EVOLUTION OF THE MILLIARCSECOND TOTAL INTENSITY AND POLARIZATION STRUCTURES OF BL LACERTAE OBJECTS

Second-epoch 5 GHz VLBI total intensity (I) and linear polarization (P) maps are presented for eight BL Lacertae objects. In addition, I and P maps are presented for 2007 + 777, which has never before been mapped at milliarsecond resolution with polarization sensitivity. There are now 19 BL Lacertae objects for which VLBI polarization maps have been made, 12 at two or more epochs. We find that the VLBI polarization structures of BL Lacertae objects are frequently variable on timescales of similar to 2 yr. There is some evidence that the quiescent polarization state for the VLBI cores has polarization position angle chi perpendicular to the jet direction, theta, and that chi aligns with theta when new jet components are emerging. Application of the Kolmogorov-Smirnov test to the distributions of VLBI component speeds in BL Lacertae objects and quasars indicates with better than 99% confidence that the speeds observed in BL Lacertae objects are lower than those in quasars. We suggest that this is a natural consequence of the fact that many of the VLBI jet components in BL Lacertae objects appear to be transverse relativistic shocks.