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Intrusion of MeV–TeV Cosmic Rays into Molecular Clouds Studied by Ionization, the Neutral Iron Line, and Gamma Rays

Low-energy (∼ MeV) cosmic rays (CRs) ionize molecular clouds and create the neutral iron line (Fe i Kα) at 6.4 keV. On the other hand, high-energy (≳GeV) CRs interact with the dense cloud gas and produce gamma rays. Based on a one-dimensional model, we study the spatial correlation among ionization rates of gas, 6.4 keV line fluxes, and gamma-ray emissions from a molecular cloud illuminated by CRs accelerated at an adjacent supernova remnant. We find that the spatial distributions of these three observables depend on how CRs intrude the cloud and on the internal structure of the cloud. If the intrusion is represented by slow diffusion, the 6.4 keV line should be detected around the cloud edge where ionization rates are high. On the other hand, if CRs freely stream in the cloud, the 6.4 keV line should be observed where gamma rays are emitted. In the former, the cooling time of the CRs responsible for the 6.4 keV line is shorter than their cloud crossing time, and it is opposite in the latter. Although we compare the results with observations, we cannot conclude whether the diffusion or the free-streaming is dominantly realized. Our predictions can be checked in more detail with future X-ray missions such as XRISM and Athena and by observations of ionization rates that cover wider fields.