Very long baseline polarimetry of BL Lacertae

We present the results of an extensive observing campaign designed to study the evolution of parsec-scale radio structure in the nucleus of the AGN ga laxy BL Lacertae. The observations spanned 17 epochs from 1994.7 to 1998.3. The VLBA observations, made at regular intervals at 15, 22, and 43 GHz, sh ow the ejection and evolution of four highly polarized superluminal compone nts (denoted S7-S10). The trajectories of all components were significantly curved, with the slowest component (S8) exhibiting the most bending. All f our components had nearly constant apparent speeds, at least between 1 and 3 mas separation from the core. Extrapolation of the core-component separat ion to a zero spacing epoch suggests that they may have been created in pai rs, with fast and slow counterparts. Each superluminal component was modera tely linearly polarized at all observed frequencies. The electric vector po sition angle (EVPA) was frequency independent but changed with core-compone nt separation for all four components, with total EVPA rotation varying fro m 30 degrees (S7) to 80 degrees (S8). There was no preferred EVPA orientati on with respect to the jet axis when components were young, i.e., close to the core. However, three of the four components' EVPAs rotated to within 20 degrees of the jet direction at later epochs, consistent with emission fro m transverse shocks. The core was nearly unpolarized at 15 GHz, except when a new superluminal component was just emerging. At higher frequencies (22 and 43 GHz) core fractional polarization was low with a quadratic frequency dependence. We applied Hardee's model of helical twisting on an adiabatica lly expanding jet to explain the observed bent component trajectories. By s earching the parameter space of allowed helical geometries, we found a best -fit set of parameters with fixed opening and line of sight angles for all four components. The preferred helical geometry was described by a line of sight angle Theta similar or equal to 9 degrees and a jet half-opening angl e Psi similar or equal to 2 degrees. individual component trajectories were fitted by varying initial conditions at the throat of the jet and the init ial helical wavelength scale. Although the position along the jet and appar ent speed are consistent with the helical model, the predicted polarization position angles are not in good agreement. We derived physical properties in the emission regions by assuming that they are due to optically thin syn chrotron radiative shocks within a underlying relativistic flow. Using the observed fractional polarization and jet-interjet intensity ratio along wit h the helix-line of sight angle as constraints, we found that slower compon ents (S8, S10) are consistent with forward shocks, while faster components (S7, S9) could be either forward or reverse shocks. Derived shock compressi on ratios vary from weak (k similar to 0.5-0.8) for slow component S8 to mo derately strong (k similar to 0.0-0.5) for fast component S9. As of epoch 1 998.2, there have been no unusual features in the parsec-scale radio struct ure following the large optical-gamma-ray flare at epoch 1997.6.