Superluminal motion in the parsec-scale jet in 3C 380

We present extensive multi-epoch VLBI observations of the quasar 3C 380 which reveal a bent parsec-scale radio jet with complex substructure which exhibits superluminal motion out to ～100 pc from the active core. The general characteristics of the jet and its motion are similar to those in several other well-studied superluminal sources, most notably a rapid increase in the opening angle at a projected distance of a few tens of parsecs from the core and apparent acceleration along the jet. This acceleration could be a simple kinematic effect associated with bulk flow at a constant speed, but at a varying angle to the line of sight. The jet is well resolved in the transverse direction for the majority of its length and resembles numerical simulations of a jet disrupted by the rapid growth of sinusoidal 'Kelvin–Helmholtz' modes. Our maps also reveal extremely rapid localized variations in brightness which have not yet been seen in other nuclear jets. We suggest that phase effects, arising from the intersection of oblique shocks, may be the cause of these dramatic changes. 3C 380 is often classified as a compact steep spectrum (CSS) source; however, in contrast to the majority of CSS sources, it is likely that 3C 380 is simply a powerful FRII source seen approximately end-on.