ROSAT HRI AND ASCA OBSERVATIONS OF THE MOST LUMINOUS X-RAY-CLUSTER RXJ1347.5-1145/

We report on the X-ray properties of the exceptional X-ray cluster RX J1347.5-1145 at z=0.451. We confirm that it is with a luminosity L(X)( bol) = 2 x 10(46) erg/s the most luminous X-ray cluster discovered to date. The mass of the cluster within 1.7 Mpc is 9.8 x 10(14) M.. A com parison of the central X-ray mass and the mass determined from a simpl e gravitational lens model shows a discrepancy of a factor of 2-3 with the X-ray mass being smaller. The temperature of the cluster is 9.3(- 1.10)(+1.1) keV. We detect a strong FeK line corresponding to a metall icity of 0.33+/-0.10 in solar units, which is an unexpectedly high val ue for a distant and hot cluster. There are several hints that the clu ster contains an extremely strong cooling flow. With the usual assumpt ions we derive formally a mass accretion rate of more than 3000 M./yr indicating that this may be the largest cooling flow detected so far. To find these extreme properties in this distant cluster which can be taken as an indication of a well relaxed and old system is of high imp ortance for the theory of formation and evolution of clusters.