Broad-band nonthermal emission from molecular clouds illuminated by cosmic rays from nearby supernova remnants

TL;DR

This paper models the non-thermal emission from molecular clouds near supernova remnants, showing that cosmic ray interactions can produce gamma rays that dominate the emission spectrum, potentially explaining some unidentified TeV sources.

Contribution

It provides a detailed calculation of multiwavelength emission from clouds near cosmic ray accelerators, highlighting the significance of gamma rays and spectral features relevant for source identification.

Findings

Gamma-ray emission can surpass other wavelengths by over an order of magnitude.

Unidentified TeV sources may be clouds illuminated by nearby cosmic ray sources.

Gamma-ray spectra can be concave, affecting spectral analysis of sources.

Abstract

Molecular clouds are expected to emit non-thermal radiation due to cosmic ray interactions in the dense magnetized gas. Such emission is amplified if a cloud is located close to an accelerator of cosmic rays and if energetic particles can leave the accelerator site and diffusively reach the cloud. We consider here the situation in which a molecular cloud is located in the proximity of a supernova remnant which is efficiently accelerating cosmic rays and gradually releasing them in the interstellar medium. We calculate the multiwavelength spectrum from radio to gamma rays which is emerging from the cloud as the result of cosmic ray interactions. The total energy output is dominated by the gamma ray emission, which can exceed the emission in other bands by an order of magnitude or more. This suggests that some of the unidentified TeV sources detected so far, with no obvious or very weak counterparts in other wavelengths, might be in fact associated with clouds illuminated by cosmic rays coming from a nearby source. Moreover, under certain conditions, the gamma ray spectrum exhibit a concave shape, being steep at low energies and hard at high energies. This fact might have important implications for the studies of the spectral compatibility of GeV and TeV gamma ray sources.