Stochastic relativistic shock-surfing acceleration

We study relativistic particles undergoing surfing acceleration at perpendicular shocks. We assume that particles undergo diffusion in the component of momentum perpendicular to the shock plane due to moderate fluctuations in the shock electric and magnetic fields. We show that dN/dE, the number of surfing-accelerated particles per unit energy, attains a power-law form, dN/dE \propto E^{-b}. We calculate b analytically in the limit of weak momentum diffusion, and use Monte Carlo test-particle calculations to evaluate b in the weak, moderate, and strong momentum-diffusion limits.