Semi-analytic Models for Electron Acceleration in Weak ICM Shocks

TL;DR

This paper develops semi-analytic models for electron acceleration in weak intracluster medium shocks, examining in situ acceleration and re-acceleration, and their implications for radio emission observed in galaxy clusters.

Contribution

It introduces a unified semi-analytic framework for modeling electron acceleration via DSA, incorporating both in situ and re-acceleration processes in weak shocks.

Findings

Radio emission amplitude depends strongly on Mach number in in situ acceleration.

Re-acceleration model shows weak dependence on Mach number, aligning better with observed smooth radio relics.

In situ acceleration may be inconsistent with observed relic surface brightness due to its Mach number dependence.

Abstract

We propose semi-analytic models for the electron momentum distribution in weak shocks that accounts for both in situ acceleration and re-acceleration through diffusive shock acceleration (DSA). In the former case, a small fraction of incoming electrons are assumed to be reflected at the shock ramp and pre-accelerated to the so-called injection momentum, $p_{\rm inj}$, above which particles can diffuse across the shock transition and participate in the DSA process. This leads to the DSA power-law distribution extending from the smallest momentum of reflected electrons, $p_{\rm ref}$, all the way to the cutoff momentum, $p_{\rm eq}$, constrained by radiative cooling. In the later case, fossil electrons, specified by a power-law spectrum with a cutoff, are assumed to be re-accelerated also from $p_{\rm ref}$ up to $p_{\rm eq}$ via DSA. We then show that, in the in situ acceleration model, the amplitude of radio synchrotron emission depends strongly on the shock Mach number, whereas it varies rather weakly in the re-acceleration model. Considering rather turbulent nature of shocks in the intracluster medium, such extreme dependence for the in situ acceleration might not be compatible with relatively smooth surface brightness of observed radio relics.