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Paper: Measurement of Low-energy Cosmic Rays
Volume: 528, New Horizons in Galactic Center Astronomy and Beyond
Page: 415
Authors: Nobukawa, K. K.
Abstract: There has been little information of Galactic low-energy cosmic rays (CRs) so far because observations of them in the solar system are affected by solar modulation and there is no effective way of indirect measurement. When low-energy CRs in the MeV energy band collide with neutral iron atoms in the interstellar medium, fluorescent X-rays at 6.4 keV are produced via inner-shell ionization. The particle-induced neutral iron line is discovered from more than 10 supernova remnants (SNRs). Some of the SNRs have gamma-ray emission. At least for three SNRs, both the neutral iron line and gamma-ray emissions can be explained by a standard acceleration model and a CR escaping scenario. Also the Galactic ridge is searched for the particle-induced line. The Galactic ridge near the giant molecular clouds Bania's Clump 2 shows enhancement of the neutral iron line emission. Its distribution and spectrum indicate that the iron line originates from interaction between MeV protons and molecular clouds. Since the diffusion length of MeV protons is short, they should be produced in situ. However, no SNR has been reported around the region. Other mechanism such as stochastic acceleration may occur. As for the Galactic center, it is too difficult to resolve the iron line emission into photoionization and particle-induced emission in practice at this moment. The X-ray astronomy satellite XRISM can detect structure characteristic of ionization by CRs, and thus we will truly distinguish the particle-induced line for the first time in the Galactic center.
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