We present the first results from an XMM-Newton observation of RX J1347.5 - 1145 ( z=0.451 ) , the most luminous X-ray cluster of galaxies currently known , with a luminosity L _ { X } = 6.0 \pm 0.1 \times 10 ^ { 45 } \mbox { erg s } ^ { -1 } in the [ 2-10 ] keV energy band .
The cluster has an overall temperature of kT = 10.0 \pm 0.3 keV and is not isothermal : the temperature profile shows a decline in the outer regions and a drop in the centre , indicating the presence of a cooling core .
The spectral analysis identifies a hot region at radii 50-200 kpc to south-east of the main X-ray peak , at a position consistent with the subclump seen in the X-ray image .
Excluding the data of the south-east quadrant , the cluster appears relatively relaxed and we estimate a total mass within 1 Mpc of 1.0 \pm 0.2 \times 10 ^ { 15 } M _ { \odot } .