Abstract
ABSTRACT Significant positional offsets of the value from 1 mas to more than 10 mas were found previously between radio (VLBI) and optical (Gaia) positions of active galactic nuclei (AGNs). They happen preferentially parallel to the parsec-scale jet direction. AGNs with VLBI-to-Gaia offsets pointed downstream the jet are found to have favourably higher optical polarization, as expected if extended optical jets dominate in the emission and shift the Gaia centroid away from the physical nucleus of the source. Upstream offsets with the suggested domination of accretion discs manifest themselves through the observed low optical polarization. Direction of linear optical polarization is confirmed to preferentially align with parsec-scale jets in AGNs with dominant jets consistent with a toroidal magnetic field structure. Our findings support the disc–jet interpretation of the observed positional offsets. These results call on an intensification of AGN optical polarization monitoring programs in order to collect precious observational data. Taken together with the continued VLBI and Gaia observations, they will allow researchers to reconstruct detailed models of the disc–jet system in AGNs on parsec scales.
Highlights
An analysis of the disk-jet system on parsec and sub-parsec scales is important for understanding nature of the central engine in active galactic nuclei
We have found that active galactic nuclei (AGN) with VLBI-Gaia offsets upstream in the jet have significantly lower fractional linear optical polarization than
Optical polarization of AGN with VLBI-Gaia offsets L5 the downstream ones. This is expected within the proposed scenario: optical emission with the centroid close to the AGN nucleus corresponds to the dominance of an accretion disk and if the Gaia centroid shifts down the parsec-scale jet, the jet emission dominates
Summary
An analysis of the disk-jet system on parsec and sub-parsec scales is important for understanding nature of the central engine in active galactic nuclei. BL Lacs do not solely drive the found difference between optical polarization fraction for upstream and downstrem offsets in the “All AGN” sample This difference is confirmed with the probability of significance 0.998 if we analyze the sample of quasars only. It is worth noting that the collected list of sources with archival polarimetric data (section 2) may be biased towards objects with higher degree of optical polarization. For this reason we repeated our analysis for the γ-ray loud and the γ-ray quiet samples of RoboPol separately. This analysis confirmed our findings, even though the found differences are less significant due to smaller sample sizes
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