Optical variability and morphology of AGN. Consequences for the future Gaia Celestial Reference Frame.

International audienceThe Gaia catalogue will contain 500 000 QSOs with very accurate optical coordinates, at level of some tens to some hundreds of micro - arcsecond. From these targets a core sample will be chosen, formed by those with the most precise positions, to define the Gaia Fundamental Reference Frame. It will be at the basis of an optical reference frame ( provisionally, the GCRF) more accurate than the current version of the (radio) ICRF. At the same time due care must be taken when choosing the core picks because quasars do are also known by their flux variability, on time scales that may span from hours u p to years. We will review some of the potential astrophysical processes that could induce these variations in the corresponding emission regions and we will present light curves that were obtained by small robotic telescopes during more than 1.5 years. We will also present high resolution images obtained at ground based optical telescopes together with the morphological analysis of the targets. Finally we will conclude by the presentation of an optical image database currently under construction in the frame of the ICRS - pc