An XMM-Newton+HST study of the likely most X-ray luminous z>=0.9 galaxy cluster

We propose XMMNewton+HST observations of the new, extremely Xray luminouscluster ClGJ120959.0+495352 (z0.90), discovered through our combined search ofRASS and SDSS data. Optical and SZ data suggest that it is an excellentcandidate for a very massive, dynamically relaxed coolingcore cluster, whichare extremely rare at high z. The XMMNewton data will provide the firstresolved Xray imaging of the system to confirm its dynamical state andconstrain both the temperature profile and global cluster properties. The HSTdata will provide complementary constraints on the cluster mass and dynamicalstate from strong and weak lensing, enabling the comparison to scaling relationsand the assessment if the cluster.s discovery is in line with standard cosmological expectations. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]