Rosati, P.Tozzi, P.Ettori, S.Mainieri, V.Demarco, R.Stanford, S. A.Lidman, C.Nonino, M.Borgani, S.Della Ceca, R.Eisenhardt, P.Holden, B. P.Norman, C.2025-06-112025-06-110004-6256http://www.scopus.com/inward/record.url?scp=1642585788&partnerID=8YFLogxKhttps://hdl.handle.net/1885/733758330We present deep Chandra and XMM-Newton observations of the galaxy cluster RDCS 1252.9-2927, which was selected from the ROSAT Deep Cluster Survey (RDCS) and confirmed by extensive spectroscopy with the Very Large Telescope at redshift z = 1.237. With the Chandra data, the X-ray emission from the intracluster medium is well resolved and traced out to 500 kpc, thus allowing a measurement of the physical properties of the gas with unprecedented accuracy at this redshift. We detect a clear 6.7 keV iron K line in the Chandra spectrum providing a redshift within 1% of the spectroscopic one. By augmenting our spectroscopic analysis with the XMM-Newton data (MOS detectors only), we significantly narrow down the 1 σ error bar to 10% for the temperature and 30% for the metallicity, with best-fit values kT = 6.0-0.5 -0.7 keV, Z = 0.36-0.10+0.12 Z ⊙. On the likely hypothesis of hydrostatic equilibrium, we measure a total mass of M500 = (1.9 ± 0.3) × 10 14 h70-1 M⊙ within R Δ=500 ≃ 536 kpc. Overall, these observations imply that RDCS 1252.9-2927 is the most X-ray luminous and likely the most massive bona fide cluster discovered to date at z > 1. When combined with current samples of distant clusters, these data lend further support to a mild evolution of the cluster scaling relations, as well the metallicity of the intracluster gas. Inspection of the cluster mass function in the current cosmological concordance model (h, Ωm, ΩΛ) = (0.7, 0.3, 0.7) and σg = 0.7-0.8 shows that RDCS 1252.9-2927 is an M* cluster at z = 1.24, in keeping with number density expectations in the RDCS survey volume.9enCosmology: observationsGalaxies: clusters: generalGalaxies: clusters: individual (RDCS 1252.9-2927)X-rays200410.1086/3798571642585788