Radio to gamma-ray emission from shell-type supernova remnants: Predictions from nonlinear shock acceleration models

Supernova remnants (SNRs) are widely believed to be the principal source of Galactic cosmic rays, produced by diffusive shock acceleration in the envi rons of the remnant's expanding blast wave. Such energetic particles can pr oduce gamma rays and lower energy photons via interactions with the ambient plasma. The recently reported observation of TeV gamma rays from SN 1006 b y the Collaboration of Australia and Nippon for a Gamma-Ray Observatory in the Outback (CANGAROO), combined with the fact that several unidentified EG RET sources have been associated with known radio/optical/X-ray-emitting re mnants, provides powerful motivation for studying gamma-ray emission from S NRs. In this paper, we present results from a Monte Carlo simulation of non linear shock structure and acceleration coupled with photon emission in she lllike SNRs. These nonlinearities are a by-product of the dynamical influen ce of the accelerated cosmic rays on the shocked plasma and result in distr ibutions of cosmic rays that deviate from pure power laws. Such deviations are crucial to acceleration efficiency considerations and impact photon int ensities and spectral shapes at all energies, producing GeV/TeV intensity r atios that are quite different from test particle predictions. The Sedov sc aling solution for SNR expansions is used to estimate important shock param eters for input into the Monte Carlo simulation. We calculate ion (proton a nd helium) and electron distributions that spawn neutral pion decay, bremss trahlung, inverse Compton, and synchrotron emission, yielding complete phot on spectra from radio frequencies to gamma-ray energies. The cessation of a cceleration caused by the spatial and temporal limitations of the expanding SNR shell in moderately dense interstellar regions can yield spectral cuto ffs in the TeV energy range that are consistent with Whipple's TeV upper li mits on those EGRET unidentified sources that have SNR associations. Supern ova remnants in lower density environments generate higher energy cosmic ra ys that produce predominantly inverse Compton emission observable at super- TeV energies, consistent with the SN 1006 detection. In general, sources in such low-density regions will be gamma-ray-dim at GeV energies.