Search for correlations between the optical and radio polarization of active galactic nuclei - II. VLBA polarization data at 12+15+22+24+43 GHz

Abstract

Previous research showed that most BL Lac objects and some quasars have aligned very long baseline interferometry (VLBI) core and optical polarizations, although some of the active galactic nuclei (AGNs) also showed no obvious relationship between their VLBI core and optical polarization angles. This may indicate that some AGNs have cospatial regions of optical and radio emission, while others do not. However, another possibility is that some of the VLBI cores had Faraday rotations of the order of several tens of thousands of rad m−2, which were not properly fitted using the three-frequency data due to nπ 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 χ0. With this is mind, we obtained additional 12+15+22+24+43 GHz plus optical observations for eight of 40 AGNs previously considered, enabling improved sampling of Faraday effects. 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, Δχ= |χopt−χ0|. Comparisons between Δχ and (i) the orientation of χ0 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 1 pc from the jet base do not yield evidence for any correlations between these properties. There is, however, some evidence that the maximum observed Δχ tends to decrease as the core-region magnetic field increases, suggesting that large misalignments in Δχ could be associated in part with relatively low core magnetic fields. Thus, although the overall distribution of Δχ for all 40 sources in our sample does show a significant peak at Δχ∼ 0, it remains unclear what distinguishes these AGN cores from those showing appreciable misalignment between optical and VLBI core polarization position angles.