High Observed Brightnesses in Radio Jets

We study the variability properties at radio frequencies of the jets thoughtto be typical of Active Galactic Nuclei, i.e. bulk Lorentz factor \Gamma ~ 10,and incoherent synchrotron emission. We assume that the radiating electrons areaccelerated at shocks within the jet, and that these shocks have a suitable(conical) geometry. In our framework, we can reproduce the variability patternof Intra Day Variables (20% in less than 1 day) as long as the observedbrightness temperature is T_B < 3x10^{17} K. The only stringent condition is aninjection timescale of the perturbation shorter than the variability timescale;the geometric condition on the viewing direction is not especially severe, andagrees with the observed occurrence of the phenomenon. For higher values of T_Bcoherent processes are probably necessary.Comment: 12 pages, aaspp4 style, accepted ApJ Part