Observational consequences of precessing relativistic jets in extragalactic radio sources

Jets of extragalactic radio sources in many cases show signs of precessional motion. This may arise when two massive compact objects are close to each other within a galaxy nucleus. The observed radio brightness distributions are modelled by slowly precessing jets, often relativistic. Interesting illustrations of their behavior are offered by the frequency of occurrence of one-sided jets and by an explanation of the ''preferential avoidance'' effect. There may be some evidence for a binary massive black hole in the nearest galaxy nucleus, the Galactic Centre. (author). 6 figs., 17 refs