Magnetic Field Geometry in `Red' and `Blue' BL Lacs

Abstract

We compare the systematics of the magnetic field geometry in the red low-energy peaked BL Lacs (LBLs) and blue high-energy peaked BL Lacs (HBLs) using VLBI polarimetric images. The LBLs are primarily radio--selected BL Lacs and the HBLs are primarily X-ray selected. In contrast to the LBLs, which show predominantly transverse jet magnetic fields, the HBLs show predominantly longitudinal fields. Thus, while the SED peaks of core-dominated quasars, LBLs and HBLs form a sequence of increasing frequency, the magnetic field geometry does not follow an analogous sequence. We briefly investigate possible connections between the observed parsec-scale magnetic field structures and circular polarization measurements in the literature on various spatial scales.