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 ofgalactic cosmic rays. Such energetic particles can produce gamma-rays and lowerenergy photons via interactions with the ambient plasma. In this paper, wepresent results from a Monte Carlo simulation of non-linear shock structure andacceleration coupled with photon emission in shell-like SNRs. Thesenon-linearities are a by-product of the dynamical influence of the acceleratedcosmic rays on the shocked plasma and result in distributions of cosmic rayswhich deviate from pure power-laws. Such deviations are crucial to accelerationefficiency and spectral considerations, producing GeV/TeV intensity ratios thatare quite different from test particle predictions. The Sedov scaling solutionfor SNR expansions is used to estimate important shock parameters for inputinto the Monte Carlo simulation. We calculate ion and electron distributionsthat spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotronemission, yielding complete photon spectra from radio frequencies to gamma-rayenergies. The cessation of acceleration caused by the spatial and temporallimitations of the expanding SNR shell in moderately dense interstellar regionscan yield spectral cutoffs in the TeV energy range; these are consistent withWhipple's TeV upper limits on unidentified EGRET sources. Supernova remnants inlower density environments generate higher energy cosmic rays that producepredominantly inverse Compton emission at super-TeV energies; such sources willgenerally be gamma-ray dim at GeV energies.Comment: 62 pages, AASTeX format, including 1 table and 11 figures, accepted for publication in The Astrophysical Journal (Vol 513, March 1, 1999