The XXL survey: first results and future
  The XXL survey currently covers two 25 sq. deg. patches with XMM observations
of ~10ks. We summarise the scientific results associated with the first release
of the XXL data set, that occurred mid 2016. We review several arguments for
increasing the survey depth to 40 ks during the next decade of XMM operations.
X-ray (z<2) cluster, (z<4) AGN and cosmic background survey science will then
benefit from an extraordinary data reservoir. This, combined with deep
multi-$\lambda$ observations, will lead to solid standalone cosmological
constraints and provide a wealth of information on the formation and evolution
of AGN, clusters and the X-ray background. In particular, it will offer a
unique opportunity to pinpoint the z>1 cluster density. It will eventually
constitute a reference study and an ideal calibration field for the upcoming
eROSITA and Euclid missions.
