  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.
