Measurement of low-energy cosmic rays in supernova remnants via neutral iron line

Supernova remnants (SNRs) have been prime candidates for Galactic cosmic-ray accelerators. Gamma-ray observations have revealed that protons and/or electrons are accelerated to energies up to 100 TeV in SNRs. In diffusive shock acceleration, suprathermal particles (low-energy cosmic rays; LECRs) are accelerated to relativistic energy through multiple crossings of the shock front. Thus, the energy spectra and fluxes of the LECRs in SNRs provide a key link to generation of the GeV and TeV cosmic rays in SNRs. However, there has been very few observation of LECRs below the MeV band due to the lack of an effective probe to investigate them. When LECRs collide with interstellar gas, they ionize neutral iron atoms and emit the neutral iron line at 6.4 keV. We have started a campaign to search for the 6.4 keV line in SNRs with the Suzaku archive data. We have already discovered the line emission from more than 10 SNRs [1-3]. The spectra and morphologies suggest that the 6.4 keV line is produced by interactions between LECR protons and the adjacent cold gas. The proton energy density is estimated to be 10-100 eV/cc, which is more than 10 times higher than that in the ambient interstellar medium. In this presentation, we also discuss an association between the 6.4 keV line and gamma rays.[1] Nobukawa et al. 2018, ApJ, 854, 87 [2] Saji et al. 2018, PASJ, in press, [3] S. Saji, Ph.D. thesis, Nagoya University, 2018