Relativistic electron beams in IDV blazars

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The observed variability of BL Lac objects and Quasars on timescales <1 day (intraday variability, IDV) have revealed radio brightness temperatures up to 10¹⁶-10²⁰ 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=10²-10⁴ 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 10¹² 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.