Particle acceleration at radiative supernova remnant shocks

Abstract

Numerous astrophysical shock waves evolve in an environment where the radiative cooling behind the shock affects the hydrodynamical structure downstream, thereby influencing the potential for particle acceleration via diffusive shock acceleration (DSA). We study the possibility for DSA to energize particles from the thermal pool and from pre-existing cosmic rays at radiative shocks, focusing on the case of supernova remnants (SNRs). We rely on a semi-analytical description of particle acceleration at collisionless shocks in the test-particle limit, estimating the total proton and electron content from SNRs throughout the radiative phase. Our results indicate that DSA can lead to significant particle acceleration during the first few tens of kyrs of the radiative phase. Although the associated multi-wavelength emission from SNRs in the radiative phase may not be detectable with current observatories in most cases, the radiative phase is found to lead to substantial deviations from the canonical p$^{-4}$ of the test-particle limit. The hardening/steepening is due to an interplay between a growing contribution of the reaccelerated term as the SNR volume expands and the effects of adiabatic and radiative losses on trapped particles as particles are confined for a longer time. The slope of the cumulative proton and electron spectra over the SNR lifetime thus depends on the environment in which the SNR shock propagates, and on the duration of the radiative phase during which DSA can take place. Overall, DSA in the radiative phase can lead to a total electron spectrum steeper than the proton spectrum, both at SNRs from thermonuclear and core-collapse SNe. Finally, we comment on the case of young radiative SNRs (in the first month to a few years after the explosion) for which the denser environments (with mass-loss rates of $\dot{M} \sim 10^{-1} - 1$ M$_{\odot}$/yr) tend to inhibit DSA.