Diffusive Shock Acceleration Simulations of Radio Relics

TL;DR

This paper presents time-dependent simulations of diffusive shock acceleration at galaxy cluster shocks, comparing results with observed radio relics to understand electron acceleration and the role of pre-existing cosmic-ray electrons.

Contribution

It introduces detailed simulations of electron acceleration at weak shocks in galaxy clusters, including energy losses and pre-existing electrons, to interpret radio relic observations.

Findings

Radio relics may indicate pre-existing low-energy cosmic-ray electrons.

Simulations match observed radio flux and spectra of specific relics.

Weak shocks with Mach number less than a few can produce observable relics.

Abstract

Recent radio observations have identified a class of structures, so-called radio relics, in clusters of galaxies. The radio emission from these sources is interpreted as synchrotron radiation from GeV electrons gyrating in microG-level magnetic fields. Radio relics, located mostly in the outskirts of clusters, seem to associate with shock waves, especially those developed during mergers. In fact, they seem to be good structures to identify and probe such shocks in intracluster media (ICMs), provided we understand the electron acceleration and re-acceleration at those shocks. In this paper, we describe time-dependent simulations for diffusive shock acceleration at weak shocks that are expected to be found in ICMs. Freshly injected as well as pre-existing populations of cosmic-ray (CR) electrons are considered, and energy losses via synchrotron and inverse Compton are included. We then compare the synchrotron flux and spectral distributions estimated from the simulations with those in two well-observed radio relics in CIZA J2242.8+5301 and ZwCl0008.8+5215. Considering that the CR electron injection is rather inefficient at weak shocks with Mach number M <~ a few, the existence of radio relics could indicate the pre-existing population of low-energy CR electrons in ICMs. The implication of our results on the merger shock scenario of radio relics is discussed.