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

Abstract

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 vFv 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. This mechanism reinforces recent claims of a similar spectral modification by magnetic field amplification around a molecular cloud by Inoue et al.(2012) and Celli et al.(2018).

8 pages, 6 figures, accepted for publication in ApJ

References in corpus (6)

• Detection of the Characteristic Pion-Decay Signature in Supernova Remnants
• Parameterization of Gamma, e^+/- and Neutrino Spectra Produced by p-p Interaction in Astronomical Environment
• Observations of the young supernova remnant RX J1713.7-3946 with the Fermi Large Area Telescope
• Hadronic gamma-rays from RX J1713.7-3946?
• A cosmic ray current driven instability in partially ionised media
• The Role of Ambipolar Diffusion in the Formation Process of Moderately Magnetized Diffuse Clouds

Cited by in corpus (5)

• TeV cosmic ray nuclei acceleration in shell-type supernova remnants with hard $\gamma$-ray spectra
• ALMA CO Observations of the Gamma-Ray Supernova Remnant RX J1713.7-3946: Discovery of Shocked Molecular Cloudlets and Filaments at 0.01 pc scales
• The interstellar medium in young supernova remnants: key to the production of cosmic X-rays and $\gamma$-rays
• Particle acceleration in interstellar shocks
• Cosmic-ray current-driven instabilities -- revisiting environmental conditions