Parsec-scale VLBA images of BL Lac at 15 GHz show that the jet contains a permanent quasi-stationary emission feature 0.26 mas ( 0.34 pc projected ) from the core , along with numerous moving features .
In projection , the tracks of the moving features cluster around an axis at position angle -166.6 ^ { \circ } that connects the core with the standing feature .
The moving features appear to emanate from the standing feature in a manner strikingly similar to the results of numerical 2-D relativistic magneto-hydrodynamic ( RMHD ) simulations in which moving shocks are generated at a recollimation shock .
Because of this , and the close analogy to the jet feature HST-1 in M 87 , we identify the standing feature in BL Lac as a recollimation shock .
We assume that the magnetic field dominates the dynamics in the jet , and that the field is predominantly toroidal .
From this we suggest that the moving features are compressions established by slow and fast mode magneto-acoustic MHD waves .
We illustrate the situation with a simple model in which the slowest moving feature is a slow-mode wave , and the fastest feature is a fast-mode wave .
In the model the beam has Lorentz factor \Gamma _ { beam } ^ { gal } \approx 3.5 in the frame of the host galaxy , and the fast mode wave has Lorentz factor \Gamma _ { Fwave } ^ { beam } \approx 1.6 in the frame of the beam .
This gives a maximum apparent speed for the moving features , \beta _ { app } = v _ { app } / c = 10 .
In this model the Lorentz factor of the pattern in the galaxy frame is approximately 3 times larger than that of the beam itself .