Shock Acceleration and γ-Ray Emitting Supernova Remnants

Diffusive shock acceleration in the environs of a remnant’s expanding shell is a popular candidate for the origin of SNR gamma-rays, as well as providing the principal source of galactic cosmic rays. In this paper, results from our study of non-linear effects in shock acceleration theory and their impact on the gamma-ray spectra of SNRs are presented. These effects describe the dynamical influence of the accelerated cosmic rays on the shocked plasma at the same time as addressing how the non-uniformities in the fluid flow force the distribution of the cosmic rays to deviate from pure power-laws. Such deviations are crucial to gamma-ray spectral determination. Our self-consistent Monte Carlo approach to shock acceleration is used to predict ion and electron distributions that spawn neutral pion decay, bremsstrahlung and inverse Compton emission components for SNRs. We demonstrate how the spatial and temporal limitations imposed by the expanding SNR shell quench acceleration above critical energies in the 500 GeV 10 TeV range, thereby spawning gamma-ray spectral cutoffs that are quite consistent with Whipple’s TeV upper limits to the EGRET unidentified sources that have SNR associations. We also discuss the role of electron injection in shocks and its impact on the significance of electromagnetic components to GeV–TeV spectral formation.