EDP Sciences
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Volume 428, Number 3, December IV 2004
Page(s) 867 - 875
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20041426

A&A 428, 867-875 (2004)
DOI: 10.1051/0004-6361:20041426

RX J1821.6+6827: A cool cluster at z = 0.81 from the ROSAT NEP survey

I. M. Gioia1, A. Wolter2, C. R. Mullis3, J. P. Henry4, H. Böhringer5 and U. G. Briel5

1  Istituto di Radioastronomia CNR, via P. Gobetti 101, 40129 Bologna, Italy
    e-mail: gioia@ira.cnr.it
2  INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
    e-mail: anna@brera.mi.astro.it
3  European Southern Observatory, Karl-Schwarzschild-Str. 2, Garching bei München 85740, Germany
    e-mail: cmullis@eso.org
4  Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
    e-mail: henry@ifa.hawaii.edu
5  Max-Planck-Institut fur Extraterrestrische Physik, Giessenbachstrasse, Postfach 1312, Garching 85741, Germany
    e-mail: hxb@xray.mpe.mpg.de,ugb@xray.mpe.mpg.de

(Received 7 June 2004 / Accepted 30 July 2004 )

We present an analysis of the properties of the cluster of galaxies RX J1821.6+6827, or NEP 5281, at a redshift $z= 0.816 \pm0.001$. 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 $\sigma_{\rm V}=775^{+182}_{-113}$ 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 $F_{2{-}10~{\rm ~keV}} = 1.24^{+0.16}_{-0.23} \times10^{-13}$ erg cm -2 s -1 and a K-corrected luminosity in the same band of $L_{2{-}10~{\rm ~keV}} = 6.32_{-0.73}^{+0.76} \times10^{44}~h^{-2}_{50}$ erg s -1 (90% confidence level). The cluster X-ray bolometric luminosity is $L_{\rm BOL,X} = 1.35^{+0.08}_{-0.21} \times10^{45}~h_{50}^{-2}$ erg s -1 ( $L_{\rm BOL,X} = 1.17^{+0.13}_{-0.18} \times10^{45}~h_{70}^{-2}$ 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  $Z_{\odot}$ 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 $L_{\rm X}-T_{\rm X}$ relation of local clusters published in the literature. The optically measured velocity dispersion is consistent with the velocity dispersion expected from the $\sigma_{\rm V}-T_{\rm X}$ 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 $z= 1.692 \pm0.008$ in the ROSAT NEP survey. The X-ray source is well fitted by an absorbed power law model with $N_{\rm H}=4.1_{-3.0}^{+3.1}\times10^{20}$ atoms cm -2 and a photon index $\Gamma=1.67\pm0.12$ typical of an active galactic nucleus.

Key words: galaxies: clusters: general -- galaxies: clusters: individual: RX J1821.6+6827 -- X-rays: general -- X-rays: individuals: RX J1821.6+6827 -- X-rays: galaxies: clusters -- galaxies: intergalactic medium

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