Abstract. We describe XMM-Newton Guaranteed Time observations of a sample of eight high redshift (0.45 < z < 0.62) clusters. The goal of these observations was to measure the luminosity and the temperature of the clusters to a precision of ∼10%, leading to constraints on the possible evolution of the luminosity-temperature (L x − T x ) relation, and ultimately on the values of the matter density, Ω M , and, to a lesser extent, the cosmological constant Ω Λ . The clusters were drawn from the SHARC and 160 Square Degree (160SD) ROSAT surveys and span a bolometric (0.0-20 keV) luminosity range of 2.0 to 14.4×10 44 erg s −1 (H 0 = 50, Ω M = 1, Ω Λ = 0). Here we describe our data analysis techniques and present, for the first time with XMM-Newton, a L x − T x relation. For each of the eight clusters in the sample, we have measured total (r < r virial ) bolometric luminosities, performed β-model fits to the radial surface profiles and made spectral fits to a single temperature isothermal model. We describe data analysis techniques that pay particular attention to background mitigation. We have also estimated temperatures and luminosities for two known clusters (Abell 2246 and RX J1325.0−3814), and one new high redshift cluster candidate (XMMU J084701.8+345117), that were detected off-axis. Characterizing the (2002) using a compilation of Chandra observations of 0.39 < z < 1.26 clusters. We conclude that there is now evidence from both XMM-Newton and Chandra for an evolutionary trend in the L x − T x relation. This evolution is significantly below the level expected from the predictions of the self-similar model for an Ω Λ = 0.0, Ω M = 1.0, cosmology, but consistent with self-similar model in an Ω Λ = 0.7, Ω M = 0.3 cosmology. Our observations lend support to the robustness and completeness of the SHARC and 160SD surveys.

Abstract. The evolution with redshift of the temperature-luminosity relation of X-ray galaxy clusters is a key ingredient to break degeneracies in the interpretation of X-ray clusters redshift number counts. We therefore take advantage of the recent measurements of the temperature-luminosity relation of distant clusters observed with XMM-Newton and Chandra satellites to examine theoretical number counts expected for different available X-rays cluster samples, namely the RDCS, EMSS, SHARC, 160 deg 2 and the MACS at redshift greater than 0.3. We derive these counts without any adjustment, using models previously normalized to the local (z ∼ 0.05) temperature distribution function (TDF) and to the high-z (z ∼ 0.33) TDF. We find that these models having Ω M in the range [0.85-1.] predict counts in remarkable agreement with the observed counts in the different samples. We illustrate that this conclusion is weakly sensitive to the various ingredients of the modeling. Therefore number counts provide a robust evidence of an evolving population. A realistic flat low density model (Ω M = 0.3), normalized to the local abundance of clusters is found to overproduce cluster abundance at high redshift (above z ∼ 0.5) by nearly an order of magnitude. This result is in conflict with the popular concordance model. The conflict could indicate a deviation from the expected scaling of the M − T relation with redshift.