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 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 Intra Day Variables (20% in less than 1 day) as long as the observed brightness temperature is TB < 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 TB coherent processes are probably necessary. Subject headings: BL Lacertae objects : individual (Mkn 421, S5 0716+714) — galaxies : jets — radio continuum : galaxies — radiation mechanisms : non thermal