Subparsec polarimetric radio observations of 3C 120: A close-up look at superluminal motion

We present two-epoch polarimetric images of the radio galaxy 3C 120 obtained with the Very Long Baseline Array at 22 and 43 GHz. Because of the proximity of 3C 120 (z = 0.033), the 43 GHz observations allow us to observe superluminal motions with the highest resolution achieved to date, 0.07 h(-1) pc. Up to ten different superluminal components, with velocities between 2.3 and 5.4 h(-1)c, can be observed in this active source, with approximately monthly ejections of new components. Polarization is observed in several components and at both frequencies, with peaks in the linearly polarized flux not always coincident with the peaks in total intensity. The orientation of the magnetic field is observed to vary with respect to the jet flow direction as a function of frequency, epoch, and position along the jet. These observations are in agreement with previous numerical simulations of superluminal sources (Gomez et al. 1997). The classical association of lower frequency components with single shocks does not appear to be valid, and multiple components may be the result of a single disturbance in the jet flow. This is a consequence of the interaction of the shocked plasma with the external medium and the underlying jet, as well as hydrodynamical processes inside the shocked region, neglected in previous analytical shock studies owing to the nonlinearity of the jet fluid dynamics.