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.
