BL Lacertae variability and superluminal motion via a helical filament/shock interface

The combination of a helical filament found by VLA observations at kpc scales in the M87 jet by Hardee, Owen, and Cornwell (1989) with a shock propagating down a jet suggests a kinematical model that could explain much of the flux variation in sources where the jet is aligned almost directly toward the observer, such as BL Lacertae objects. This model also naturally generates a superluminal pattern speed. The assumptions made by a first-order approximate model are discussed. Reasonable values for jet opening angle, viewing angle, shock speed, and radiative particle speed are used to match the model both to flux variations and superluminal motion of BL Lacertae itself with reasonable success. 33 refs