2016A&A...592A...2P

Context. The XXL Survey is the largest survey carried out by the XMM-Newton satellite and covers a total area of 50 square degrees distributed over two fields. It primarily aims at investigating the large-scale structures of the Universe using the distribution of galaxy clusters and active galactic nuclei as tracers of the matter distribution. The survey will ultimately uncover several hundreds of galaxy clusters out to a redshift of ∼2 at a sensitivity of ∼10^–14erg/s/cm² in the [0.5-2]keV band.
Aims. This article presents the XXL bright cluster sample, a subsample of 100 galaxy clusters selected from the full XXL catalogue by setting a lower limit of 3x10^–14erg/s/cm² on the source flux within a 1' aperture.
Methods. The selection function was estimated using a mixture of Monte Carlo simulations and analytical recipes that closely reproduce the source selection process. An extensive spectroscopic follow-up provided redshifts for 97 of the 100 clusters. We derived accurate X-ray parameters for all the sources. Scaling relations were self-consistently derived from the same sample in other publications of the series. On this basis, we study the number density, luminosity function, and spatial distribution of the sample.
Results. The bright cluster sample consists of systems with masses between M₅₀₀=7x10¹³ and 3x10¹⁴M_☉, mostly located between z=0.1 and 0.5. The observed sky density of clusters is slightly below the predictions from the WMAP9 model, and significantly below the prediction from the Planck 2015 cosmology. In general, within the current uncertainties of the cluster mass calibration, models with higher values of σ₈ and/or Ω_M appear more difficult to accommodate. We provide tight constraints on the cluster differential luminosity function and find no hint of evolution out to z∼1. We also find strong evidence for the presence of large-scale structures in the XXL bright cluster sample and identify five new superclusters.