RX J1821.6+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 RX J1821.6+6827, or NEP 5281, at a redshift . 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 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 erg cm-2 s-1 and a K-corrected luminosity in the same band of erg s-1 (90% confidence level). The cluster X-ray bolometric luminosity is erg s-1 ( 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 while the best fit X-ray temperature is keV. This emission weighted X-ray temperature is a little lower, barely within the uncertainties, than the predicted temperature, keV, from the relation of local clusters published in the literature. The optically measured velocity dispersion is consistent with the velocity dispersion expected from the relationship. We also examine the point X-ray source RX J1821.9+6818, or NEP 5330, located to the south east of the cluster which was identified as a QSO at in the ROSAT NEP survey. The X-ray source is well fitted by an absorbed power law model with atoms cm-2 and a photon index typical of an active galactic nucleus.