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 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_-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 ×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 ×10⁴⁴ h^-2₅₀ erg s^-1 (90% confidence level). The cluster X-ray bolometric luminosity is L_BOL,X = 1.35^+0.08_-0.21 ×10⁴⁵ h₅₀^-2 erg s^-1 (L_BOL,X = 1.17^+0.13_-0.18 ×10⁴⁵ h₇₀^-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_⊙ 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 RX J1821.9+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^+3.1×10²⁰ atoms cm^-2 and a photon index Γ=1.67±0.12 typical of an active galactic nucleus.

Data presented here used the XMM-Newton facility.