Mirror diffusion of cosmic rays in highly compressible turbulence near supernova remnants

Abstract: Recent gamma-ray observations reveal inhomogeneous diffusion of cosmic rays (CRs) in the interstellar medium (ISM). This is expected as the diffusion of CRs depends on the properties of turbulence, which can vary widely in the multi-phase ISM. We focus on the mirror diffusion arising in highly compressible turbulence in molecular clouds (MCs) around supernova remnants (SNRs), where the magnetic mirroring effect results in significant suppression of diffusion of CRs near CR sources. Significant energy loss via proton-proton interactions due to slow diffusion flattens the low-energy CR spectrum, while the high-energy CR spectrum is steepened due to the strong dependence of mirror diffusion on CR energy. The resulting broken power law spectrum of CRs matches well the gamma-ray spectrum observed from SNR/MC systems, e.g., IC443 and W44.