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

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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 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 . The calculated integral γ-ray flux from Cas A, , is dominated by -decay γ-rays due to relativistic protons. It extends up to roughly if CR diffusion is as strong as the Bohm limit. At TeV energies it satisfactorily agrees with the value detected by the HEGRA collaboration.