4C 39.25: Witnessing the interaction between a moving and a stationary component

We present multi-epoch multi-frequency VLBI images of the compact polarized radio structure of the quasar 4C 39.25. Between 1995 and 1999, the total intensity images at 15, 22, and 43 GHz have remained almost unperturbed, while there have been significant changes in the polarized intensity maps together with a related rotation of the polarization angle. These changes in the polarized structure trace in great detail the interaction between a moving and a standing component. We interpret the stationary feature as a bend in the jet trajectory in a plane that does not contain the observer, and the moving one as a shock turning around the bend.