Non-thermal neutral iron line emission in the supernova remnants G304.6+0.1 and G346.6-0.2 interacting with molecular clouds

Abstract The non-thermal neutral iron line can be a probe of low-energy cosmic rays; when low-energy cosmic rays ionize iron atoms in the interstellar medium, the atoms would emit the non-thermal neutral iron line. In this paper, we investigated the non-thermal neutral iron line in a couple of supernova remnants (SNRs), G304.6$+$0.1 and G346.6-0.2, which are known to be associated with molecular clouds. Analyzing the Suzaku archive data, we discovered enhancements of the neutral iron line with a significance level of $\sim \! 3\sigma$ in the two SNRs. In addition, we conducted a detailed study of CO clouds associated with G304.6$+$0.1 to examine in detail the evidence for the shock–cloud interaction and found cavity-like structures in the position–velocity diagram of CO not only in a velocity range of $-31$ to $-14$ km s$^{-1}$, which corresponds to the interacting cloud reported in a previous study, but also in a velocity range of $-42$ to $-30$ km s$^{-1}$. The enhancements of the neutral iron line of both SNRs are spatially associated with the SNR shells and the CO clouds that interact with the SNRs. There are no irradiating bright X-ray sources around the SNRs, and the photoionization cannot explain the neutral iron line intensities. The most plausible origin of the enhancements is the bombardment of low-energy cosmic rays, which were accelerated by the SNR shells and directly injected into the molecular clouds.