Particle Acceleration in Relativistic Flows

A property common to several different astrophysical sources of high-energy gamma-rays is the presence of bulk motion at relativistic speed. The intrinsic spectra of the nonthermal radiating particles also show interesting similarities, with a pronounced hardening towards lower energies. This suggests two distinct acceleration mechanisms could be at work in these sources. At high energies, the stochastic first-order Fermi process at shocks seems to provide a reasonable explanation. I will briefly review the status of this mechanism before discussing the possibility that, at lower energies, non-stochastic acceleration in the induced electric field of a relativistic current sheet plays a role.