2003MNRAS.341..405S

We present the results of a multiwavelength campaign during which BL Lac was observed at 17 regular epochs from 1998.23 to 2001.28. We used the Very Long Baseline Array (VLBA) in an observing mode sensitive to linear polarization at wavelength 7 mm with a resolution of the order of 0.2 mas. For the first time such observations were complemented by near-simultaneous polarization-sensitive James Clerk Maxwell Telescope (JCMT) observations at a wavelength close to 1 mm. Optical polarimetry was also obtained for the final year of observations. The VLBA images reveal a remarkable oscillating `nozzle' structure with a period of ∼2 yr. The orientation of the nozzle tracks the polarization position angle measured at the JCMT, in which we detect a very similar period. We argue that these results require the same jet direction at the small scale probed by the 1-mm emission as at the larger scale probed by the 7-mm emission; there is no evidence to suggest wiggling of the jet between these scales. The picture that emerges is of a straight but precessing jet where the components form a straight line near the core, but form an apparently curved locus at larger separations where components were ejected in different directions. Two further lines of evidence support this view. First, tracing the evolution of bright components showed straight paths and ejection angles consistent with the nozzle direction at the ejection time. Secondly, a simple model of a precessing nozzle was used to predict the inner jet structure. At most epochs the agreement between the model and data was good. On angular scales >2 mas from the core a transition occurs and most components bend to follow the usual southeasterly direction on deca-milliarcsecond scales. This precession is clearly analogous to that found in the galactic jet source SS 433 albeit with a higher jet speed and a smaller precession cone opening angle. The precession observed in SS 433 is believed to be caused by the interaction of a companion star with the accretion disc around the compact object. This suggests the possibility that the black hole from which the jet from BL Lac emerges is part of a binary system.