More than 90% of the Galactic gas-related gamma-ray emissivity above 1 GeV is attributed to the decay of neutral pions formed in collisions between cosmic rays and interstellar matter, with lepton-induced processes becoming increasingly important bel
ow 1 GeV. Given the high-quality measurements of the gamma-ray emissivity of local interstellar gas between ~50 MeV and ~4 GeV obtained with the Large Area Telescope on board the Fermi space observatory, it is timely to re-investigate this topic in detail, including the hadronic production mechanisms. The emissivity spectrum will allow the interstellar cosmic-ray spectrum to be determined reliably, providing a reference for origin and propagation studies as well as input to solar modulation models. A method for such an analysis and illustrative results are presented.
Context: Cosmic rays are thought to be accelerated at supernova remnant (SNR) shocks, but conclusive evidence is lacking. Aims: New data from ground-based gamma-ray telescopes and the Large Area Telescope on the Fermi Gamma-ray Space Telescope are us
ed to test this hypothesis. A simple model for gamma-ray production efficiency is compared with measured gamma-ray luminosities of SNRs, and the GeV to TeV fluxes ratios of SNRs are examined for correlations with SNR ages. Methods: The supernova explosion is modeled as an expanding spherical shell of material that sweeps up matter from the surrounding interstellar medium (ISM). The accumulated kinetic energy of the shell, which provides the energy available for nonthermal particle acceleration, changes when matter is swept up from the ISM and the SNR shell decelerates. A fraction of this energy is assumed to be converted into the energy of cosmic-ray electrons or protons. Three different particle radiation processes---nuclear pion-production interactions, nonthermal electron bremsstrahlung, and Compton scattering---are considered. Results: The efficiencies for gamma-ray production by these three processes are compared with gamma-ray luminosities of SNRs. Our results suggest that SNRs become less gamma-ray luminous at >~ 10^4 yr, and are consistent with the hypothesis that supernova remnants accelerate cosmic rays with an efficiency of ~10% for the dissipation of kinetic energy into nonthermal cosmic rays. Weak evidence for an increasing GeV to TeV flux ratio with SNR age is found.
