We report on the discovery of the X-ray luminous cluster XMMU J100750.5+125818 at redshift 1.082 based on 19 spectroscopic members, which displays several strong lensing features. S ED modeling of the lensed arc features from multicolor imagi ng with the VLT and the LBT reveals likely redshifts ∼2.7 for the most prominent of the lensed background galaxies . Ma s estimates are derived for different radii from the velocity dispersion of the cluster memb rs,M200 ≃ 1.8 × 1014 M⊙, from the X-ray spectral parameters, M500 ≃ 1.0× 1014 M⊙, and the largest lensing arc, MSL ≃ 2.3× 1013 M⊙. The projected spatial distribution of cluster galaxies ap pears to be elongated, and the brightest galaxy lies o ff center with respect to the X-ray emission indicating a not ye t relaxed structure. XMMU J100750.5+125818 offers excellent diagnostics of the inner mass distribution of a distant cluster with a combination of strong and weak lensing, optical and X-ray spectroscopy.

We investigate the scientific impact of the Wide Field X-ray Telescope mission. We present simulated images and spectra of X-ray sources as observed from the three surveys planned for the nominal 5-year WFXT lifetime. The goal of these simulations is to provide WFXT images of the extragalactic sky in different energy bands based on accurate description of AGN populations, normal and star forming galaxies, groups and clusters of galaxies. The images are realized using a detailed PSF model, instrumental and physical backgrounds/foregrounds, accurate model of the effective area and the related vignetting effect. Thanks to this comprehensive modelization of the WFXT properties, the simulated images can be used to evaluate the flux limits for detection of point and extended sources, the effect of source confusion at very faint fluxes, and in general the efficiency of detection algorithms. We also simulate the spectra of the detected sources, in order to address specific science topics which are unique to WFXT. Among them, we focus on the characterization of the Intra Cluster Medium (ICM) of high-z clusters, and in particular on the measurement of the redshift from the ICM spectrum in order to build a cosmological sample of galaxy clusters. The end-to-end simulation procedure presented here, is a valuable tool in optimizing the mission design. Therefore, these simulations can be used to reliably characterize the WFXT discovery space and to verify the connection between mission requirements and scientific goals. Thanks to this effort, we can conclude on firm basis that an X-ray mission optimized for surveys like WFXT is necessary to bring X-ray astronomy at the level of the optical, IR, submm and radio wavebands as foreseen in the coming decade.

We report on the discovery of the X-ray luminous cluster XMMU J100750.5+125818 at redshift 1.082 based on 19 spectroscopic members, which displays several strong lensing features. SED modeling of the lensed arc features from multicolor imaging with the VLT and the LBT reveals likely redshifts ~2.7 for the most prominent of the lensed background galaxies. Mass estimates are derived for different radii from the velocity dispersion of the cluster members, M_200 ~ 1.8 10^{14} Msun, from the X-ray spectral parameters, M_500 ~ 1.0 10^{14} Msun, and the largest lensing arc, M_SL ~ 2.3 10^{13} Msun. The projected spatial distribution of cluster galaxies appears to be elongated, and the brightest galaxy lies off center with respect to the X-ray emission indicating a not yet relaxed structure. XMMU J100750.5+125818 offers excellent diagnostics of the inner mass distribution of a distant cluster with a combination of strong and weak lensing, optical and X-ray spectroscopy.

Statistical studies of ROSAT selected cluster samples at z 1. We are currently engaged in the XMM-Newton Distant Cluster Project (XDCP) [1], a serendipitous X-ray survey with the goal to construct a statistically complete sample of at least 30 galaxy clusters at z>1 for evolutionary and cosmological studies. In these proceedings we show that the prospects of finding very distant cooling core clusters are very promising for ongoing XMM surveys if these systems exist at lookback times of more than 8 Gyrs.

We report on an ongoing search for distant clusters of galaxies in archival XMM-Newton observations. The aim of the survey is to establish a sample of similar to 30 clusters at redshifts z > 1. Here we describe the strategy of the survey from X-ray source detection to optical follow up observations of distant cluster candidates. First results of a pilot survey and the discovery of the most distant X-ray selected cluster at z=1.4 are presented.