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G. R. Denn, R. L. Mutel (University of Iowa)
We present the results of a polarization sensitive VLBA monitoring program of the AGN BL~Lacertae spanning epochs 1995.4 - 1998.3. The images show the ejection of four highly polarized superluminal components (denoted S7, S8, S9, and S10 chronologically).
The trajectories of the components are clearly non-radial. Extrapolation of the position of the components to their respective birth epochs indicates that some of the components are apparently created in pairs, with a fast component and a slow component; the birth epochs are t0 = (1994.12 ±0.13, 1994.20 ±0.05) for (S7, S8) and t0 = (1996.35 ±0.09, 1996.47 ±0.10) for (S9, S10). The apparent velocities (\betaapp) for the components are S7: (4.1±.1)h-1, S8: (2.4±.6)h-1, S9: (6.1±.6)h-1, and S10:(3.1±.2)h-1.
There has been no obvious radio counterpart to the large optical-gamma ray outburst at epoch 1997.6 (from three subsequent epochs.)
The core component polarization was low and found to satisfy a \nu2 frequency dependence expected from a turbulent foreground Faraday screen with characteristic spatial scale s0 ~0.3 a.u.
We have applied the helical model of Hardee (1987) to explain the components' non-radial trajectories. The best fit trajectories indicate that the angle of the helical axis to the line of sight is consistent with \Theta ~q 9\arcdeg and the jet half-opening angle is \Psi ~q 2\arcdeg.
Using the polarized maps, we have applied a relativistic shock model (Hughes et al. 1985) to the components and have determined that the three slower components (S7, S8, and S10) are weak shocks (compression k \approx 0.7) and that the fastest component (S9) is a strong shock (k \approx 0.4).
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