Shock-Cloud Interaction in Gamma-Ray Supernova Remnants:
Evidence for Cosmic-Ray Acceleration
Speaker: Hidetoshi Sano (NAOJ)
The origin of cosmic rays, consisting of mainly protons, has been a long-standing issue in modern astrophysics since their discovery by V. Hess in 1912. Supernova remnants (SNRs) are promising candidates for acceleration sites of Galactic cosmic rays with an energy up to ~3 × 1015 eV. However, the principal acceleration sites of cosmic rays are still being debated due to a lack of observational evidence. We argue that investigating the interstellar gas associated with gamma-ray SNRs holds a key to understand the origin of cosmic rays, because the interstellar gas act as a target for cosmic-ray protons producing hadronic gamma-rays via neutral pion decay. We revealed good spatial correspondence between the interstellar gas and gamma-rays for dozens of SNRs. This provides one of the essential conditions for gamma-rays to be predominantly of hadronic origin, because in such a case, the gamma-ray flux is proportional to the target-gas density. It is also noteworthy that shock-cloud interaction also suppresses the gamma-rays from cosmic-ray electrons through the magnetic field amplification. In this talk, I will introduce our ISM studies of gamma-ray SNRs in the past ten years, as well as ongoing or future research beyond the previous studies.