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 \pm 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 ^ { \prime } 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 } \times 10 ^ { -13 } erg cm ^ { -2 } s ^ { -1 } and a K-corrected luminosity in the same band of L _ { 2 - 10 keV } = 6.32 _ { -0.73 } ^ { +0.76 } \times 10 ^ { 44 } h ^ { -2 } _ { 50 } erg s ^ { -1 } ( 90 % confidence level ) .
The cluster X-ray bolometric luminosity is L _ { BOL,X } = 1.35 ^ { +0.08 } _ { -0.21 } \times 10 ^ { 45 } h _ { 50 } ^ { -2 } erg s ^ { -1 } ( L _ { BOL,X } = 1.17 ^ { +0.13 } _ { -0.18 } \times 10 ^ { 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 _ { \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 velocity dispersion is consistent with the velocity dispersion expected from the \sigma _ { V } - T _ { X } relationship .
We also examine the point X-ray source RXJ 1821.9 + 6818 , or NEP 5330 , located to the south east of the cluster which was identified as a QSO at z = 1.692 \pm 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 } \times 10 ^ { 20 } atoms cm ^ { -2 } and a photon index \Gamma = 1.67 \pm 0.12 typical of an active galactic nucleus .