Evidence for a neutral iron line generated by MeV protons from supernova remnants interacting with molecular clouds

TL;DR

This study detects neutral iron Kα line emissions in five supernova remnants interacting with molecular clouds, indicating the presence of low-energy cosmic-ray protons with high energy density, supporting their role as cosmic-ray accelerators.

Contribution

First detection of Fe I Kα line emissions in multiple SNRs, linking them to LECRp interactions with molecular clouds, and estimating high proton energy densities.

Findings

Fe I Kα line detected in 5 SNRs

Proton energy density estimated to be >10-100 eV/cm³

Line emissions linked to LECRp interactions with cold gas

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.