The observed variability of BL Lac objects and Quasars on timescales \la 1 day ( intraday variability , IDV ) have revealed radio brightness temperatures up to T _ { b } \sim 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 \ga 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 } \sim 10 ^ { 2 } -10 ^ { 4 } reduces the intrinsic variability timescale \Delta t ^ { \prime } to the observed value \Delta t \sim \Delta t ^ { \prime } / \gamma _ { 0 } so that the intrinsic brightness temperature is reduced by a factor of order \sim 1 / \gamma _ { 0 } ^ { 2 } , easily below the Inverse Compton limit of T _ { b } \la 10 ^ { 12 } K. When looking at a single event we expect the variability time scales \Delta t 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 \propto \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 .