Gamma rays from molecular clouds illuminated by accumulated diffusive protons from supernova remnant W28

TL;DR

This paper models gamma-ray emissions from molecular clouds near supernova remnant W28 by considering accumulated diffusive protons, explaining observed spectral features and differences in gamma-ray brightness.

Contribution

It introduces an accumulative diffusion model for protons from SNR W28 and explains gamma-ray spectra and spectral breaks in associated molecular clouds.

Findings

Successfully fitted gamma-ray spectra of four sources

Explained the GeV spectral break in source N

Accounted for faint GeV emission in sources A and C

Abstract

W28 is one of the archetype supernova remnants (SNRs) interacting with molecular clouds. H.E.S.S. observation found four TeV sources which are coincident with the molecular clouds (MCs) around W28, but Fermi LAT detected no prominent GeV counterparts for two of them. An accumulative diffusion model is established in this Letter and the energetic protons colliding the nearby MCs are considered to be an accumulation of the diffusive protons escaping from the shock front throughout the history of the SNR expansion. We have fitted the gamma ray spectra of the four sources and naturally explained the GeV spectral break of the northeastern source (source N) and the nonsignificant GeV emission of the southern sources A and C. The distances of sources A and C from the SNR centre are found to be much larger than those of sources N and B, which may be the basic reason for the faint GeV gamma rays of the two former sources.