Radio to Gamma-Ray Emission from Shell-type Supernova Remnants: Predictions from Non-linear Shock Acceleration Models

TL;DR

This paper models gamma-ray and lower energy photon emissions from shell-type supernova remnants using non-linear shock acceleration simulations, revealing spectral features and cutoffs consistent with observations.

Contribution

It introduces a Monte Carlo simulation incorporating non-linear shock effects to predict detailed photon spectra from SNRs, accounting for cosmic ray feedback.

Findings

Spectral deviations from power-laws affect gamma-ray predictions.

Spectral cutoffs in TeV range depend on environmental density.

Inverse Compton dominates in low-density SNRs at super-TeV energies.

Abstract

Supernova remnants (SNRs) are widely believed to be the principal source of galactic cosmic rays. Such energetic particles can produce gamma-rays and lower energy photons via interactions with the ambient plasma. In this paper, we present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like SNRs. These non-linearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays which deviate from pure power-laws. Such deviations are crucial to acceleration efficiency and spectral considerations, producing GeV/TeV intensity ratios that are quite different from test particle predictions. The Sedov scaling solution for SNR expansions is used to estimate important shock parameters for input into the Monte Carlo simulation. We calculate ion and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission, yielding complete photon spectra from radio frequencies to gamma-ray energies. The cessation of acceleration caused by the spatial and temporal limitations of the expanding SNR shell in moderately dense interstellar regions can yield spectral cutoffs in the TeV energy range; these are consistent with Whipple's TeV upper limits on unidentified EGRET sources. Supernova remnants in lower density environments generate higher energy cosmic rays that produce predominantly inverse Compton emission at super-TeV energies; such sources will generally be gamma-ray dim at GeV energies.