The cosmological analysis of X-ray cluster surveys - II. Application of the CR-HR method to the XMM archive

We have processed 2774 high galactic observations from the XMM archive (as of 2010 May) and extracted a serendipitous catalogue of some 850 clusters of galaxies based on purely X-ray criteria, following the methodology developed for the XMM-Large-Scale Survey. Restricting the sample to the highest signal-to-noise ratio objects (347 clusters), we perform a cosmological analysis using only the X-ray information. The analysis consists in the modelling of the observed colour-magnitude [count rate and hardness ratio (CR-HR)] diagram constructed from cluster instrumental count rates measured in the [0.5-2], [1-2] and [0.5-1] keV bands. A Monte Carlo Markov chain procedure simultaneously fits the cosmological parameters, the evolution of the cluster scaling laws and the selection effects.

Our results are consistent with the σ₈ and Ω_m values obtained by the 5-year Wilkinson Microwave Anisotropy Probe (WMAP5) and point towards a negative evolution of the cluster scaling relations with respect to the self-similar expectation. We are further able to constrain the cluster fractional radius x_c,0=r_c/R_500c to x_{c, 0}= 0.24 ± 0.04. This study stresses again the critical role of selection effects in deriving cluster scaling relations, even in the local universe. Finally, we show that the CR-HR method applied to the eRosita all-sky survey - provided that cluster photometric redshifts are available - will enable the determination of the equation of state of the dark energy at the level of the Dark Energy Task Force (DETF) stage IV predictions; simultaneously, the evolution of the cluster scaling relations will be unambiguously determined.

The XMM CLuster Archive Super Survey (X-CLASS) serendipitous cluster catalogue is available online at http://xmm-lss.in2p3.fr:8080/l4sdb/ .