Very Long Baseline Polarimetry of BL Lacertae

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Abstract
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 galaxy 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, show the ejection and evolution of four highly polarized superluminal components (denoted S7-S10). The trajectories of all components were significantly curved, with the slowest component (S8) exhibiting the most bending. All four components had nearly constant apparent speeds, at least between 1 and 3 mas separation from the core. Extrapolation of the core-component separation to a zero spacing epoch suggests that they may have been created in pairs, with fast and slow counterparts. Each superluminal component was moderately linearly polarized at all observed frequencies. The electric vector position angle (EVPA) was frequency independent but changed with core-component separation for all four components, with total EVPA rotation varying from 30° (S7) to 80° (S8). There was no preferred EVPA orientation 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° of the jet direction at later epochs, consistent with emission from 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 adiabatically expanding jet to explain the observed bent component trajectories. By searching 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 Θ 9° and a jet half-opening angle Ψ 2°. Individual component trajectories were fitted by varying initial conditions at the throat of the jet and the initial helical wavelength scale. Although the position along the jet and apparent 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 synchrotron radiative shocks within a underlying relativistic flow. Using the observed fractional polarization and jet-interjet intensity ratio along with the helix-line of sight angle as constraints, we found that slower components (S8, S10) are consistent with forward shocks, while faster components (S7, S9) could be either forward or reverse shocks. Derived shock compression ratios vary from weak (k ~ 0.5-0.8) for slow component S8 to moderately strong (k ~ 0.0-0.5) for fast component S9. As of epoch 1998.2, there have been no unusual features in the parsec-scale radio structure following the large optical-gamma-ray flare at epoch 1997.6.