A multishock scenario for the formation of radio relics

TL;DR

This study uses magneto-hydrodynamic simulations and a Fokker-Planck approach to show that re-acceleration of fossil electrons, especially through multiple shocks, is crucial for explaining radio relic emissions in galaxy clusters, surpassing traditional DSA models.

Contribution

The paper introduces a combined simulation and spectral modeling approach demonstrating the importance of fossil electron re-acceleration and multiple shocks in radio relic formation, challenging previous DSA-based assumptions.

Findings

Re-acceleration of fossil electrons is key to relic emission.

Multiple shocks significantly boost relic luminosity.

DSA of thermal electrons overestimates acceleration efficiency.

Abstract

Radio relics are giant sources of diffuse synchrotron radio emission in the outskirts of galaxy clusters that are associated with shocks in the intracluster medium. Still, the origin of relativistic particles that make up relics is not fully understood. For most relics, diffusive shock acceleration (DSA) of thermal electrons is not efficient enough to explain observed radio fluxes. In this paper, we use a magneto-hydrodynamic simulation of galaxy clusters in combination with Lagrangian tracers to simulate the formation of radio relics. Using a Fokker-Planck solver to compute the energy spectra of relativistic electrons, we determine the synchrotron emission of the relic. We find that re-acceleration of fossil electrons plays a major role in explaining the synchrotron emission of radio relics. Particles that pass through multiple shocks contribute significantly to the overall luminosity of a radio relic and greatly boosts the effective acceleration efficiency. Furthermore, we find that the assumption that the luminosity of a radio relic can be explained with DSA of thermal electrons leads to an overestimate of the acceleration efficiency by a factor of more than $10^3$.