An Attempt at a Unified Model for the Gamma-Ray Emission of Supernova Remnants

Shocks of supernova remnants (SNRs) are important (and perhaps the dominant) agents for the production of the Galactic cosmic rays. Recent γ-ray observations of several SNRs have made this case more compelling. However, these broadband high-energy measurements also reveal a variety of spectral shapes demanding more comprehensive modeling of emissions from SNRs. According to the locally observed fluxes of cosmic-ray protons and electrons, the electron-to-proton number ratio is known to be about 1%. Assuming such a ratio is universal for all SNRs and identical spectral shape for all kinds of accelerated particles, we propose a unified model that ascribes the distinct γ-ray spectra of different SNRs to variations of the medium density and the spectral difference between cosmic-ray electrons and protons observed from Earth to transport effects. For low-density environments, the γ-ray emission is inverse-Compton dominated. For high-density environments like systems of high-energy particles interacting with molecular clouds, the γ-ray emission is π⁰-decay dominated. The model predicts a hadronic origin of γ-ray emission from very old remnants interacting mostly with molecular clouds and a leptonic origin for intermediate-age remnants whose shocks propagate in a low-density environment created by their progenitors via, e.g., strong stellar winds. These results can be regarded as evidence in support of the SNR origin of Galactic cosmic rays.