arXiv subjects

A. M. Bykov

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

83 records · Page 5Linked to original sources

arXiv2005

Hard Extended X-ray Source in the IC 443 SNR Resolved by Chandra: A Fast Ejecta Fragment or a New Pulsar Wind Nebula?

A Chandra observation of the isolated hard X-ray source XMMU J061804.3+222732, located in the region of apparent interaction of the supernova remnant IC 443 with a molecular cloud, resolved the complex structure of the source in a few bright clumps embedded in an extended emission of a ~ 30 arcsec size. The X-ray spectra of the clumps and the extended emission are dominated by a hard power-law component with a photon index of 1.2--1.4. In addition, we see some indications of an optically thin thermal plasma of a ~ 0.3 keV temperature. The observed X-ray morphology and spectra are consistent with those expected for an isolated supernova ejecta fragment interacting with a dense ambient medium. A possible alternative interpretation is a pulsar wind nebula associated with either IC 443 or another SNR, G189.6+3.3.

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arXiv2005

Multi-fluid shocks in clusters of galaxies: entropy, sigma_ v-T, M-T and L_x-T scalings

The nonthermal phenomena in clusters of galaxies are considered in the context of the hierarchical model of cosmic structure formation by accretion and merging of the dark matter (DM) substructures.Accretion and merging processes produce large-scale gas shocks. The plasma shocks are expected to be collisionless. In the course of cluster's aggregation, the shocks, being the main gas-heating agent, generate turbulent magnetic fields and accelerate energetic particles via collisionless multi-fluid plasma relaxation processes. The intracluster gas heating and entropy production rate by a collisionless shock may differ significantly from that in a single-fluid collisional shock. Simple scaling relations for postshock ion temperature and entropy as functions of shock velocity in strong collisionless multi-fluid shocks are presented. We show that the multi-fluid nature of the collisionless shocks results in high gas compression, reduced entropy production and modified sigma_v-T, M-T and L_x-T scalings. The scaling indexes estimated for a simple model of a strong accretion multi-fluid shock are generally consistent with observations. Soft X-ray and extreme ultraviolet photons dominate the emission of strong accretion shock precursors that appear as large-scale filaments. Magnetic fields, turbulence and energetic particles constitute the nonthermal components contributing into the pressure balance, energy transport and emission of clusters. Nonthermal emission of energetic particles could be a test to constrain the cluster properties.

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arXiv2004

Dynamics of the Flows Accreting onto a Magnetized Neutron Star

Non-stationary column accretion onto a surface of a magnetized neutron star is studied with a numerical code based on modified first-order Godunov method with splitting. Formation and evolution of shocks in the column is modeled for accretion rates ranging from 10^{15} g/s to 10^{16} g/s and surface magnetic fields ranging from 5*10^{11} G to 10^{13} G. Non-stationary solutions with plasma deceleration at collisionless oscillating shocks are found. The kinetic energy of the accreting flow efficiently transforms into a cyclotron radiation field. Collisionless stopping of the flow allows a substantial part of accreting CNO nuclei to avoid spallation and reach the surface. The nuclei survival fraction depends on the surface magnetic field, being higher at lower magnetic fields.

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arXiv2004

Hard X-ray Emission Clumps in the gamma-Cygni Supernova Remnant: an INTEGRAL-ISGRI View

Spatially resolved images of the galactic supernova remnant G78.2+2.1 (gamma-Cygni) in hard X-ray energy bands from 25 keV to 120 keV are obtained with the IBIS-ISGRI imager aboard the International Gamma-Ray Astrophysics Laboratory INTEGRAL. The images are dominated by localized clumps of about ten arcmin in size. The flux of the most prominent North-Western (NW) clump is (1.7 +/- 0.4) 10^{-11} erg/cm^2/s in the 25-40 keV band. The observed X-ray fluxes are in agreement with extrapolations of soft X-ray imaging observations of gamma-Cygni by ASCA GIS and spatially unresolved RXTE PCA data. The positions of the hard X-ray clumps correlate with bright patches of optical line emission, possibly indicating the presence of radiative shock waves in a shocked cloud. The observed spatial structure and spectra are consistent with model predictions of hard X-ray emission from nonthermal electrons accelerated by a radiative shock in a supernova interacting with an interstellar cloud, but the powerful stellar wind of the O9V star HD 193322 is a plausible candidate for the NW source as well.

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arXiv2004

Superbubbles and Energetic Particles in the Galaxy. I: Collective effects of particle acceleration

Observations indicate that most massive stars in the Galaxy appear in groups, called OB associations, where their strong wind activity generates large structures known as superbubbles, inside which the subsequent supernovae (SNe) explode, in tight space and time correlation. Acknowledging this fact, we investigate four main questions: 1) does the clustering of massive stars and SN explosions influence the particle acceleration process usually associated with SNe, and induce collective effects which would not manifest around isolated supernova remnants?; 2) does it make a difference for the general phenomenology of Galactic Cosmic Rays (GCRs), notably for their energy spectrum and composition?; 3) can this help alleviate some of the problems encountered within the standard GCR source model?; and 4) Is the link between superbubbles and energetic particles supported by observational data, and can it be further tested and constrained? We argue for a positive answer to all these questions. Theoretical, phenomenological and observational aspects are treated in separate papers. Here, we discuss the interaction of massive stellar winds and SN shocks inside superbubbles and indicate how this leads to specific acceleration effects. We also show that due to the high SN explosion rate and low diffusion coefficient, low-energy particles experience repeated shock acceleration inside superbubbles.

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arXiv2003

Faint Hard X-ray Sources in the Galactic Center Region: Supernova Ejecta Fragments Population

Long Chandra observations of 17 arcmin x 17 arcmin field in the Galactic Center (GC) region reported by Muno et. al. (2003) have revealed a population of about 2,000 hard faint X-ray sources limited by luminosities Lx > 10(31} erg/s. We show that fast moving knots (FMKs) of supernovae ejecta could comprise a sizeable fraction of the sources. Each supernova event can produce hundreds of FMKs. The presence of ~ 3 supernova remnants of age ~ 1,000 years in the GC region could provide the required number of the FMKs. Simulated X-ray spectra of the FMKs contain both thermal and nonthermal components. The nonthermal spectra are power-laws with hard photon indexes 0 < Gamma < 1.5 and prominent lines of some metals (e.g. Fe). The logN-logS distribution and the hardness ratios of the FMKs in the GC region are consistent with the Chandra findings. Spatial distribution of the FMKs in the GC region should appear as an ensemble of coherent shell and jet-like structures of a few arcmin-scale.

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arXiv2003

XMM-Newton study of hard X-ray sources in IC443

We present \xmm observations of hard X-ray emission from the field of IC443, a supernova remnant interacting with a molecular cloud. The hard emission from the field is dominated by 12 isolated sources having 2--10 keV flux $\ga 10^{-14}$ $\enf$. Only a fraction of the sources are expected to be extragalactic or stars on statistical grounds, while the others may be associated with the remnant. We have analyzed near-infrared K band and also DSS optical data for all of the detected sources, finding that six X-ray sources are located in a relatively small $15^\prime\times 15^\prime$ region where there is strong 2.2 $μ{\rm m}$ infrared emission, indicating interaction with a molecular cloud. The source 1SAX J0618.0+2227, the brightest in this region (excluding the plerion), is resolved with \xmm into two sources, one of which is extended and has a hard power law spectrum photon index $\lsim$ 1.5) and shows some indications of spectral line signatures (e.g. Si), while the other is point-like and has a featureless spectrum of steeper photon index $\sim$ 2.2. Possible interpretations of some of the discrete sources in terms of interaction between the SNR and the molecular cloud are discussed.

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arXiv2002

X-ray Line Emission from Supernova Ejecta Fragments

We show that fast moving isolated fragments of a supernova ejecta composed of heavy elements should be sources of K_alpha X-ray line emission of the SN nuclear-processed products. Supersonic motion of the knots in the intercloud medium will result in a bow-shock/knot-shock structure creation. Fast nonthermal particles accelerated by Fermi mechanism in the MHD collisionless shocks diffuse through a cold metallic knot, producing the X-ray emission. We modeled the X-ray emission from a fast moving knot of a mass M ~ 10^{-3} \Msun, containing about 10^{-4} \Msun of any metal impurities like Si, S, Ar, Ca, Fe. The fast electron distribution was simulated using the kinetic description. We accounted for nonlinear effects of shock modification by the nonthermal particles pressure. The K_alpha line emission is most prominent for the knots propagating through dense molecular clouds. The bow shock should be a radiative wave with a prominent infrared and optical emission. In that case the X-ray line spectrum of an ejecta fragment is dominated by the low ionization states of the ions with the metal line luminosities reaching L_x \gsim 10^{31} erg/s. High resolution XMM and Chandra observations are able to detect the line emission from the knots at distances up to a few kpcs. The knots propagating through tenuous interstellar matter are of much fainter surface brightness but long-lived. The line spectra with higher ionization states of the ions are expected in that case. Compact dense knots could be opaque for some X-ray lines and that is important for their abundances interpretation. The ensemble of unresolved knots of galactic supernovae can contribute substantially to the iron line emission observed from the Galactic Center region and the Galactic ridge.

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arXiv2001

The plerion nebula in IC 443: the XMM-Newton view

\xmm ~observations of the X-ray feature 1SAX J0617.1+2221 in the IC443 supernova remnant are reported.We resolve the structure of the nebula into a compact core with a hard spectrum of photon index $γ= 1.63^{+0.11}_{-0.10}$ in the 2--10 keV energy range. The nebula also has an extended ($\sim 8\arcmin \times 5\arcmin$) X-ray halo, much larger than the radio emission extension. The photon index softens, following a linear scaling with distance from the centroid, similar to other known X-ray plerions. The index range is compatible with synchrotron burn-off models. All the observational evidence points toward a confirmation of the plerionic nature of the nebula, as recently suggested by a \ch observation, but with characteristics more similar to "non Crab-like" plerions. We discuss the implications on the synchrotron nebula magnetic field if the $>100$ MeV emission reported by {\it CGRO EGRET} is produced by the synchrotron emission. We also constrain the thermal emission of the central object, arguing that the surface temperature should be around 0.1 keV, although other possible fits cannot be excluded on the base of the \xmm data

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arXiv2000

Hard X-ray emission from IC443: evidence for a shocked molecular clump?

We report BeppoSAX observations of spatially resolved hard X-ray emission from IC 443, a supernova remnant interacting with a molecular cloud. The emission is shown to come from two localized features spatially correlated with bright molecular emission regions. Both hard X-ray features have soft X-ray counterparts, in one case shifted by ~ 2 arcmin toward the remnant interior. The spectra of X-ray emission from both isolated features have photon index ~2 in the MECS regime. The emission detected from the remnant with PDS detector extends up to 100 keV. We discuss the observed properties of the hard X-ray features in relation to non-thermal emission from shocked molecular clumps and pulsar wind nebula.

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arXiv2000

Nonthermal Emission from a Supernova Remnant in a Molecular Cloud

In evolved supernova remnants (SNRs) interacting with molecular clouds, such as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a forward shock of moderate Mach number, a cooling layer, a dense radiative shell and an interior region filled with hot tenuous plasma is expected. We present a kinetic model of nonthermal electron injection, acceleration and propagation in that environment and find that these SNRs are efficient electron accelerators and sources of hard X- and gamma-ray emission. The energy spectrum of the nonthermal electrons is shaped by the joint action of first and second order Fermi acceleration in a turbulent plasma with substantial Coulomb losses. Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal electrons produce multiwavelength photon spectra in quantitative agreement with the radio and the hard emission observed by ASCA and EGRET from IC 443. We distinguish interclump shock wave emission from molecular clump shock wave emission accounting for a complex structure of molecular cloud. Spatially resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and 3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST would distinguish the contribution of the energetic lepton component to the gamma-rays observed by EGRET.

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