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The radio-sources observed by VLBI are jetted Active Galactic Nuclei [AGN]. Most of them show a perceptible variability in their astrometric position time series (generally in the range of 0.1-1 milli-arcsecond). Such variability may be due to extrinsic effects to the source (instrumental, from the antenna network or geophysical) or may be caused by astrophysical phenomena affecting the central VLBI region of those objects, such as radio components moving from the main radio core along the jet or the influence of a supermassive binary black hole system. In order to learn more, we conducted an analysis of the direction(s) on the sky extracted from the signal in the source position time series. This analysis revealed that the astrometric variability generally happens along at least one preferred direction. However, for some sources two directions are distinguished. In parallel, Gaia provides optical astrometric position with an accuracy equivalent to VLBI, enabling to investigate radio-optical position offset. Comparing directions derived from radio position time series with those derived from radio-optical position offsets brings further insights into AGN astrophysical phenomena within AGN. In the future, understanding the underlying physics of AGN will be essential for the realisation of future celestial frames because of the need to identify even more stable fiducial marks on the celestial sphere.

Linking VLBI astrometric measurements of extragalactic radio-sources to astrophysical phenomena