RXJ 1821.6+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 RXJ 1821.6+6827, or NEP 5281, at a redshift z=0.816+/-0.001. RXJ 1821.6+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 sigma_V=775(+182,-113) 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 F(2-10 keV)=1.24(+0.16,-0.23) x 10-13 erg cm2 s-1 and a K-corrected luminosity in the same band of L(2-10 keV)=6.32(+76,-0.73)x10^44 h_50^-2 erg s-1 (90% confidence level). The cluster X-ray bolometric luminosity is L(BOL,X)=1.35(+0.08,-0.21)x10^45 h_50^-2 erg s-1. 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_sun while the best fit X-ray temperature is T=4.7(+1.2,-0.7) keV. This emission weighted X-ray temperature is a little lower, barely within the uncertainties, than the predicted temperature, T=6.34(+0.13,-0.35) keV, from the L_X-T_X relation of local clusters published in the literature. The optically measured velocitydispersion 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. 
I. M. Gioia (1), A. Wolter (2) C. R. Mullis (3), J. P. Henry (4), H.   Boehringer (5), U. G. Briel (6) (1) (Istituto di Radi...