Search for correlations between the optical and radio polarization of   AGNs II: VLBA polarization data at 12+15+22+24+43 GHz
  Previous research showed most BL Lac objects and some quasars have aligned VLBI-core and optical polarizations, although some of the AGNs also showed no obvious relationship between VLBI-core and optical polarization angles. This may indicate that some AGNs have co-spatial regions of optical and radio emission, while others do not. Another possibility is that some of the VLBI cores had Faraday rotations of several tens of thousand of rad/m^2, which were not properly fit using the three-frequency data due to n\pi ambiguities in the observed polarization angles, leading to incorrect subtraction of the effects of the core Faraday rotation, and so incorrect intrinsic radio polarization angles \chi_0. We obtained additional 12+15+22+24+43GHz plus optical observations for 8 of 40 AGNs previously considered. Our results indicate that, although some VLBI radio cores have comparatively high rotation measures, this alone cannot explain the misalignments found between the radio core and optical VLBI polarization angles, \Delta\chi = |\chi_opt - \chi_0|. Comparison between \Delta\chi and (i) the orientation of \chi_0 relative to the jet direction, (ii) the degree of polarization of the core, (iii) a depolarization factor, (iv) the core rotation measures and (v) the core magnetic fields 1pc from the jet base do not yield evidence for any correlations between these properties. There is some evidence that the maximum observed \Delta\chi tends to decrease as the core-region magnetic field increases, suggesting that misalignments in \Delta\chi could be associated in part with relatively low core magnetic fields. Thus, although the overall distribution of \Delta\chi for all 40 sources in our sample does show a significant peak at \Delta\chi\sim0, it remains unclear what distinguishes these AGN cores from those showing appreciable misalignment between optical and VLBI-core polarization position angles. 
J. C. Algaba, D. C. Gabuzda, P. S. Smith
astro-ph.CO