Broad–band non–thermal emission from molecular clouds illuminated by cosmic rays from nearby supernova remnants

• Published in: Monthly notices of the Royal Astronomical Society Vol. 396; no. 3; pp. 1629 - 1639
• Main Authors: Gabici, S., Aharonian, F. A., Casanova, S.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.07.2009; Wiley-Blackwell; Oxford University Press
• Subjects: Astronomy; Cosmic rays; Earth, ocean, space; Exact sciences and technology; Gamma rays; ISM: clouds; Radiation; radiation mechanisms: non‐thermal; supernova remnants; Supernovae; radiation mechanisms: non-thermal; ISM: clouds; cosmic rays; supernova remnants; Gamma emission; Total energy; Molecular clouds; Gamma spectra; Cosmic ray interactions; Cosmic radiation; Nonthermal radiation; TeV range; Radiation mechanism; Supernova remnants; Gamma radiation; Gamma ray source; Dense gas; Relativistic particle; Compatibility; Radio spectrum; Interstellar matter
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2009.14832.x

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 a 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 exhibits 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.