Studies of the Jet in Bl Lacertae. I. Recollimation Shock and Moving Emission Features
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 a position angle of –166º.6 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 two-dimensional relativistic magneto-hydrodynamic (RMHD) simulations in which moving shocks are generated at a recollimation shock (RCS). Because of this, and the close analogy to the jet feature HST-1 in M87, we identify the standing feature in BL Lac as an RCS. 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 Γ_(beam)^(gal) ≈ 3.5 in the frame of the host galaxy and the fast mode wave has Lorentz factor Γ_(Fwave)^(beam) ≈ 1.6 in the frame of the beam. This gives a maximum apparent speed for the moving features, β_(app) = v_(app)/c = 10. In this model the Lorentz factor of the pattern in the galaxy frame is approximately three times larger than that of the beam itself.
Additional Information© 2014 American Astronomical Society. Received 2013 December 19; accepted 2014 April 2; published 2014 May 14. We thank the anonymous referee whose comments substantially improved the paper. We are grateful to Ken Kellermann and the rest of the MOJAVE team for their comments and for their years of work in producing the data base that makes this work possible. T.H. was supported in part by a grant from the Jenny and Antti Wihuri foundation and by the Academy of Finland project number 267324. Y.Y.K. is partly supported by the Russian Foundation for Basic Research (project 13-02-12103), Research Program OFN-17 of the Division of Physics, Russian Academy of Sciences, and the Dynasty Foundation. A.B.P. was supported by the "Non stationary processes in the Universe" Program of the Presidium of the Russian Academy of Sciences. T.G.A. acknowledges support by DFG project number Os 177/2-1. The VLBA is a facility of the National Radio Astronomy Observatory, a facility of the National Science Foundation that is operated under cooperative agreement with Associated Universities, Inc. The MOJAVE program is supported under NASA-Fermi grant 11-Fermi11-0019. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of NASA's Astrophysics Data System.
Published - 0004-637X_787_2_151.pdf
Submitted - 1404.0976v1.pdf