Astrometric and photometric variability in quasars

International audienceQuasars are the choicest objects to define a quasi-inertial reference frame. At the same time they are active galactic nuclei powered by a massive black hole. As the astrometric precision approaches the limit set by the forthcoming GAIA mission, the astrometric stability can be investigated. Though the optical emission from the core region usually exceeds the other components by a factor of hundred, the variability of those components must surely imply in some measure of variability on the astrometric barycenter. To investigate the correlation between long term optical variability and what is dubbed as the random walk of the astrometric center, a program is being pursued at the WFI/ESO2.2m. A sample of quasars was selected by the large amplitude and long term optical variability. The observations are typically made every two months. The treatment is all differential, comparing the quasar position and brightness against a basket of selected stars for which the average relative distances and magnitudes remain constant. The provisional results for four objects bring strong support to the hypothesis of a degree of relationship between astrometric and photometric variability