Particle Acceleration at Relativistic Shocks in Extragalactic Systems
  Diffusive shock acceleration (DSA) at relativistic shocks is expected to be
an important acceleration mechanism in a variety of astrophysical objects
including extragalactic jets in active galactic nuclei and gamma ray bursts.
These sources remain strong and interesting candidate sites for the generation
of ultra-high energy cosmic rays. In this paper, key predictions of DSA at
relativistic shocks that are salient to the issue of cosmic ray ion and
electron production are outlined. Results from a Monte Carlo simulation of such
diffusive acceleration in test-particle, relativistic, oblique, MHD shocks are
presented. Simulation output is described for both large angle and small angle
scattering scenarios, and a variety of shock obliquities including superluminal
regimes when the de Hoffman-Teller frame does not exist. The distribution
function power-law indices compare favorably with results from other
techniques. They are found to depend sensitively on the mean magnetic field
orientation in the shock, and the nature of MHD turbulence that propagates
along fields in shock environs. An interesting regime of flat spectrum
generation is addressed, providing evidence for its origin being due to shock
drift acceleration. The impact of these theoretical results on gamma-ray burst
and blazar science is outlined. Specifically, Fermi gamma-ray observations of
these cosmic sources are already providing significant constraints on important
environmental quantities for relativistic shocks, namely the frequency of
scattering and the level of field turbulence.
