Optical Polarization Properties of Compact Radio Sources

There is a strong correlation between the orientation of optical polarization and the direction of the VLBI jet in compact radio sources. The optical E vector is parallel to the VLBI jet, for both the highly polarized blazars and low (optical) polarization quasars (LPQs). The high optical polarization of blazars probably arises from synchrotron emission within an optically thin region where the magnetic field is normal to the VLBI jet. The origin of the optical polarization in LPQs is uncertain. It may result from scattering of continuum emission in, for instance, an optically thin accretion disk normal to the VLBI jet or it could be associated with synchrotron emission. For such good alignment of VLBI jet axis and optical E vector to exist, curvature in the source structure exceeding ten degrees, between the region producing the parsec scale radio emission and the very much smaller region (at least in blazars) producing the polarized optical emission, must be rare.