Can long-term periodic variability and jet helicity in 3C 120 be explained by jet precession?

Abstract

Optical variability of 3C 120 is discussed in the framework of jet precession. Specifically, we assume that the observed long-term periodic variability is produced by the emission from an underlying jet with a time-dependent boosting factor driven by precession. The differences in the apparent velocities of the different superluminal components in the milliarcsecond jet can also be explained by the precession model as being related to changes in the viewing angle. The evolution of the jet components has been used to determine the parameters of the precession model, which also reproduce the helical structure seen at large scales. Among the possible mechanisms that could produce jet precession, we consider that 3C 120 harbours a super-massive black hole binary system in its nuclear region and that torques induced by misalignment between the accretion disc and the orbital plane of the secondary black hole are responsible for this precession; we estimated upper and lower limits for the black holes masses and their mean separation.