Low-energy Cosmic Rays in Supernova Remnants Interacting with Molecular Clouds

Abstract

Information on low-energy cosmic rays (CRs) below the MeV band has been very limited due to difficulty of measurements in contrast to high-energy (GeV–TeV) CRs, which are observable via gamma-rays. Only ionization rate provides indirect information on low-energy CRs. The neutral iron line at 6.4 keV can be a new effective probe. Utilizing the Suzaku archive data, we searched supernova remnants (SNRs) for the particle-induced 6.4 keV line. Since accelerated CR particles can produce the 6.4 keV line when they interact with molecular clouds, we focused on SNRs interacting with molecular clouds. The 6.4 keV line probably induced by CR particles has been found from more than 10 SNRs. More than a half of the SNRs also show gamma-ray emission. We found that both the 6.4 keV line and gamma-ray emissions can be explained by a CR escaping scenario at least for W28, W44, and IC 443. It indicates that low-energy CRs producing the 6.4 keV line have been accelerated at the SNRs together with high-energy CRs.