Relativistic electron beams in IDV blazars
  The observed variability of BL Lac objects and Quasars on timescales <1 day (intraday variability, IDV) have revealed radio brightness temperatures up to 10^{16}-10^{20} K. These values challenge the beaming model with isotropic comoving radio emission beyond its limits, requiring bulk relativistic motion with Lorentz factors >100. We argue in favor of a model where an anisotropic distribution of relativistic electrons streams out along the field lines. When this relativistic beam is scattered in pitch angle and/or hits a magnetic field with components perpendicular to the beam velocity it starts to emit synchrotron radiation and redistribute in momentum space. The propagation of relativistic electrons with Lorentz factor gamma=10^2-10^4 reduces the intrinsic variability timescale by a factor gamma^{-1}, so that the intrinsic brightness temperature is reduced by a factor of order gamma^{-2}, easily below the Inverse Compton limit of 10^{12} K. When looking at a single event we expect the variability time scales to be independent of frequency for a monoenergetic electron beam. The production of variable X- and gamma-ray flux is briefly discussed. 
A. R. Crusius-W"atzel, H. Lesch (Institut f"ur Astronomie und   Astrophysik der Universit"at M"unchen)