Effects of Multi-scale Plasma Waves on Electron Preacceleration at Weak Quasi-perpendicular Intracluster Shocks

Radio relics associated with merging galaxy clusters indicate the acceleration of relativistic electrons in merger-driven shocks with low sonic Mach numbers ($M_{\rm s}\lesssim 3$) in the intracluster medium (ICM). Recent studies have suggested that electron injection to diffusive shock acceleration (DSA) could take place through the so-called Fermi-like acceleration in the shock foot of $\beta=P_{\rm gas}/P_{\rm B}\approx 20-100$ shocks and the stochastic shock drift acceleration (SSDA) in the shock transition of $\beta\approx 1-5$ shocks. Here we explore how the SSDA can facilitate electron preacceleration in weak quasi-perpendicular ($Q_{\perp}$) shocks in $\beta\approx 20-100$ plasmas by performing particle-in-cell simulations in the two-dimensional domain large enough to encompass ion-scale waves. We find that in supercritical shocks with $M_{\rm s}\gtrsim M_{\rm AIC}^*\sim 2.3$, multi-scale waves are excited by the ion and electron temperature anisotropies in the downstream of the shock ramp, and that through stochastic pitch-angle scattering off the induced waves, electrons are confined in the shock transition for an extended period. Gaining energy through the gradient-drift along the motional electric field, electrons could be preaccelerated all the way to injection to DSA at such ICM shocks. Our findings imply that the electron DSA process at weak ICM shocks could explain the origin of radio relics. However, a further investigation of electron acceleration at subcritical shocks with $M_{\rm s}< 2.3$ is called for, since the Mach numbers of some observed radio relic shocks derived from radio or X-ray observations are as low as $M_{\rm s}\sim 1.5$.
Comment: 17 pages, 9 figures, to appear in ApJ