Particle Acceleration in Supernova Remnants and the Production of Thermal and Nonthermal Radiation

If highly efficient, cosmic ray production can have a significant effect onthe X-ray emission from SNRs as well as their dynamical evolution. Usinghydrodynamical simulations including diffusive shock acceleration, we producespectra for both the thermal and nonthermal forward shock emission. For a givenambient density and explosion energy, we find that the position of the forwardshock at a given age is a strong function of the acceleration efficiency,providing a signature of cosmic-ray production. Using an approximate treatmentfor the ionization state of the plasma, we investigate the effects of slow vs.rapid heating of the postshock electrons on the ratio of thermal to nonthermalX-ray emission at the forward shock. We also investigate the effects ofmagnetic field strength on the observed spectrum for efficient cosmic-rayacceleration. The primary effect of a large field is a considerable flatteningof the nonthermal spectrum in the soft X-ray band. Spectral index measurementsfrom X-ray observations may thus be indicators of the postshock magnetic fieldstrength. The predicted gamma-ray flux from inverse-Compton (IC) scattering andneutral pion decay is strongly affected by the ambient conditions and, for theparticular parameters used in our examples, the IC emission at E ~ 1 TeVexceeds that from pion decay, although at both lower and higher energies thistrend is reversed for cases of high ambient density. More importantly, highmagnetic fields produce a steepening of the electron spectrum over a wideenergy range which may make it more difficult to differentiate between IC andpion-decay emission solely by spectral shape.Comment: 30 pages, 12 figures, submitted to ApJ January 200