Observational Evidence of an Internal Shock Collision in the Relativistic Jet of the Nearby Radio Galaxy 3C 111

Abstract

We report 10 epochs of 22 GHz East Asia To Italy: Nearly Global VLBI (EATING VLBI) observations of the nearby radio galaxy 3C~111 obtained from 2020 to 2023. Combined with VLBA data at 15 and 43 GHz, these observations yield a 64-epoch, multi-frequency sequence of high-resolution radio images. We used high-resolution Bayesian Stokes~I imaging and Gaussian component modeling to derive light curves and knot kinematics. We find evidence that a faster, later-emitted knot catches up with a slower, earlier knot at a deprojected distance of about 10 pc from the central engine. The knot component responsible for this interaction propagated at relativistic speed and exhibited a rise-and-decay evolution over approximately six months. The Stokes~I flux evolution supports the internal shock interpretation, while archival polarization images reveal a temporary increase in fractional linear polarization in the interaction region near the Stokes~I flux maximum. The timing of the 2021 GeV flare suggests that this earlier high-energy event is more naturally associated with activity near the core. These results provide direct evidence for an internal shock in an active galactic nucleus jet and support a hybrid picture in which internal and standing shocks coexist in relativistic jets and may play different roles in their variability.