Correlation of long-term optical color variability of radio-loud quasars with their VLBI astrometric characteristics

Aims. The International Celestial Reference Frame (ICRF) is realized through geodetic very long baseline interferometry (VLBI) positions of thousands of extragalactic objects, mainly radio-loud active galactic nuclei. As previous studies have suggested a link between the optical variability of these sources and optical-radio position offsets and variability, we want to explore the possible relationships between these astrometric quantities and the color variability that characterizes VLBI sources. Methods. We computed the BP/RP and color-magnitude correlations of 2584 ICRF sources using the Gaia DR3 photometric time series at the GBP (BP), GRP (RP), and G bands, and we compared them with astrometric properties (Gaia-VLBI optical-radio offsets and positional variability derived from VLBI coordinate time series). We also searched for possible biases coming from the length of the photometric series, the magnitude, the redshift, and the jet viewing angle when available as well as correlations between the color variability and the central engine properties (black hole mass and accretion luminosity). Results. We find that the most astrometrically stable sources and the sources with smaller optical-radio shifts have lower values of BP/RP correlation and low color-magnitude correlations, corresponding to the undefined-when-brighter (or redder-stable-when-brighter and bluer-stable-when-brighter) classes of optical variability. These preferred astrometric objects often belong to the spectral class FSRQ and have a z of about one and higher. We found a significant correlation for both of the astrometric stability measures with the jet viewing angle. The shorter optical-radio offsets occur in active galactic nuclei that contain more massive black holes and more luminous accretion disks, which may be caused by the tendency of smaller viewing angles to be associated with higher redshifts. Conclusions. Our results strengthen the hypothesis that the Doppler boosting effect on luminosity and geometry is the driving physical mechanism at the origin of the observed optical-radio position offsets and the astrometric variability suggested by previous studies. The results define additional predictors of absolute astrometric performance of reference frame candidates, and they supplement the optical variability criterion. Moreover, this work demonstrates that the relatively short time span (a few years) and irregular sampling of the Gaia light curves are sufficient to predict the astrometric stability of new reference frame objects. This study also shows that absolute astrometry with the geodetic VLBI is a valuable tool to gain new insight into the physical processes responsible for quasar activity in the inner parts of the relativistic engines.