Evidence for a Neutral Iron Line Generated by MeV Protons from Supernova Remnants Interacting with Molecular Clouds

Abstract

Supernova remnants (SNRs) have been prime candidates for Galactic cosmic-ray accelerators. When low-energy cosmic-ray protons (LECRp) collide with interstellar gas, they ionize neutral iron atoms and emit the neutral iron line (Fe i K α ) at 6.40 keV. We search for the iron K-shell line in seven SNRs from the Suzaku archive data of the Galactic plane in the region. All of these SNRs interact with molecular clouds. We discover Fe i K α line emissions from five SNRs (W28, Kes 67, Kes 69, Kes 78, and W44). The spectra and morphologies suggest that the Fe i K α line is produced by interactions between LECRp and the adjacent cold gas. The proton energy density is estimated to be ≳10–100 eV cm −3 , which is more than 10 times higher than that in the ambient interstellar medium.