New Results from VLBI Polarization Observations of BL Lacertae Objects

Abstract

The major distinguishing features of BL Lacertae Objects are weak or absent line emission and strong and variable optical, infrared, and radio polarization (Angel and Stockman 1980; Kollgaard 1994). The radio emission and much of the optical emission is believed to be synchrotron radiation. In nearly every BL Lacertae object in which polarization structure has been detected, the polarization position angles χ in knots in the jets are nearly parallel to the VLBI structural axis. Assuming the jet components to be optically thin, the inferred magnetic fields B are nearly perpendicular to the jet direction θ; perhaps the most natural interpretation of this is that the knots are associated with shocks that compress an initially tangled B field as they propagate down the VLBI jet, enhancing B transverse to the compression (Laing 1980; Hughes, Aller, & Aller 1989). The superluminal speeds observed in the jets of BL Lacertae objects are on average lower than those observed in a comparably core-dominated population of quasars (Gabuzda et al. 1994). The distribution of core polarization position angles χ _core at λ = 6 cm is bi-modal, suggesting that χ _core is roughly perpendicular to θ when the cores are quiescent, and aligns with θ at epochs when polarization from newly emerging shock components is blended with the core polarization (Gabuzda et al. 1994).