Gamma rays from molecular clouds illuminated by cosmic rays escaping from interacting supernova remnants

TL;DR

This paper provides an analytical model explaining gamma-ray spectra from molecular clouds illuminated by cosmic rays escaping supernova remnants, supporting the idea that SNRs are the main sources of Galactic cosmic rays.

Contribution

It introduces an analytical solution for cosmic ray distribution from finite regions, explaining observed spectral features and the typical gamma-ray break energy in middle-aged SNRs.

Findings

CR spectra from different SNRs can be similar despite spectral differences

The break energy of gamma-ray spectra is explained by stellar wind dynamics

Supports SNRs as primary sources of Galactic cosmic rays

Abstract

Recently, gamma-ray telescopes AGILE and Fermi observed several middle-aged supernova remnants (SNRs) interacting with molecular clouds. It is likely that their gamma rays arise from the decay of neutral pions produced by the inelastic collision between cosmic rays (CRs) and nucleons, which suggests that SNRs make the bulk of Galactic CRs. In this letter, we provide the analytical solution of the distribution of CRs that have escaped from a finite-size region, which naturally explains observed broken power-law spectra of the middle-aged SNRs. In addition, the typical value of the break energy of the gamma-ray spectrum, 1-10 GeV, is naturally explained from the fact that the stellar wind dynamics shows the separation between the molecular clouds and the explosion center of about 10 pc. We find that a runaway-CR spectrum of the four middle-aged SNRs (W51C, W28, W44 and IC 443) interacting with molecular clouds could be the same, even though it leads to different gamma-ray spectra. This result is consistent with that of recent studies of the Galactic CR propagation, and supports that SNRs are indeed the sources of Galactic CRs.