2005A&A...435..497B

A very detailed monitoring of a radio flare in the Seyfert I galaxy IIIZw2 with the VLA and the VLBA is presented. The relative astrometry in the VLBA observations was precise to a few µas. The spectral and spatial evolutions of the source are closely linked, and these observations allowed us to study a textbook example of a synchrotron self-absorbed jet in great detail. We observed a phase where the jet gets frustrated with neither expansion nor spectral evolution. Then the jet breaks free and starts to expand with apparent superluminal motion. This expansion is accompanied by a strong spectral evolution. The results are a good confirmation of synchrotron theory and equipartition for jets.