Precession in the Inner Jet of 3C 345

Abstract

VLBI observations have shown that the parsec-jet of 3C 345 is formed by several components, ejected from the core with superluminal velocities and travelling along bent trajectories on the plane of the sky. We interpret the differences in velocity and position angle among the different features at formation time as the result of parsec-scale precession of the relativistic jet and calculate the aperture angle of the precession cone, the angle between the cone axis and the line of sight and the Lorentz factor associated with the jet bulk motion. We assumed a precession period of 10.1 yr, which is one of the B-band light curve long-term periods reported in the literature. We propose that boosting of the underlying jet emission, which is time-dependent due to precession, is responsible for this long-term optical variability. Jet precession with periods of several years can be produced in super-massive black hole binary systems, when the secondary black hole is in an orbit non-coplanar with the primary accretion disk, inducing torques in the inner parts of the disk. Assuming that this mechanism is responsible for the jet precession in 3C 345, we estimate upper and lower limits for the masses of the two black holes, as well as their mean separation. We found a correlation between the formation of jet components and the occurrence of strong optical flares, as well as a very strong anti-correlation between the intensity of these flares and the time required for the components to reach the maximum flux density at radio frequencies.