Studies of the Jet in BL Lacertae. II. Superluminal Alfvén Waves
We study the kinematics of ridge lines on the pc-scale jet of the active galactic nucleus BL Lac. We show that the ridge lines display transverse patterns that move superluminally downstream, and that the moving patterns are analogous to waves on a whip. Their apparent speeds β_(app) (units of c) range from 3.9 to 13.5, corresponding to βgal/wave=0.981−0.998 in the galaxy frame. We show that the magnetic field in the jet is well-ordered with a strong transverse component, and assume that it is helical and that the transverse patterns are Alfvén waves propagating downstream on the longitudinal component of the magnetic field. The wave-induced transverse speed of the jet is non-relativistic (βgal/tr≲0.09). In 2010 the wave activity subsided and the jet then displayed a mild wiggle that had a complex oscillatory behaviour. The Alfvén waves appear to be excited by changes in the position angle of the recollimation shock, in analogy to exciting a wave on a whip by shaking the handle. A simple model of the system with plasma sound speed β_s=0.3 and apparent speed of a slow MHD wave β_(app,S)=4 yields Lorentz factor of the beam Γ_(beam)∼4.5, pitch angle of the helix (in the beam frame) α∼67^∘, Alfvén speed β_A∼0.64, and magnetosonic Mach number M_(ms)∼4.7. This describes a plasma in which the magnetic field is dominant and in a rather tight helix, and Alfvén waves are responsible for the moving transverse patterns.
Additional Information© 2015 American Astronomical Society. Received 2014 September 8; accepted 2015 January 14; published 2015 April 6. We are grateful to M. Perucho for reading the manuscript and offering helpful suggestions, and to the MOJAVE team for comments on the manuscript, and for years of work in producing the data base that makes this work possible. TGA acknowledges support by DFG project number Os 177/2-1. TH was partly supported by the Jenny and Antti Wihuri foundation and by the Academy of Finland project number 267324; TS was partly supported by the Academy of Finland project 274477. YYK 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. ABP was supported by the "Non-stationary processes in the Universe" Program of the Presidium of the Russian Academy of Sciences. 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 NNX12A087G. This study makes use of 43 GHz VLBA data from the VLBA-BU Blazar Monitoring Program (VLBA-BU-BLAZAR15), funded by NASA through the Fermi Guest Investigator Program. 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_803_1_3.pdf
Submitted - 1409.3599v1.pdf