A detailed X-ray analysis of an XMM-Newton observation of the high-redshift (z=0.89) galaxy cluster ClJ1226.9+3332 is presented. The X-ray temperature is found to be 11.5{+1.1}{-0.9}keV, the highest X-ray temperature of any cluster at z>0.6. In contrast to MS1054-0321, the only other very hot cluster currently known at z>0.8, ClJ1226.9+3332 features a relaxed X-ray morphology, and its high overall gas temperature is not caused by one or several hot spots. The system thus constitutes a unique example of a high redshift, high temperature, relaxed cluster, for which the usual hydrostatic equilibrium assumption, and the X-ray mass is most reliable. A temperature profile is constructed (for the first time at this redshift) and is consistent with the cluster being isothermal out to 45% of the virial radius. Within the virial radius (corresponding to a measured overdensity of a factor of 200), a total mass of (1.4+/-0.5)*10^15 M_solar is derived, with a gas mass fraction of 12+/-5%. The bolometric X-ray luminosity is (5.3+/-0.2)*10^45 erg/s. The probabilities of finding a cluster of this mass within the volume of the discovery X-ray survey are 8*10^{-5} for Omega_M=1 and 0.64 for Omega_M=0.3, making Omega_M=1 highly unlikely. The entropy profile suggests that entropy evolution is being observed. The metal abundance (of Z=0.33{+0.14}{-0.10} Z_solar), gas mass fraction, and gas distribution are consistent with those of local clusters; thus the bulk of the metals were in place by z=0.89.
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