Article

STUDIES OF THE JET IN BL LACERTAE. II. SUPERLUMINAL ALFVÉN WAVES

M. H. CohenDepartment of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA;D. L. MeierDepartment of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA;T. G. ArshakianByurakan Astrophysical Observatory, Byurakan 378433, Armenia and Isaac Newton Institute of Chile, Armenian BranchE. Clausen-BrownMax-Planck-Institut fr Radioastronomie, Auf dem Hgel 69, D-53121 Bonn, GermanyD. C. HomanDepartment of Physics, Denison University, Granville, OH 43023, USAT. HovattaAalto University Metshovi Radio Observatory, Metshovintie 114, FI-02540 Kylml, FinlandY. Y. KovalevAstro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, RussiaM. L. ListerDepartment of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USAA. B. PushkarevJ. L. RichardsDepartment of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USAT. SavolainenAalto University Metshovi Radio Observatory, Metshovintie 114, FI-02540 Kylml, Finland

2015en

ABI

Abstract

We study the kinematics of ridge lines on the parsec-scale jet of the active galactic nucleus BL Lacertae. 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 app (units of c) range from 3.9 to 13.5, corresponding to b = -0.981 0.998 wave gal 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 Alfvn waves propagating downstream on the longitudinal component of the magnetic field. The wave-induced transverse speed of the jet is non-relativistic (b 0.09 tr gal ). In 2010 the wave activity subsided and the jet then displayed a mild wiggle that had a complex oscillatory behavior. The Alfvn 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 s =0.3 and apparent speed of a slow MHD wave app,S =4 yields Lorentz factor of the beam beam 4.5, pitch angle of the helix (in the beam frame) 67, Alfvn speed 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 Alfvn waves are responsible for the moving transverse patterns.

Identifiers

DOI: 10.1088/0004-637x/803/1/3

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