On cosmic-ray production efficiency at supernova remnant shocks propagating into realistic diffuse interstellar medium

TL;DR

This study uses 3D magnetohydrodynamics simulations to show that cosmic-ray production efficiency at supernova remnant shocks is overestimated when using traditional methods, due to shock rippling caused by upstream density fluctuations.

Contribution

It demonstrates that shock rippling affects the estimation of cosmic-ray efficiency, highlighting the need to account for shock obliqueness and turbulence in models.

Findings

CR efficiency overestimated by 10-40% using traditional methods

Shock rippling causes energy transfer to turbulence and thermal energy

Upstream density fluctuations significantly impact shock dynamics

Abstract

Using three-dimensional magnetohydrodynamics simulations, we show that the efficiency of cosmic-ray (CR) production at supernova remnants (SNRs) is over-predicted if it could be estimated based on proper motion measurements of H$\alpha$ filaments in combination with shock-jump conditions. Density fluctuations of upstream medium make shock waves rippled and oblique almost everywhere. The kinetic energy of the shock wave is transferred into that of downstream turbulence as well as thermal energy which is related to the shock velocity component normal to the shock surface. Our synthetic observation shows that the CR acceleration efficiency as estimated from a lower downstream plasma temperature, is overestimated by 10-40%, because rippled shock does not immediately dissipate all upstream kinetic energy.