Oral Presentation

Bell-Instability-Mediated Spectral Modulation of Gamma Rays from a Supernova Remnant Interacting with Molecular Cloud

Author(s): Tsuyoshi Inoue (Nagoya University)

Presenter: Inoue Tsuyoshi (Nagoya University)

Supernova remnants (SNRs) are believed to be the site of galactic cosmic ray acceleration. However, the details of the cosmic ray acceleration are still not well understood. Gamma ray observation is a promising method to study cosmic ray acceleration in the SNRs, because a hadronic gamma ray can trace high energy cosmic rays above ∼GeV energy. Conventional theory predicts that the hadronic gamma ray shows a flat energy flux spectrum from the pion-creation threshold energy to the maximum energy of diffusive shock acceleration. In this paper, by employing numerical simulations that solve a hybrid system of the magnetohydrodynamics of a molecular cloud and diffusive propagation of cosmic rays, we demonstrate that the hadronic gamma ray spectrum can be harder than the conventional one and that the modulated spectrum becomes consistent with observations. The modification mechanism is explained as follows: The cosmic rays accelerated at the supernova blast wave shock propagate into a clump of a molecular cloud. The cosmic ray streaming in the cloud induces the so-called Bell instability that induces Alfven waves in the cloud. The induced magnetic field fluctuations prevent further cosmic ray incursion by diminishing the diffusion coefficient for the cosmic rays below ∼ 1 TeV energy.