Gamma-Ray Production in Supernova Remnants

The bulk of cosmic rays of up to about 100 TeV are thought to be accelerated by the first-order Fermi mechanism at supernova shocks, producing a power-law spectrum. Both electrons and protons should be accelerated, but their ratio on acceleration is not well known. Recently, the EGRET instrument on the Compton Gamma Ray Observatory has observed supernova remnants IC 443 and γ Cygni at GeV energies. On the assumption that the observed γ-rays are produced by accelerated particles in the remnants (rather than, for example, from a central compact object), we model the contributions from pion production, bremsstrahlung, and inverse Compton scattering on the cosmic microwave, diffuse galactic, and locally produced radiation fields. In the case of the same spectral index for both electrons and nuclei and a cutoff at 80 TeV, we find that a spectral index of accelerated particles that is close to 2.4 and a ratio of electrons to protons in the range 0.2-0.3 give a good fit to the observed spectra. For lower cutoff energies, flatter spectra are possible. We also investigate the case in which the electron spectrum is steeper than that of nuclei. We discuss the implications of our results for observations at air shower energies, and for the propagation of cosmic rays.