Semi-analytic Models for Electron Acceleration in Weak ICM Shocks

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.