Re-acceleration of Cosmic Ray Electrons by Multiple ICM Shocks

TL;DR

This paper investigates how multiple shock re-acceleration in galaxy clusters enhances cosmic ray electron acceleration and affects observed radio relic properties, explaining discrepancies in shock Mach number estimates.

Contribution

It introduces a detailed model of CR electron re-acceleration considering energy losses and magnetic field decompression, highlighting the significance of multiple shocks in radio relic formation.

Findings

Repeated shock re-acceleration significantly boosts CR electron spectra.

Enhanced and flatter radiation spectra are produced behind the last shock.

Shock Mach number estimates from radio data can overestimate the actual Mach number.

Abstract

Radio relics could be generated by multiple shocks induced in the turbulent intracluster medium during galaxy mergers. Kang (2021) demonstrated that the re-acceleration of cosmic ray (CR) protons via diffusive shock acceleration (DSA) by multiple shocks could enhance the acceleration efficiency and flatten the CR spectrum, compared to a single episode of DSA. Here we examine the CR electron acceleration through multiple re-acceleration by considering energy losses and decompression of the particle distribution and magnetic fields in the postshock region between consecutive shock passages. We find that the accumulated effects of repeated re-acceleration are significant, if preceding shocks are stronger than the last shock and the shock passage interval is $\lesssim20$ Myr. In such cases, both the CR spectrum and the ensuing radiation spectrum behind the last shock are enhanced and become flatter than the canonical DSA power-law forms. As a result, the shock Mach number estimated from radio observations tends be higher than the actual Mach number of the last shock. Thus, multiple episodes of DSA may explain the enhanced acceleration efficiency for CR electrons and the discrepancy of shock Mach numbers, $M_{\rm X} \lesssim M_{\rm rad}$, inferred for some observed radio relics.