Gamma-rays from molecular clouds illuminated by accumulated diffusive protons. II: interacting supernova remnants

TL;DR

This paper refines a diffusion model to explain gamma-ray spectral features in supernova remnants interacting with molecular clouds, revealing suppressed cosmic ray diffusion near these remnants and matching observed GeV and TeV data.

Contribution

The study improves the accumulative diffusion model by including finite cloud volume effects and applies it to multiple SNRs, explaining spectral breaks and platforms with diffusion parameters.

Findings

Diffusion coefficient index  range of 0.5-0.7

Cosmic ray diffusion near SNRs is two orders of magnitude lower than the galactic average

Model successfully explains GeV spectral breaks and TeV data

Abstract

Recent observations reveal that spectral breaks at ~GeV are commonly present in Galactic gamma-ray supernova remnants (SNRs) interacting with molecular clouds and that most of them have a spectral ($E^2dF/dE$) "platform" extended from the break to lower energies. In paper I (Li & Chen 2010), we developed an accumulative diffusion model by considering an accumulation of the diffusive protons escaping from the shock front throughout the history of the SNR expansion. In this paper, we improve the model by incorporating finite-volume of MCs, demonstrate the model dependence on particle diffusion parameters and cloud size, and apply it to nine interacting SNRs (W28, W41, W44, W49B, W51C, Cygnus Loop, IC443, CTB 37A, and G349.7+0.2). This refined model naturally explains the GeV spectral breaks and, especially, the "platform"s, together with available TeV data. We find that the index of the diffusion coeffcient \delta\ is in the range of 0.5-0.7, similar to the galactic averaged value, and the diffusion coefficient for cosmic rays around the SNRs is essentially two orders of magnitude lower than the Galactic average, which is a good indication for the suppression of cosmic ray diffusion near SNRs.