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

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 beta(app) (units of c) range from 3.9 to 13.5, corresponding to beta(gal)(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 Alfven waves propagating downstream on the longitudinal component of the magnetic field. The wave-induced transverse speed of the jet is non-relativistic (beta(gal)(tr) less than or similar to 0.09 ). In 2010 the wave activity subsided and the jet then displayed a mild wiggle that had a complex oscillatory behavior. The Alfven 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 beta(s) = 0.3 and apparent speed of a slow MHD wave beta(app,) (S) = 4 yields Lorentz factor of the beam Gamma(beam) similar to 4.5, pitch angle of the helix (in the beam frame) alpha similar to 67 degrees, Alfven speed beta(A) similar to 0.64, and magnetosonic Mach number M-ms similar to 4.7. This describes a plasma in which the magnetic field is dominant and in a rather tight helix, and Alfven waves are responsible for the moving transverse patterns.