We present X-ray observations of two high redshift clusters of galaxies carried out with the BeppoSAX satellite. One cluster, RXJ0152.7-1357 at z similar or equal to 0.83, was selected from the ROSAT Deep Cluster Survey sample, as one of the most X-ray luminous systems known at z > 0.5. The optical and ROSAT-PSPC data show a complex morphology with at least two cores. Our SAX observations yield a gas temperature kT = 6.46(-1.19)(+1.74) keV and a metallicity A = 0.53(-0.24)(+0.29), with a prominent iron K-alpha line. The second cluster, MS 2053.7-0449 at z similar or equal to 0.58, was selected from the EMSS sample. Given the poor statistics no constraints on the metallicity can be derived fi om the present data. Large uncertainties are associated to the gas temperature (kT = 6.7(-2.3)(+6.8) keV), which has been obtained after fixing the abundance to 0.3 solar. Combining these results with those obtained for similarly high redshift (z = 0.5) clusters of galaxies with broad band X-ray spectra, we discuss the high redshift L-bol - T relationship. The data can easily be accommodated with a non-evolving (or mildly evolving) L-bol - T relation, a result which, when combined with the little observed evolution in the bulk of the X-ray cluster population, gives support to low Omega cosmological models
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