Polarized Radio Outbursts in Bl-Lacertae - Part Two - the Flux and Polarization of a Piston-Driven Shock

Abstract

We present a discussion of the outbursts of the variable source BL Lac that occurred during 1982 and 1983 and were seen in both total and polarized flux at 4.8, 8.0, and 14.5 GHz. These outbursts are interpreted in terms of compressions that propagate from an optically thick to an optically thin part of a beam. Each is assumed to be driven by a hot "piston." The observations are consistent with the interpretation that these compressions are weak shocks (Mach number < 2), the emission from which is polarized as a result of compression of an initially random magnetic field. The rapid decay of the polarized flux is shown to be evidence that the beam is unconfined and that the emitting electrons have a steep energy spectrum. Furthermore, when considered in conjunction with published VLBI data, our work provides evidence that the superluminal components of BL Lac are relativistically moving flow features. To supplement the discussion of the observations, we present a discussion of the degree of polarization for emission by a power-law distribution of electrons in a compressed random magnetic field. We conclude that highly polarized components should be a common feature of collimated flows.