Superluminal Motion of the Radio Core of BL Lacertae: 1980.4-1988.3

The compact radio source associated with the galaxy BL Lacertae has been mapped using intercontinental VLBI arrays at frequencies of 5.0 and 10.6 GHz at 23 epochs from 1980.4 to 1988.3. We have identified four superluminally moving components and a nearly stationary knot which persisted during a prolonged period of quiescence. Each of the moving components is coincident with a flux and polarization outburst. The first three components (S1, S2, S3) were ejected along position angle 188^deg^ +/- 2^deg^ from an assumed stationary core, while S5 moved along position angle 196^deg^ +/- 3^deg^. A nearly stationary component (S4) was detected during a period of quiescence between late 1984 and early 1986. The trajectories of S2 and S5 are consistent with rectilinear motion along the initial position angle, while S1 and S3 showed a projected curvature of ~ 10^deg^ at later epochs. The apparent velocities of all four components were constant throughout their evolution within measurement errors, with values v = (3.7 +/- 0.3)h^-1^c (S1), (3.6 +/- 0.3)h^-1^c (S2, S3), and (3.3 +/- 0.3)h^-1^c (S5). For all moving components, the flux density decreases and the angular size increases as the components evolve. The mean projected opening angle is 30^deg^ +/- 10^deg^. The 5 GHz maps show an expanding component which we have identified with component S2 at 10.6 GHz. The core-component separation is ~ 0.3 mas smaller at 5 GHz compared with that at 10.6 GHz, in agreement with conical beaming models which predict a spectral index gradient at the throat of the cone. The observations are discussed in terms of relativistic beaming models with propagating shocks. We find that the observations favor models with a Lorentz factor {GAMMA} ~ 4 and a viewing angle between the observers line of sight and jet axis of 19^deg^ <~ φ <~ 31^deg^. By applying the gasdynamical model of Daly and Marscher, the observations are consistent with a jet opening angle ψ ~ 19^deg^ and a ratio of initial jet to ambient medium pressure greater than 6.6. These results are similar to the best-fit solution of total flux and polarization observations using the model of Hughes, Aller, and Aller.