Radio Polarization and Flux Density Variability of BL Lacertae: Oblique Shocks

The results of more than 20 years of UMRAO multifre-quency single antenna observations of BL Lac reveal a series of polarized outbursts. Comparison of these results with VLBI maps shows that for many bursts the electric vector of the polarized emission (EVPA) is parallel to the observed jet structure, and several of these bursts have been quantitatively modeled by weak, transverse shocks propagating along a relativistic ow. However, in the past decade the EVPA has exhibited variations of tens of degrees from burst to burst which cannot be accounted for by transverse shocks. During this same period VLBI observations by Mutel, Denn and others have shown curvature and orientation changes in the radio jet structures. This behavior suggests the presence of oblique shocks. The required strength of the shocks, and the range of angles which specify the orientation of the shock fronts which are required to account for these variations in EVPA (and still reproduce the observed degree of polarization) are discussed. We nd that oblique shocks with strengths comparable to the transverse shocks, already used in our mod-eling, appear capable of producing the observed polarization behavior.