Multiwavelength Probes of the Environs of Relativistic Shocks in Blazar Jets

Abstract

Diffusive shock acceleration (DSA) at relativistic shocks is likely to be an important acceleration mechanism in various astrophysical jet sources, including radio-loud AGN. An important recent development for blazar science is the ability of Fermi-LAT data 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 X-ray band and Fermi data can be used to probe diffusive acceleration in relativistic, oblique, MHD shocks in blazar jets. The spectral index of the non-thermal particle distributions resulting from Monte Carlo simulations of DSA, and the fraction of thermal particles accelerated to non-thermal energies, depend sensitively on the particles' mean free path scale, and also on the magnetic field obliquity to the shock normal. We investigate self-consistently the radiative synchrotron/Compton signatures of the resulting thermal and non-thermal particle distributions. Important constraints on the frequency of particle scattering and the level of field turbulence are identified for the blazar AO 0235+164. The possible interpretation that turbulence levels decline with remoteness from jet shocks, and a significant role for non-gyroresonant diffusion, are discussed.

Gamma rays from blazars

“…The results also suggest that most of BL Lac objects are quite inefficient in emitting the radiation, since the derived cooling time of the electrons is generally much longer then the dynamical timescale and thus most of the energy stored in the relativistic particles is lost [20,21]. Somewhat paradoxically for source emitting conspicuous VHE radiation, a relatively low efficiency is also found to characterize the acceleration process itself, see [22]. Another important point is that the emission regions appear to be matter-dominated, the magnetic field providing a negligible contribution to the power carried by the jet (more on this later).…”