7mm VLBA observations of BL Lac object OJ287

Analysis of the image sequence reveals a sharp jet-position-angle swing by >100{deg} during [2004,2006], as viewed in the plane of the sky, which we interpret as the crossing of the jet from one side of the line of sight to the other during a softer- and longer-term swing of the inner jet.

Modulating such long-term swing, our images also show for the first time a prominent erratic wobbling behavior of the innermost ~0.4 mas of the jet with fluctuations in position angle of up to ~40{deg} over timescales ~2 yr. This is accompanied by highly superluminal motions along non-radial trajectories, which reflect the remarkable non-ballistic nature of the jet plasma on these scales. The erratic nature and short timescales of the observed behavior rule out scenarios such as binary black hole systems, accretion disk precession, and interaction with the ambient medium as possible origins of the phenomenon on the scales probed by our observations, although such processes may cause longer-term modulation of the jet direction.