Title: Cosmology with XMM serendipitous clusters using a forward modelling method
The XMM Cluster Archive Super Survey (X-CLASS) is a serendipitously-detected X-ray-selected sample of 845 galaxy clusters based on 2774 XMM archival observations and covering approximately 90 square degrees spread across the high-Galactic latitude (|b|>20 deg) sky. We performed an extensive photometric redshift followup campaign with GROND, a seven channel (grizJHK) simultaneous imager on the MPG 2.2m telescope at the ESO La Silla Observatory. The median redshift of the sample was found to be z=0.39 with an accuracy of dz=0.02(1+z). The z-CR-HR method is a novel approach to interpreting cluster number counts. It was developed to simplify the cosmological analysis of X-ray cluster surveys and is based purely on directly observable quantities, namely the redshift (z), count rate (CR) and hardness ratio (HR). In this talk, I will provide a brief overview of the X-CLASS/GROND cluster catalogue and redshift followup. I will also present constraints on the cosmological parameters Omega_M and sigma8, along with the evolution of the X-ray scaling relations, derived with the z-CR-HR method. Beyond the results obtained for this sample, this study is a testing ground for aspects of the eROSITA cosmological analysis, in particular for the many low signal-to-noise clusters in the eROSITA All-sky Survey.