Discovery of a 6.4 keV emission line in the supernova remnant IC 443: fluorescence induced by low-energy cosmic rays?

IC 443 is a middle-aged supernova remnant (SNR) in our galaxy. A gamma-ray spectrum of IC 443 shows a pion-decay signature, which means that cosmic-ray protons are accelerated in the SNR (Ackermann et al. 2013, Sci, 339, 807). Since pion production requires cosmic-ray protons with > 280 MeV, gamma-ray observations cannot constrain low-energy cosmic rays (LECRs). Observations of H_3^+ absorption lines revealed that the ionization rate of H_2 is large in IC 443 (Indriolo et al. 2010, ApJ, 724, 1357). This suggests that a large amount of LECRs are produced. However, H_3^+ observations cannot provide the information on energy density of LECRs. When LECRs collide with the surrounding interstellar material, they ionize the neutral iron in it and emit a fluorescent X-ray at 6.4 keV (Tatischeff et al. 2012, A&A, 546, 24). Since IC 443 interacts with molecular clouds, accelerated particles can collide with them to produce the 6.4 keV line. Analyzing the Suzaku archive data of IC 443, we discovered the 6.4 keV line with a significance level of ≥ 3 σ. We measured the spatial intensity distribution of the 6.4 keV line and found that the high-intensity region contains the site where the large ionization rate was observed. The 6.4 keV line would be produced by LECRs with the energy density of 10 - 100 eV/cc for protons and 0.01 - 0.1 eV/cc for electrons.