula of PSR J0437-4715 and their subsequent diffusion through the interstellar magnetic fields. The leptons accelerated at the pulsar wind termination surface are injected into re-acceleration in colliding shock flows. Modelled spectra of synchrotron emission from the accelerated electrons and positrons are consistent with the far-ultraviolet and X-ray observations of the nebula carried out with the Hubble Space Telescope and Chandra X-ray Observatory. These observations are employed to constrain the absolute fluxes of relativistic leptons, which are escaping from the nebula and eventually reaching the Solar System after energy-dependent diffusion through the local interstellar medium accompanied by synchrotron and Compton losses. It is shown that accelerated leptons from the nebula of PSR J0437-4715 can be responsible both for the enhancement of the positron fraction above a few GeV detected by PAMELA and AMS-02 spectrometers and for the TeV range lepton fluxes observed with H.E.S.S., VERITAS, Fermi, CALET, and DAMPE.

astro-ph.HE↗

arXiv2019

Shocks and non-thermal particles in clusters of galaxies

Galaxy clusters grow by gas accretion, mostly from mergers of substructures, which release powerful shock waves into cosmic plasmas and convert a fraction of kinetic energy into thermal energy, amplification of magnetic fields and into the acceleration of energetic particles. The modeling of the radio signature of cosmic shocks, combined with the lack of detected gamma-rays from cosmic ray (CR) protons, poses challenges to our understanding of how cosmic rays get accelerated and stored in the intracluster medium. Here we review the injection of CRs by cosmic shocks of different strengths, combining the detailed "microscopic" view of collisionless processes governing the creation of non-thermal distributions of electrons and protons in cluster shocks (based on analytic theory and particle-in-cell simulations), with the "macroscopic" view of the large-scale distribution of cosmic rays, suggested by modern cosmological simulations. We discuss time dependent non-linear kinetic models of particle acceleration by multiple internal shocks with large scale compressible motions of plasma. The models produce soft CR spectra containing a noticeable energy density in the super-thermal protons of energies below a few GeV which are difficult to constrain by Fermi observations. We consider the effect of plasma composition on CR injection and super-thermal particle population in the hot intracluster matter which can be constrained by fine high resolution X-ray spectroscopy of Fe ions.

astro-ph.HE↗

arXiv2018

Maximus: a Hybrid Particle-in-Cell Code for Microscopic Modeling of Collisionless Plasmas

A second-order accurate divergence-conserving hybrid particle-in-cell code Maximus has been developed for microscopic modeling of collisionless plasmas. The main specifics of the code include a constrained transport algorithm for exact conservation of magnetic field divergence, a Boris-type particle pusher, a weighted particle momentum deposit on the cells of the 3D spatial grid, an ability to model multispecies plasmas and an adaptive time step. The code is efficiently parallelized for running on supercomputers by means of the message passing interface (MPI) technology; an analysis of parallelization efficiency and overall resource intensity is presented. A Maximus simulation of the shocked flow in the Solar wind is shown to agree well with the observations of the Ion Release Module (IRM) aboard the Active Magnetospheric Particle Tracer Explorers interplanetary mission.

physics.comp-ph↗

arXiv2018

Cosmic ray production in supernovae

We give a brief review of the origin and acceleration of cosmic rays (CRs), emphasizing the production of CRs at different stages of supernova evolution by the first-order Fermi shock acceleration mechanism. We suggest that supernovae with trans-relativistic outflows, despite being rather rare, may accelerate CRs to energies above 10$^{18}$ eV over the first year of their evolution. Supernovae in young compact clusters of massive stars, and interaction powered superluminous supernovae, may accelerate CRs well above the PeV regime. We discuss the acceleration of the bulk of the galactic CRs in isolated supernova remnants and re-acceleration of escaped CRs by the multiple shocks present in superbubbles produced by associations of OB stars. The effects of magnetic field amplification by CR driven instabilities, as well as superdiffusive CR transport, are discussed for nonthermal radiation produced by nonlinear shocks of all speeds including trans-relativistic ones.

astro-ph.HE↗