RXJ1821.6+6827:: A cool cluster at z=0.81 from the ROSAT NEP survey

We present an analysis of the properties of the cluster of galaxies RXJ1821.6+6827, or NEP 5281, at a redshift z=0.816+/-0.001. RXJ1821.6+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-Hawaii (CFH) telescopes. The value for the cluster velocity dispersion is sigma(V)=775(-113)(+182) km s(-1). 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 2-10 keV energy band of F2-10 (keV)=1.24(-0.23)(+0.16)x10(-13) erg cm(-2) s(-1) and a K-corrected luminosity in the same band of L2-10 (keV)=6.32(-0.73)(+0.76)x10(44) h(50)(-2) erg s(-1) (90% confidence level). The cluster X-ray bolometric luminosity is L-BOL,L-X=1.35(-0.21)(+0.08)x10(45) h(50)(-2) erg s(-1) (L-BOL,L-X=1.17(-0.18)(+0.13)x10(45) h(70)(-2) erg s(-1) 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 0.28-1.42 Z(circle dot) while the best fit X-ray temperature is T=4.7(-0.7)(+1.2) keV. This emission weighted X-ray temperature is a little lower, barely within the uncertainties, than the predicted temperature, T=6.34(-0.35)(+0.13) keV, 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 sigma(V)-T-X relationship. We also examine the point X-ray source RXJ1821.9+6818, or NEP 5330, 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)(+3.1)x10(20) atoms cm(-2) and a photon index Gamma=1.67+/-0.12 typical of an active galactic nucleus.