Gaia initial QSO catalogue: The variability and compactness indexes

The manifold Gaia scientific output relies on precise astrometry accurate to submas standards. This depends on building a fundamental reference frame formed by pointlike, position stable, and allsky homogeneous grid points. In one word, quasars. The Gaia CU3 Initial Quasar Catalogue Working Package was established to beforehand produce one such list, although ultimately the satellite multiband photometry aided by astrometric monitoring has the potential to pick up a clean sample of quasars.