Molecular Clouds as a Probe of Cosmic-Ray Acceleration in a Supernova Remnant

TL;DR

This paper models cosmic-ray acceleration and escape in supernova remnants, using molecular cloud gamma-ray spectra to infer properties of particle acceleration and diffusion.

Contribution

It introduces a detailed model of cosmic-ray acceleration in SNRs within low-density cavities and compares predictions with gamma-ray observations.

Findings

Cosmic-ray diffusion coefficient is less than 1% of typical values.

Particle spectral index is approximately 2.3.

Particles likely accelerated at the end of the Sedov phase.

Abstract

We study cosmic-ray acceleration in a supernova remnant (SNR) and the escape from it. We model nonthermal particle and photon spectra for the hidden SNR in the open cluster Westerlund 2, and the old-age mixed-morphology SNR W 28. We assume that the SNR shock propagates in a low-density cavity, which is created and heated through the activities of the progenitor stars and/or previous supernova explosions. We indicate that the diffusion coefficient for cosmic-rays around the SNRs is less than ~1% of that away from them. We compare our predictions with the gamma-ray spectra of molecular clouds illuminated by the cosmic-rays (Fermi and H.E.S.S.). We found that the spectral indices of the particles are ~2.3. This may be because the particles were accelerated at the end of the Sedov phase, and because energy dependent escape and propagation of particles did not much affect the spectrum.