Relativistic electron beams in IDV blazars

The observed variability of BL Lac objects and Quasars on timescales less than or similar to 1 day (intraday variability, IDV) have revealed radio brightness temperatures up to T(b) similar 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 Gamma greater than or similar to 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(0) similar to 10(2) - 10(4) reduces the intrinsic variability timescale Delta t' to the observed value Delta t similar to Delta t'/gamma(0) so that the intrinsic brightness temperature is reduced by a factor of order similar to 1/gamma(0)(2), easily below the Inverse Compton limit of T(b) less than or similar to 10(12) K. When looking at a single event we expect the variability time scales at to be independent of frequency for a monoenergetic electron beam, whereas for a beam with a spread out distribution of energies (e.g. power-law) parallel to the magnetic field the timescales are shortened towards higher frequencies according to Delta t proportional to nu(-0.5). The observations seem to favor monoenergetic relativistic electrons which explain several properties of variable blazar spectra. The production of variable X- and gamma-ray flux is briefly discussed.