Hybrid Simulations of Proton Acceleration at Oblique High-$\beta$ Shocks

Abstract

Collisionless shocks in the intracluster and intergalactic medium (ICM/IGM) are expected to energize both electrons and ions. While electron acceleration is revealed by prominent radio emission, $\gamma$-ray emission from hadronic interactions remains undetected, suggesting that high-$\beta$ (ratio of thermal to magnetic pressure), low-Mach-number shocks cannot accelerate protons efficiently. We present three-dimensional hybrid simulations, in which ions are treated kinetically and electrons as a fluid, of quasi-perpendicular (magnetic obliquity $\vartheta = 80^\circ$) shocks with sonic Mach numbers $M_s \sim 3{-}15$ and plasma $\beta \gtrsim 15$, representative of cluster environments. We find that weak shocks ($M_s \lesssim 5$) fail to develop significant nonthermal populations, with cosmic ray (CR) acceleration efficiencies $\varepsilon_{\rm CR} \lesssim 0.1\%$. In contrast, stronger shocks ($M_s \gtrsim 10$) develop clear power-law tails with slopes $q \sim 4.0 $ and reach $\varepsilon_{\rm CR} \sim 3\%$. These results suggest that weak, oblique ICM shocks are generally unlikely to accelerate protons efficiently. However, reducing $\vartheta$ to $\sim 45^\circ$ leads to substantially higher acceleration efficiencies, indicating that magnetic obliquity plays a critical role in determining proton acceleration. Our findings provide a microphysical framework for interpreting radio relic observations, whose polarization suggests that electrons are accelerated at oblique shocks, and the absence of cluster $\gamma$-ray detections.