Evidence for a Neutral Iron Line Generated by MeV Protons from Supernova Remnants Interacting with Molecular Clouds
Koyama, Katsuji; Uchida, Hiroyuki; Nobukawa, Masayoshi; Tanaka, Takaaki; Uchiyama, Hideki; Nobukawa, Kumiko K.; Yamauchi, Shigeo; Tsuru, Takeshi G.; Okon, Hiromichi
Japan
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 Kα) 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 6^\circ ≲ l≲ 40^\circ ,| b| < 1^\circ region. All of these SNRs interact with molecular clouds. We discover Fe I Kα line emissions from five SNRs (W28, Kes 67, Kes 69, Kes 78, and W44). The spectra and morphologies suggest that the Fe I Kα 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.