Slow Diffusion of Cosmic-Rays around a Supernova Remnant

TL;DR

This study uses Monte-Carlo simulations to show that cosmic-ray protons near supernova remnants are confined longer than expected due to wave interactions, affecting their escape and observational signatures.

Contribution

It demonstrates that cosmic-ray scattering by Alfven waves near SNRs delays their escape, a novel insight supported by simulation and consistent with recent observations.

Findings

Cosmic-rays form a slowly expanding bubble around SNRs.

Wave scattering prevents early escape of cosmic-rays.

Cosmic-ray escape occurs at a late stage of SNR evolution.

Abstract

We study the escape of cosmic-ray protons accelerated at a supernova remnant (SNR). We are interested in their propagation in interstellar medium (ISM) after they leave the shock neighborhood where they are accelerated, but when they are still near the SNR with their energy density higher than that in the average ISM. Using Monte-Carlo simulations, we found that the cosmic-rays with energies of <~TeV excite Alfven waves around the SNR on a scale of the SNR itself if the ISM is highly ionized. Thus, even if the cosmic-rays can leave the shock, scattering by the waves prevents them from moving further away from the SNR. The cosmic-rays form a slowly expanding cosmic-ray bubble, and they spend a long time around the SNR. This means that the cosmic-rays cannot actually escape from the SNR until a fairly late stage of the SNR evolution. This is consistent with some results of Fermi and H.E.S.S. observations.