Stochastic Acceleration in Relativistic Parallel Shocks

We present results of test-particle simulations of both the first- and the second-order Fermi acceleration (i.e., stochastic acceleration) at relativistic parallel shock waves. We consider two scenarios for particle injection: (1) particles injected at the shock front, then accelerated at the shock by the first-order mechanism and subsequently by the stochastic process in the downstream region; and (2) particles injected uniformly throughout the downstream region into the stochastic process, mimicking injection from the thermal pool by cascading turbulence. We show that regardless of the injection scenario, depending on the magnetic field strength, plasma composition, and the turbulence model employed, the stochastic mechanism can have considerable effects on the particle spectrum on temporal and spatial scales too short to be resolved in extragalactic jets. Stochastic acceleration is shown to be able to produce spectra that are significantly flatter than the limiting case of N(E)~E-1 of the first-order mechanism. Our study also reveals a possibility of reacceleration of the stochastically accelerated spectrum at the shock, as particles at high energies become more and more mobile as their mean free path increases with energy. Our findings suggest that the role of the second-order mechanism in the turbulent downstream of a relativistic shock with respect to the first-order mechanism at the shock front has been underestimated in the past, and that the second-order mechanism may have significant effects on the form of the particle spectra and its evolution.