The XMM-NEWTON Ω project. I. The X-ray luminosity-temperature relation at z>0.4

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 × 10⁴⁴ ergs (H₀=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 L_x-T_x relation as L_x = L₆ ((T)/(6 keV))^α, we find L₆=15.9 ^+7.6_-5.2 × 10⁴⁴ ergs and α =2.7±0.4 for an Ω_Λ=0.0, Ω_M =1.0, H₀=50 km s^-1 Mpc^-1 cosmology at a typical redshift z ∼ 0.55. Comparing with the low redshift study by Markevitch (1998), we find α to be in agreement, and assuming L_x-T_x to evolve as (1+z)^{A }, we find A=0.68±0.26 for the same cosmology and A= 1.52^+0.26_-0.27 for an Ω_Λ=0.7, Ω_M=0.3 cosmology. Our A values are very similar to those found previously by Vikhlinin et al. (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.

Table \ref{tab:L} and Appendix are only available in electronic form at http://www.edpsciences.org