Monthly 43 GHz VLBA polarimetric monitoring of 3C 120 over 16 epochs: Evidence for trailing shocks in a relativistic jet

We present a 16 month sequence of monthly polarimetric 43 GHz VLBA images o f the radio galaxy 3C 120. The images probe the inner regions of the radio jet of this relatively nearby superluminal radio galaxy at a linear resolut ion of 0.07 h(65)(-1) pc (H-0 = 65 h(65) km s(-1) Mpc). We follow the motio n of a number of features with apparent velocities between 4.01 +/- 0.08 an d 5.82 +/- 0.13 h(65)(-1) c. A new superluminal knot, moving at 4.29 +/- 0. 16 h(65)(-1) c, is observed to be ejected from the core at a time coinciden t with the largest flare ever observed for this source at millimeter wavele ngths. Changes in the position angle of this component, as well as a progre ssive rotation of its magnetic polarization vector, suggest the presence of a twisted (resembling a helix in projection) configuration of the underlyi ng jet magnetic field and jet geometry. We identify several knots that appe ar in the wake of the new superluminal component, moving at proper motions similar to4 times slower than any of the other moving knots observed in 3C 120. These features have properties similar to those of the "trailing" shoc ks seen in relativistic, time-dependent, hydrodynamical, and emission simul ations of compact jets. Such trailing compressions are triggered by pinch-m ode jet-body instabilities caused by the propagation of a strong perturbati on, which we associate with the new strong superluminal component.