SIMBAD references

An earlier comparison of simultaneous optical and 6 cm VLBI observations of polarization showed a correlation of their position angles which were either aligned or perpendicular. Gabuzda et al. (2005) proposed that the radio cores with their polarization aligned with optical polarization were predominantly optically thin, while the radio cores whose polarization was perpendicular to optical polarization were optically thick. We present the first results from new coordinated optical and high-frequency VLBA polarization observations of 15 AGNs designed to test this hypothesis.