High Observed Brightnesses in Radio Jets

We study the variability properties at radio frequencies of the jets thought to be typical of Active Galactic Nuclei, i.e. bulk Lorentz factor $\Gamma\approx$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 Intra Day Variables (20% in less than 1 day) as long as the observed brightness temperature is $T_{B} < 3~10^{17}$ 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.