Cohen, Marshall

Recollimation Shock and the Fire Hose in BL Lacertae.

Author list: M. H. Cohen and the MOJAVE BL Lac Team

The MOJAVE group has made 15 GHz images of BL Lacertae at 109 epochs, using VLBA data.  A quasi-stationary bright spot (Component7) is usually seen at r~0.26 mas from the core. This component is the counterpart of component A2 seen at 43 GHz (Jorstad et al 2005). Fast components with apparent speeds up to 10c come from or through component 7; several appear every year. We identify component 7 as a recollimation shock and suggest that it might be ejecting components in a manner similar to the strong MHD shock that forms in a super-fast-magnetosonic jet flow and converts it to a trans-magnetosonic flow (Clarke et al 1986, Lind et al 1989).  The PA of the inner jet varies irregularly at 15 GHz. Several large swings occurred, with time scale of about 2 years and amplitude up to 25 degrees.  In these cases the jet appeared to follow the PA ballistically for two or three mas, with the shift in PA moving downstream as a superluminal pattern, analogous to water streaming from a swinging hose.  In 2009 the activity relaxed and in 2010-2012 the jet was stable and showed a mild stationary oscillation with wavelength about 2 mas and amplitude about 0.5 mas. At most epochs the polarization had a longitudinal EVPA, and we suggest that the magnetic field is in the form of a tight helix. If this is the case, the superluminal components might be fast MHD waves or shocks traveling parallel to the central axis.