Diffusive shock acceleration (DSA) at relativistic shocks is very likely to be an important acceleration mechanism in various astrophysical jet sources, including radio-loud AGN. An important aspect of the Fermi legacy for blazar science is the ability of the LAT to pin down the power-law index of the high energy portion of emission in these sources, and therefore also the index of the underlying non-thermal particle population. This diagnostic potential was not possible prior to Fermi launch, when gamma-ray information was dominated by the highly-absorbed TeV band. This paper highlights how multiwavelength spectra including Fermi data can be used to probe diffusive acceleration in relativistic, oblique, MHD shocks in blazars.

A brief summary of recent results of Monte-Carlo simulations of DSA at relativistic shocks is given. The spectral index of the resulting nonthermal particle distributions and the fraction of thermal particles accelerated to non-thermal energies, depend sensitively on the particles’ mean free path scale, and also on the shock obliquity. We investigate self-consistently the radiative (synchrotron + Compton) signatures of the resulting thermal + nonthermal particle distributions. Important constraints on the frequency of particle scattering and the level of field turbulence are identified for blazars such as Mrk 501 and the Bl Lac object AO 0235+164. The possible interpretation that turbulence levels decline with remoteness from the shock, and a significant role for non-gyroresonant diffusion, are discussed.