2000ApJ...541...66L

A significant fraction of flat-spectrum, radio-loud quasars display most of the characteristics of relativistically beamed, high-optical polarization blazars yet are weakly polarized in the optical regime (m_opt≲3%). We have conducted a high-resolution polarization study with the VLBA at 22 and 43 GHz to look for differences in the parsec-scale magnetic field structures of 18 high and low optically polarized, compact radio-loud quasars (HPQs and LPRQs, respectively). We find a strong correlation between the polarization level of the unresolved parsec-scale radio core at 43 GHz and overall optical polarization of the source, which suggests a common (possibly cospatial) origin for the emission at these two wavelengths. The electric vectors of the polarized 43 GHz radio cores are roughly aligned with the inner jet direction, indicating magnetic fields perpendicular to the flow. Similar orientations are seen in the optical, which suggests that the polarized flux at both wavelengths is caused by one or more strong transverse shocks located very close to the base of the jet. In LPRQs, these shocks appear to be weak near the core and gradually increase in strength down the jet. The LPRQs in our sample tend to have less luminous radio cores than the HPQs and jet components with magnetic fields predominantly parallel to the jet. The components in HPQ jets, on the other hand, tend to have perpendicular magnetic field orientations. These differences cannot be accounted for by a simple model in which HPQs and LPRQs are the same type of object seen at different angles to the line of sight. A more likely scenario is that LPRQs represent a quiescent phase of blazar activity in which the inner jet flow does not contain strong shocks. Our high-resolution observations have shown that high rotation measures (up to 3000 rad.m^–2) previously seen in the nuclear regions of HPQs are present in LPRQs as well. The low-redshift quasars in our sample tend to have jet components with larger 43/22 GHz depolarization ratios than those found in the high-redshift sources. This may be caused by small-scale magnetic field fluctuations in the Faraday screens that are being smeared out in the high-redshift sources by the poorer spatial resolution of the restoring beam.