Cosmic Ray streaming from SNRs and gamma ray emission from nearby molecular clouds

TL;DR

This paper models cosmic ray acceleration and gamma ray emission near supernova remnants, considering streaming instability and turbulence, to explain observed gamma ray spectra from nearby molecular clouds.

Contribution

It presents a self-consistent model of cosmic ray acceleration and gamma ray production near SNRs, incorporating streaming instability and turbulence effects.

Findings

Cosmic rays can produce gamma rays consistent with observations.

The energy spectrum of gamma rays matches the observed data.

The model confirms the role of streaming instability in cosmic ray acceleration.

Abstract

High-energy gamma ray emission has been detected recently from supernovae remnants (SNRs) and their surroundings. The existence of molecular clouds near some of the SNRs suggests that the gamma rays originate predominantly from p-p interactions with cosmic rays accelerated at a closeby SNR shock wave. Here we investigate the acceleration of cosmic rays and the gamma ray production in the cloud self-consistently by taking into account the interactions of the streaming instability and the background turbulence both at the shock front and in the ensuing propagation to the clouds. We focus on the later evolution of SNRs, when the conventional treatment of the streaming instability is valid but the magnetic field is enhanced due to either Bell's current instability and/or due to the dynamo generation of magnetic field in the precursor region. We calculate the time dependence of the maximum energy of the accelerated particles. This result is then used to determine the diffusive flux of the runaway particles escaping the shock region, from which we obtain the gamma spectrum consistent with observations. Finally, we check the self-consistency of our results by comparing the required level of diffusion with the level of the streaming instability attainable in the presence of turbulence damping. The energy range of cosmic rays subject to the streaming instability is able to produce the observed energy spectrum of gamma rays.