Diagnosing relativistic particle acceleration in AGN jets

Abstract

Mildly relativistic, oblique shocks are generally considered to be among the most plausible sites of relativistic particle acceleration and the subsequent production of strongly variable, polarized multi-wavelength emission from relativistic jet sources such as blazars, via diffusive shock acceleration (DSA). This paper summarizes recent results on a self-consistent coupling of DSA and radiation transfer in blazar jets. We demonstrate that the observed spectral energy distributions (SEDs) of blazars strongly constrain the nature of hydromagnetic turbulence responsible for pitch-angle scattering by requiring a strongly energy-dependent pitch-angle mean free path. In the case of low-frequency-peaked blazars, we find that the scaling of the pitch-angle-scattering mean-free-path, λpas sometimes needs to be as strong as λpas ∝ p3, where p is particle momentum. The prominent soft X-ray excess in the SED of the BL Lac object AO 0235+164 can be modelled as the signature of bulk Compton scattering of external radiation fields by the thermal electron population, which places additional constraints on the level of hydromagnetic turbulence. A prediction of this interpretation is that the soft X-ray emission in this soft excess is expected to be highly polarized.