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$\alpha$) at 6.40keV. We search for the iron K-shell line in seven SNRs from the Suzaku archive data of the Galactic plane in the $6^{\circ }\lesssim l\lesssim 40^{\circ },|b|<1^{\circ }$ region. All these SNRs interact with molecular clouds. We discover Fe I K$\alpha$ line emissions from five SNRs (W28, Kes 67, Kes 69, Kes 78, and W44). The spectra and morphologies suggest that the Fe I K$\alpha$ line is produced by interactions between LECRp and the adjacent cold gas. The proton energy density is estimated to be $\gtrsim$ 10-100 eV cm${}^{-3}$, which is more than 10 times higher than that in the ambient interstellar medium.

10 pages, 5 figures, 3 tables. Accepted for publication in The Astrophysical Journal

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