2001A&A...374..784B

VLBI monitoring of PKS 0420-014 at 3.6 cm performed during the period 1989.32-1992.48 reveals superluminal jet components in a strongly curved jet. We present a binary black hole model capable of simultaneously explaining the flux-density variations and the motions of jet components on helical trajectories in active galactic nuclei. From VLBI observations we derive the characteristics of the parsec-scale jet of PKS 0420-014. Moreover, we use the optical light curve of the quasar PKS 0420-014 over the time span 1989.67-1993.93 to determine the characteristics of the binary black hole system. We propose that galactic and extragalactic radio sources with jets are due to the existence of binary systems. Extragalactic binary systems are the results of mergers and provide a natural explanation why only a small fraction of quasars are radio sources and why extragalactic radio sources are associated with elliptical galaxies. These binary systems will be one of the primary targets for the Laser Interferometer Space Antenna (LISA).