Observations of supernova remnants and molecular clouds from the mm to the gamma-ray domain: bridging low and high energy cosmic rays

TL;DR

Recent multi-wavelength observations from gamma-ray to submillimeter domains provide strong evidence that supernova remnants accelerate cosmic rays across a broad energy spectrum, especially when interacting with molecular clouds.

Contribution

This paper synthesizes recent observational data and physical processes to support the paradigm of supernova remnants as cosmic ray accelerators from MeV to TeV energies.

Findings

Enhanced gamma-ray emission from neutral pion decay observed.

Increased ionization levels in molecular clouds near supernova remnants.

Correlations between supernova remnants and cosmic ray acceleration evidence.

Abstract

New evidence that cosmic rays (hadronic component) are accelerated by supernova remnant shocks all the way from low energies to high energies, has come from recent works combining gamma-ray observations in the sub-GeV to TeV domain on the one hand, and in the submm-mm domain on the other hand. These observations concern the specific cases of supernova remnants interacting with molecular cloud complexes, that have long been suspected to be ideal laboratories to study in situ cosmic ray acceleration and diffusion. Indeed, enhanced gamma-ray emission from neutral pion decay, as well as enhanced ionization (both by at least one order of magnitude with respect to average galactic values) have been observed in several regions of massive star formation housing supernova remnants interacting with molecular cloud complexes. This paper summarizes the main physical and chemical processes at work, as well as recent observations, that further support the paradigm of cosmic ray acceleration by supernova remnants all the way from the MeV domain up to several tens of TeV, although much work remains to be done to understand cosmic ray penetration and diffusion inside and around molecular clouds, and reveal the actual spectrum of the accelerated cosmic rays.