A Diffusive Shock Acceleration Model for Protons in Weak Quasi-parallel Intracluster Shocks

Abstract

Low sonic Mach number shocks form in the intracluster medium (ICM) during the formation of the large-scale structure of the universe. Nonthermal cosmic-ray (CR) protons are expected to be accelerated via diffusive shock acceleration (DSA) in those ICM shocks, although observational evidence for the $γ$-ray emission of hadronic origin from galaxy clusters has yet to be established. Considering the results obtained from recent plasma simulations, we improve the analytic test-particle DSA model for weak quasi-parallel ($Q_\parallel$) shocks, previously suggested by \citet{kang2010}. In the model CR spectrum, the transition from the postshock thermal to CR populations occurs at the injection momentum, $p_{\rm inj}$, above which protons can undergo the full DSA process. As the shock energy is transferred to CR protons, the postshock gas temperature should decrease accordingly and the subshock strength weakens due to the dynamical feed of the CR pressure to the shock structure. This results in the reduction of the injection fraction, although the postshock CR pressure approaches an asymptotic value when the CR spectrum extends to the relativistic regime. Our new DSA model self-consistently accounts for such behaviors and adopts better estimations for $p_{\rm inj}$. With our model DSA spectrum, the CR acceleration efficiency ranges $η\sim10^{-3}-0.01$ for supercritical, $Q_\parallel$-shocks with sonic Mach number $2.25\lesssim M_{\rm s}\lesssim5$ in the ICM. Based on \citet{ha2018b}, on the other hand, we argue that proton acceleration would be negligible in subcritical shocks with $M_{\rm s}<2.25$.