We report on the XMM-Newton observation of RX J1053.7+5735, one of the most distant (z = 1.26) X-ray selected clusters of galaxies, which also shows an unusual double-lobed X-ray morphology, indicative of possible cluster-cluster interaction. The cluster was discovered during our ROSAT deep pointings in the direction of the Lockman Hole. The XMM-Newton observations were performed with the European Photon Imaging Camera (EPIC) during the performance verification phase. Total effective exposure time was ∼100 ksec. The best fit temperature based on a simultaneous fit of spectra from all EPIC cameras (pn+MOS) is 4.9-0.9+1.5 keV. Metallicity is poorly constrained even using the joint fit of all spectra, with an upper limit on the iron abundance of 0.62 solar. Using the best fit model parameters, we derived an unabsorbed (0.2-10) keV flux of f0.2-10 = 3.0 × 10-14 erg cm-2 s-1, corresponding to a bolometric luminosity of Lbol = 3.4 × 1044 h50-2 erg s-1. Despite the fact that it was observed at fairly large off-axis angle, the temperature errors are much smaller compared with those of typical measurements based on ASCA or Beppo-Sax observations of high-z (z > 0.6) clusters, demonstrating the power of the XMM for determining the X-ray temperature for high-z clusters. The measured temperature and luminosity show that one can easily reach the intrinsically X-ray faint and cool cluster regime comparable with those of z ∼ 0.4 clusters observed by past satellites. The new cluster temperature and Lbol we have measured for RX J1053.7+5735 is consistent with a weak/no evolution of the Lbol-Tx relation out to z ∼ 1.3, which lends support to a low Ωm universe, although more data-points of z > 1 clusters are required for a more definitive statement. The caution has to be also exercised in interpreting the result, because of the uncertainty associated with the dynamical status of this cluster.
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