Diffusion of Cosmic Rays in a Multiphase Interstellar Medium Swept-Up by a Supernova Remnant Blast Wave

TL;DR

This study uses 3D MHD simulations to analyze cosmic ray diffusion in supernova remnants, finding Bohm diffusion dominates and superdiffusive effects are limited at relevant scales.

Contribution

It provides the first detailed numerical analysis of cosmic ray diffusion in a realistic, multiphase interstellar medium swept by a supernova remnant blast wave.

Findings

CR diffusion is well described by Bohm diffusion for particles above 30 TeV.

Superdiffusive behavior (distance ∝ t^0.75) may occur but has limited impact at turbulence scales.

Efficient acceleration of high-energy CRs is supported by Bohm diffusion dominance.

Abstract

Supernova remnants (SNRs) are one of the most energetic astrophysical events and are thought to be the dominant source of Galactic cosmic rays (CRs). A recent report on observations from the Fermi satellite has shown a signature of pion decay in the gamma-ray spectra of SNRs. This provides strong evidence that high-energy protons are accelerated in SNRs. The actual gamma-ray emission from pion decay should depend on the diffusion of CRs in the interstellar medium. In order to quantitatively analyse the diffusion of high-energy CRs from acceleration sites, we have performed test particle numerical simulations of CR protons using a three-dimensional magnetohydrodynamics (MHD) simulation of an interstellar medium swept-up by a blast wave. We analyse the diffusion of CRs at a length scale of order a few pc in our simulated SNR, and find the diffusion of CRs is precisely described by a Bohm diffusion, which is required for efficient acceleration at least for particles with energies above 30 TeV for a realistic interstellar medium. Although we find the possibility of a superdiffusive process (travel distance proportional to t^0.75) in our simulations, its effect on CR diffusion at the length scale of the turbulence in the SNR is limited.