arXiv subjects

O. Petruk

Publications and source records attributed to O. Petruk.

50 records · Page 3Linked to original sources

arXiv2008

Aspect angle for interstellar magnetic field in SN 1006

A number of important processes taking place around strong shocks in supernova remnants (SNRs) depend on the shock obliquity. The measured synchrotron flux is a function of the aspect angle between interstellar magnetic field (ISMF) and the line of sight. Thus a model of non-thermal emission from SNRs should account for the orientation of the ambient magnetic field. We develop a new method for the estimation of the aspect angle, based on the comparison between observed and synthesized radio maps of SNRs, making different assumptions about the dependence of electron injection efficiency on the shock obliquity. The method uses the azimuthal profile of radio surface brightness as a probe for orientation of ambient magnetic field because it is almost insensitive to the downstream distribution of magnetic field and emitting electrons. We apply our method to a new radio image of SN 1006 produced on the basis of archival VLA and Parkes data. The image recovers emission from all spatial structures with angular scales from few arcseconds to 15 arcmin. We explore different models of injection efficiency and find the following best-fitting values for the aspect angle of SN 1006: phi=70 +/- 4.2 deg if the injection is isotropic, phi=64 +/- 2.8 deg for quasi-perpendicular injection (SNR has an equatorial belt in both cases) and phi=11 +/- 0.8 deg for quasi-parallel injection (polar-cap model of SNR). In the last case, SN 1006 is expected to have a centrally-peaked morphology contrary to what is observed. Therefore, our analysis provides some indication against the quasi-parallel injection model.

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arXiv2007

On the Origin of Asymmetries in Bilateral Supernova Remnants

AIMS: We investigate whether the morphology of bilateral supernova remnants (BSNRs) observed in the radio band is determined mainly either by a non-uniform interstellar medium (ISM) or by a non-uniform ambient magnetic field. METHODS: We perform 3-D MHD simulations of a spherical SNR shock propagating through a magnetized ISM. Two cases of shock propagation are considered: 1) through a gradient of ambient density with a uniform ambient magnetic field; 2) through a homogeneous medium with a gradient of ambient magnetic field strength. From the simulations, we synthesize the synchrotron radio emission, making different assumptions about the details of acceleration and injection of relativistic electrons. RESULTS: We find that asymmetric BSNRs are produced if the line-of-sight is not aligned with the gradient of ambient plasma density or with the gradient of ambient magnetic field strength. We derive useful parameters to quantify the degree of asymmetry of the remnants that may provide a powerful diagnostic of the microphysics of strong shock waves through the comparison between models and observations. CONCLUSIONS: BSNRs with two radio limbs of different brightness can be explained if a gradient of ambient density or, most likely, of ambient magnetic field strength is perpendicular to the radio limbs. BSNRs with converging similar radio arcs can be explained if the gradient runs between the two arcs.

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arXiv2006

Artificial broadening of the high-energy end of electron spectrum in supernova remnants

The observed spectrum of a supernova remnant (SNR) is a superposition of many ``local'' spectra emitted by regions of SNRs that are under different physical conditions. The question remains as to whether the broadening of the high-energy end of the observed nonthermal spectrum of SNRs, like in G347.3-0.5 and SN 1006, can be an artifact of observations or it is a consequence of the microphysics involved in the acceleration process. In this note we study the influence of parameters variations (inside the volume and over the surface of SNR) on the shape of the high-energy end of the synchrotron (and also inverse Compton) spectrum. We consider three possibilities for these parameter variations: i) gradients downstream of the shock with constant maximum energy of the accelerated electrons and the potential variation in time of the injection efficiency, ii) then we add the possibility of the maximum energy depending on time, and finally iii) the possible obliquity dependences of maximum energy and injection efficiency. It is shown that gradients of density and magnetic field strength downstream of the shock are ineffective in modifying the shape of the synchrotron spectrum, even if an SNR evolves in the nonuniform interstellar medium and/or the injection efficiency varies in time. The time dependence of the maximum energy of the electrons accelerated by the shock is also not able to make the observed spectrum much broader. The only possibility of producing considerable broadening in the spectrum is the variation in the maximum energy of electrons over the surface of SNR. In such a case, the obliquity dependence of the injection efficiency also affects the shape of the spectrum, but its role is less significant.

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arXiv2006

Influence of Thermalisation on Electron Injection in Supernova Remnant Shocks

Within a test-particle description of the acceleration process in parallel nonrelativistic shocks, we present an analytic treatment of the electron injection. We estimate the velocity distribution of the injected electrons as the product of the post-shock thermal distribution of electrons times the probability for electrons with a given velocity to be accelerated; the injection efficiency is then evaluated as the integral of this velocity distribution. We estimate the probability of a particle to be injected as that of going back to the upstream region at least once. This is the product of the probability of returning to the shock from downstream times that of recrossing the shock from downstream to upstream. The latter probability is expected to be sensitive to details of the process of electron thermalisation within the (collisionless) shock, a process that is poorly known. In order to include this effect, for our treatment we use results of a numerical, fully kinetic study, by Bykov & Uvarov (1999). According to them, the probability of recrossing depends on physics of thermalisation through a single free parameter (Gamma), which can be expressed as a function of the Mach number of the shock, of the level of electron-ion equilibration, as well as of the spectrum of turbulence. It becomes apparent, from our analysis, that the injection efficiency is related to the post-shock electron temperature, and that it results from the balance between two competing effects: the higher the electron temperature, the higher the fraction of downstream electrons with enough velocity to return to the shock and thus to be ready to cross the shock from downstream to upstream; at the same time, however, the higher the turbulence, which would hinder the crossing.

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arXiv2006

On the transition of the adiabatic supernova remnant to the radiative stage in a nonuniform interstellar medium

Methods for estimation of different reference times which appear in the description of transition of a strong adiabatic shock into the radiative era are reviewed. The need for consideration of an additional transition subphase in between the end of the adiabatic era and the beginning of the radiative ''pressure-driven snowplow'' stage for a shock running in the uniform or nonuniform medium is emphasized. This could be of importance in particular for studying the interaction of supernova remnants (SNRs) with molecular clouds and therefore for understanding the processes of the cosmic ray production in such systems. The duration of this subphase - about 70% of SNR age at its beginning - is almost independent of the density gradient for media with increasing density and is longer for higher supernova explosion energy and for smaller density in the place of explosion. It is shown as well that if the density of the ambient medium decreases then the cooling processes could differ from the commonly accepted scenario of the ''thin dense radiative shell'' formation. This property should be studied in the future because it is important for models of nonspherical SNRs which could be only partially radiative.

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arXiv2004

Analytic Solutions for the Evolution of Radiative Supernova Remnants

We present the general analytic solution for the evolution of radiative supernova remnants in a uniform interstellar medium, under thin-shell approximation. This approximation is shown to be very accurate approach to this task. For a given set of parameters, our solution closely matches the results of numerical models, showing a transient in which the deceleration parameter reaches a maximum value of 0.33, followed by a slow convergence to the asymptotic value 2/7. Oort (1951) and McKee and Ostriker (1977) analytic solutions are discussed, as special cases of the general solution we have found.

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arXiv2002

Model for Synchrotron Emission from Shell Supernova Remnants in Nonuniform Interstellar Medium and Nonuniform Magnetic Field

Possibility to model the high energy synchrotron emission (in X- and gamma-rays) from supernova remnants is an important task for modern astronomy and astrophysics, because it may be responsible for the nonthermal X-rays and TeV gamma-rays observed recently from a number of SNRs. This emission allows as to look in the processes of particle acceleration on SNR shocks and generation of cosmic rays. In this paper, a model for the synchrotron emission from shell SNR in nonuniform interstellar medium and nonuniform magnetic field is presented. This model is a generalization of the model of Reynolds and Chevalier developed for a spherical SNR in the uniform medium and uniform magnetic field. The model will be used for studies on the thermal and nonthermal X-ray images and spectra from nonspherical SNRs in different interstellar magnetic field configurations.

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arXiv2001

A New Model for the Thermal X-ray Composites and the Neutral Pion Decay Gamma-Rays from Supernova Remnants

Recent nonthermal X-ray and gamma-ray observations, attributed to electron emission processes, for the first time give an experimental confirmation that electrons are accelerated on SNR shocks up to the energy 10^{14} eV. We have no direct observational confirmations about proton acceleration by SNR. Different models of gamma-emission from SNRs predict different emission mechanisms as dominating. Only decays of neutral pion created in proton-nucleon interactions allow us to look inside the CR nuclear component acceleration processes. A new model for the thermal X-ray composites strongly suggest that thermal X-ray peak inside the radio shell of SNR tells us about entering of one part of SNR shock into a denser medium compared with other parts of the shell. This makes a TXCs promising sites for gamma-ray generation via neutral pion decays. Detailed consideration of SNR-cloud interaction allows to increase an expected proton induced gamma-ray flux from SNR at least on an order of magnitude, that allows to adjust the theoretical proton origin gamma-luminosities with observed fluxes at least for a few SNRs even for low density (n=10-100 cm^{-3}) cloud.

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arXiv2001

Thermal X-ray emission of the remnants of ashperical Supernova explosions

Evolution of adiabatic remnants of an aspherical supernova explosion in uniform medium are considered. Thermal X-ray emission of such remnants are investigated. It is shown that integral thermal X-ray characteristics (X-ray luminosity and spectrum) of the objects do not allow us to reveal the assymetry in the explosion because these characteristics are close to their Sedov counterparts. Surface distribution of X-ray emission is sensitive to anisotropy of the explosion and nonuniformity of the interstellar medium.

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arXiv2001

Thermal X-ray composites as an effect of projection

A new possibility to explain the nature of thermal X-ray composites (TXCs), i.e. a class of supernova remnants (SNRs) with a thermal X-ray centrally-filled morphology within a radio shell, as a projection effect of the 2- or 3-dimensional shell-like SNR evolved in a nonuniform medium with scale-height <10 pc is proposed. Both X-ray and radio morphologies, as well as the basic theoretical features of this kind of SNR and the surrounding medium, are considered. Theoretical properties of a shell-like SNR evolved at the edge of a molecular cloud correspond to the observed properties of TXCs if the gradient of the ambient density does not lie in the projection plane and the magnetic field is nearly aligned with the line of sight. So, at least a part of objects from the class may be interpreted within the framework of the considered effect. The proposed model suggests that SNRs with barrel-like radio and centrally-brightened thermal X-ray morphologies should exist. The model allows us to consider TXCs as prospective sources of proton origin gamma-rays.

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arXiv2000

Approximations of the self-similar solution for blastwave in a medium with power-law density variation

Approximations of the Sedov self-similar solution for a strong point explosion in a medium with the power-law density distribution ρ^o\propto r^{-m} are reviewed and their accuracy are analyzed. Taylor approximation is extended to cases m\neq 0. Two approximations of the solution are presented in the Lagrangian coordinates for spherical, cylindrical and plane geometry. These approximations may be used for the investigation of the ionization structure of the adiabatic flow, i.e., inside adiabatic supernova remnants.

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arXiv1999

Supernova remnants as cosmic ray accelerators. SNR IC 443

We examine the hypothesis that some supernova remnants (SNRs) may be responsible for some unidentified gamma-ray sources detected by EGRET instrument aboard the Compton Gamma Ray Observatory. If this is the case, gamma-rays are produced via pion production and decay from direct inelastic collisions of accelerated by SNR shock wave ultrarelativistic protons with target protons of the interstellar medium. We develop a 3-D hydrodynamical model of SNR IC 443 as a possible cosmic gamma-ray source 2EG J0618+2234. The derived parameters of IC 443: the explosion energy E_o=2.7*10^{50} erg, the initial hydrogen number density n(0)=0.21 cm^{-3}, the mean radius R=9.6 pc and the age t=4500 yr result in too low gamma-ray flux, mainly because of the low explosion energy. Therefore, we investigate in detail the hydrodynamics of IC 443 interaction with a nearby massive molecular cloud and show that the reverse shock wave considerably increases the cosmic ray density in the interaction region. Meantime, the Rayleigh-Taylor instability of contact discontinuity between the SNR and the cloud provides an effective mixing of the containing cosmic ray plasma and the cloud material. We show that the resulting gamma-ray flux is consistent with the observational data.

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arXiv1999

Evolution of supernova remnants in the interstellar medium with a large-scale density gradient. II. The 2-D modelling of the evolution and X-ray emission of supernova remnant RCW86

The results of the 2D modelling of Supernova remnant (SNR) RCW86 are given. Models of this remnant, which for the first time interpret the anisotropy of surface brightness as a result of the evolution of adiabatic SNR in the interstellar medium with a large-scale gradient of density, are considered. Estimations on the basic characteristics of RCW86 and the surrounding medium which follow from these models are found. It is shown that the observed surface brightness distribution of RCW86 may be obtaned in the both proposed up till now models with different initial assumptions: one about a Supernova explosion in 185 A.D. and another about an explosion in the OB-asossiation. In order to obtain the observational contrast of surface brightness it is necessary to have a medium with the characteristic scale of nonuniformity 11 pc if the age of RCW86 is 1800 years or 20-25 pc when the SNR is distant from us on 2.8 kpc. The preshock density contrast between the southwestern and northeastern parts of RCW86 is in range 3.5-4.5.

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arXiv1999

Evolution of supernova remnants in the interstellar medium with a large-scale density gradient. I. General properties of the morphological evolution and X-ray emission

The large-scale gradient of the interstellar medium (ISM) density distribution essentially affects the evolution of Supernova remnants (SNRs). In a non-uniform ISM, the shape of SNR becomes essentially non-spherical, and distributions of gas parameters inside the remnant become strongly anisotropic. The well-known self-similar Sedov solutions may not be applied to modelling such non-spherical objects. Therefore we propose a new approximate analytical method for full hydrodynamical description of 3D point-like explosions in non-uniform media with arbitrary density distribution. On the basis of this method, we investigate the general properties of evolution of 2D non-spherical adiabatic SNRs in ISM with large-scale density gradient. It is shown that the real shape of adiabatic SNR becomes more non-spherical with age, but the visible shape remains close to spherical even for strong real asymmetry (ratio of maximal to minimal shock radii) and surface brightness contrast. It is shown also that the values of the parameters of X-ray radiation from the entire SNR (luminosity, spectral index) are close to those in the Sedov case with the same initial data. However, the surface distribution of the X-ray emission parameters is very sensitive to the initial density distribution around the SN progenitor. Therefore, the X-ray maps give important information about physical conditions inside and outside of the non-spherical SNR.

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