Gamma-ray emission from Cassiopeia A produced by accelerated cosmic rays

Free access article

Issue		A&A Volume 400, Number 3, March IV 2003


Page(s)		971 - 980
Section		Formation, structure and evolution of stars
DOI		http://dx.doi.org/10.1051/0004-6361:20030033

A&A 400, 971-980 (2003)
DOI: 10.1051/0004-6361:20030033

Gamma-ray emission from Cassiopeia A produced by accelerated cosmic rays

E. G. Berezhko¹, G. Pühlhofer² and H. J. Völk²

¹ Institute of Cosmophysical Research and Aeronomy, 31 Lenin Ave., 677891 Yakutsk, Russia
e-mail: berezhko@ikfia.ysn.ru
² Max Planck Institut für Kernphysik, Postfach 103980, 69029 Heidelberg, Germany
e-mail: Gerd.Puehlhofer@mpi-hd.mpg.de

(Received 23 September 2002 / Accepted 7 January 2003 )

Abstract
The nonlinear kinetic model of cosmic ray (CR) acceleration in supernova remnants (SNRs) is used to describe the relevant properties of Cassiopeia A (Cas A). In order to reproduce the SNR's observed size, expansion rate and thermal X-ray emission we employ a piecewise homogeneous model for the progenitor's circumstellar medium developed by Borkowski et al. (1996). It consists of a tenuous inner wind bubble, a dense shell of swept-up red supergiant wind material, and a subsequent red supergiant wind region. A quite large SNR interior magnetic field $B_\mathrm{d}\approx 1\,\mathrm{mG}$ is required to give a good fit for the radio and X-ray synchrotron emission. The steep radio spectrum is consistent with efficient proton acceleration which produces a significant shock modification and leads to a steep electron spectrum at energies $\epsilon_{\mathrm{e}}<1\,\mathrm{GeV}$ . The calculated integral $\gamma$ -ray flux from Cas A, $F_{\gamma}\propto \epsilon_{\gamma}^{-1}$ , is dominated by $\pi^0$ -decay $\gamma$ -rays due to relativistic protons. It extends up to roughly $30\,\mathrm{TeV}$ if CR diffusion is as strong as the Bohm limit. At TeV energies it satisfactorily agrees with the value $5.8\times 10^{-13}~\mathrm{cm}^{-2}~\mathrm{s}^{-1}$ detected by the HEGRA collaboration.

Key words: supernovae: individual: Cassiopeia A -- cosmic rays -- gamma rays: theory -- acceleration of particles -- shock waves -- radiation mechanisms: non-thermal

Offprint request: H. J. Völk, Heinrich.Voelk@mpi-hd.mpg.de

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