1999ApJ...511..136S

We study the variability properties at radio frequencies of the jets thought to be typical of active galactic nuclei, i.e., bulk Lorentz factor Γ~10 and incoherent synchrotron emission. We assume that the radiating electrons are accelerated at shocks within the jet, and that these shocks have a suitable (conical) geometry. In our framework, we can reproduce the variability pattern of intraday variables (20% in less than 1 day) as long as the observed brightness temperature is T_B<3x10¹⁷ K. The only stringent condition is an injection timescale of the perturbation shorter than the variability timescale; the geometric condition on the viewing direction is not especially severe, and agrees with the observed occurrence of the phenomenon. For higher values of T_B, coherent processes are probably necessary.