The VLBI Jets of BL Lac Objects

Abstract

Very Long Baseline Interferometry (VLBI) can be used toconstruct images of the radio emission of active galactic nuclei on scalesof parsecs. Although BL Lacertac objects distinguished from othercore-dominated AGN primarily in the absence of strong optical lineemission - have "core-jet" structures similar to those seen in othertypes of compact extragalactic radio sources, their parsec-scale structureshows a number of distinctive features, such as the dominanceof transverse magnetic fields. This B-field structure may be due tothe presence of relativistic shocks: alternatively, it may reflect thedominance of the toroidal comportent of an underlying helical B field.The superluminal speeds observed in the VLBI jets of BL Lac objectsare somewhat lower than those in quasars, suggesting that the typicalLorentz factors for the jet flows are smaller.