Temporal evolution of nonthermal spectra from supernova remnants

Assuming that supernova shocks accelerate nonthermal particles, we model the temporally evolving nonthermal particle and photon spectra at different stages in the lifetime of a standard shell-type supernova remnant (SNR). A characteristic vF(v) spectrum of an SNR consists of a peak at radio through optical energies from nonthermal electron synchrotron emission and another high-energy gamma-ray peak due primarily to secondary pion production, nonthermal electron bremsstrahlung, and Compton scattering. We find that supernova remnants are capable of producing maximum gamma-ray luminosities greater than or similar to 10(35) ergs s(-1) if the density of the local interstellar medium is greater than or similar to 10 cm(-3). This emission will persist for greater than or similar to 10(5) yr after the supernova explosion because of the long energy loss timescales for electrons with kinetic energy similar to 1 GeV. This long gamma-ray lifetime implies that SNRs with a wide range of ages could be gamma-ray sources and could constitute some of the unidentified EGRET sources.