On the acceleration of cosmic rays at the post-adiabatic shocks of supernova remnants

TL;DR

This paper investigates how the post-adiabatic phase of supernova remnants enhances cosmic ray acceleration, leading to harder spectra and higher maximum particle energies, supported by observational radio data.

Contribution

It analyzes the effects of post-adiabatic shock dynamics on cosmic ray acceleration, revealing increased efficiency and spectral deviations from classical models.

Findings

Acceleration efficiency increases in the post-adiabatic phase.

Relativistic particle spectra harden at high energies.

Observational radio evidence supports the theoretical predictions.

Abstract

When a supernova remnant (SNR) interacts with the dense material of an interstellar cloud, its shock wave decelerates rapidly, and the post-shock temperature drops to levels that permit efficient cooling of the shocked plasma. At this stage, the shock enters the post-adiabatic phase of its evolution. During this phase, the internal structure of the SNR undergoes significant changes, particularly in the immediate post-shock region, at spatial scales relevant to cosmic ray acceleration. Once the shock enters the post-adiabatic regime, the efficiency of diffusive shock acceleration increases due to a higher plasma compression, to a change in the direction of the advection velocity, and to an increased rate of momentum gain. As a result, the momentum spectrum of relativistic particles hardens, deviating from a pure power law at high energies. Particles could reach higher maximum values compared to classical predictions. We highlight the dynamics of post-adiabatic flows in SNRs, study their impact on particle acceleration, and present supporting observational evidence in the radio band.