A systematic study on escaping of cosmic rays from SNR shocks through observations of thermal X-ray plasmas

Galactic cosmic rays are thought to originate from supernova remnants (SNRs). However, it is still poorly understood how they escape from SNR shock regions. Here we propose a new idea to measure timescale of proton escaping (Suzuki et al. 2018, PASJ, 70, 75). Recent studies have indicated that most of the SNRs emitting hadronic GeV gamma-rays are interacting with molecular clouds (MCs) (e.g. Ackermann et al. 2013). Interestingly, such SNRs often contain rapidly-cooled plasmas (RCPs: plasmas with unusually low electron temperatures), likely due to thermal conduction into MCs (e.g. Matsumura et al. 2018). Therefore, thermal X-ray properties can be the key to understanding the processes of proton escaping. We conducted a systematic analysis of all the identified RCP SNRs associated with GeV-emission, in order to study proton escaping by utilizing thermal X-ray properties. We discovered a positive correlation between elapsed time after therapid cooling (RCP age) and spectral softness in the GeV energy band (Suzuki et al. 2018), and a negative correlation between the RCP ages and cut-off energies in the GeV spectra. These results imply that the maximum energy of the accelerated protons gradually decreases along with the RCP ages, which is consistent with the scenario that the collision between the SNR shocks and MCs causes both proton escaping and rapid cooling of the plasma. In this analytical model, time evolution of proton escaping was calculated assuming several parameter sets of the shock velocities and environments of an SNR. We discuss the physical parameters that naturally explain the observational correlations.