2004A&A...428..867G

We present an analysis of the properties of the cluster of galaxies RX J1821.6+6827, or NEP5281, at a redshift z=0.816 ±0.001. RX J1821.6+6827 was discovered during the optical identification of the X-ray sources in the North Ecliptic Pole (NEP) region of the ROSAT All-Sky Survey and it is the highest redshift cluster of galaxies of the NEP survey. We have measured spectroscopic redshifts for twenty cluster galaxies using the Keck-I and the Canada-France-Hawai'i (CFH) telescopes. The value for the cluster velocity dispersion is σ_V=775⁺¹⁸²_–113km/s. The cluster was also observed by XMM-Newton. Both the optical and X-ray data are presented in this paper. The cluster has an unabsorbed X-ray flux in the 2-10keV energy band of F_2–10keV=1.24^+0.16_–0.23x10^–13erg/cm²/s and a K-corrected luminosity in the same band of L_2-10 keV_=6.32-0.73+0.76^x1044^h-2^₅₀erg/s (90% confidence level). The cluster X-ray bolometric luminosity is L_BOL,X=1.35^+0.08_–0.21x10⁴⁵h₅₀^–2erg/s (L_BOL,X=1.17^+0.13_–0.18x10⁴⁵h₇₀^–2erg/s in the concordance cosmology). The data do not allow fitting both metal abundance and temperature at the same time. The abundance is unconstrained and can vary in the range 0.28-1.42Z_☉ while the best fit X-ray temperature is T=4.7^+1.2_–0.7keV. This emission weighted X-ray temperature is a little lower, barely within the uncertainties, than the predicted temperature, T=6.34^+0.13_–0.35keV, from the L_X-T_X relation of local clusters published in the literature. The optically measured velocity dispersion is consistent with the velocity dispersion expected from the σ_V-T_X relationship. We also examine the point X-ray source RX J1821.9+6818, or NEP5330, located to the south east of the cluster which was identified as a QSO at z=1.692±0.008 in the ROSAT NEP survey. The X-ray source is well fitted by an absorbed power law model with N_H=4.1_–3.0^+3.1x10²⁰atoms/cm² and a photon index Γ=1.67±0.12 typical of an active galactic nucleus.