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Astron. Astrophys. Suppl. Ser. 144, 187-194

An X-ray and optical study of the cluster A33

S. Colafrancesco 1 - C.R. Mullis 2 - A. Wolter 3 - I.M. Gioia 2,10,11 - T. Maccacaro 3 - A. Antonelli 1 - F. Fiore 1 - J. Kaastra 4 - R. Mewe 4 - Y. Rephaeli 5 - R. Fusco-Femiano 6 - V. Antonuccio-Delogu 7 - F. Matteucci 8 - P. Mazzotta 9

Send offprint request: S. Colafrancesco

1 - Osservatorio Astronomico di Roma via dell'Osservatorio 2, I-00040 Monteporzio, Italy
2 - Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, U.S.A.
3 - Osservatorio Astronomico di Brera, Via Brera 26, Milano, Italy
4 - SRON, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
5 - School of Physics and Astronomy, Tel Aviv University, 69978, Israel
6 - IAS - CNR, Via Fosso del Cavaliere, I-00133 Roma, Italy
7 - Osservatorio Astrofisico di Catania, Via A. Doria, Catania, Italy
8 - Osservatorio Astronomico di Trieste, Via dell'Osservatorio, Trieste, Italy
9 - Dipartimento di Fisica, Università di Roma "Tor Vergata'', Via della Ricerca Scientifica 1, I-00133 Roma, Italy
10 - Home institution: Istituto di Radioastronomia del CNR, Via Gobetti 101, I-40129 Bologna, Italy
11 - Visiting Astronomer at the W.M. Keck Observatory, jointly operated by the California Institute of Technology, the University of California and the National Aereonautics and Space Administration

Received February 18, 1999; accepted January 4, 2000


We report the first detailed X-ray and optical observations of the medium-distant cluster A33 obtained with the Beppo-SAX satellite and with the UH 2.2 m and Keck II telescopes at Mauna Kea. The information deduced from X-ray and optical imaging and spectroscopic data allowed us to identify the X-ray source 1SAXJ0027.2-1930 as the X-ray counterpart of the A33 cluster. The faint, $F_{2-10~{\rm keV}} \approx 2.4~10^{-13}$ erg s-1 cm-2, X-ray source 1SAXJ0027.2-1930, $\sim 2$ arcmin away from the optical position of the cluster as given in the Abell catalogue, is identified with the central region of A33. Based on six cluster galaxy redshifts, we determine the redshift of A33, z=0.2409; this is lower than the value derived by Leir & Van Den Bergh (1977). The source X-ray luminosity, $L_{2-10~{\rm keV}} = 7.7~10^{43}$ erg s-1 cm-2, and intracluster gas temperature, T = 2.9 keV, make this cluster interesting for cosmological studies of the cluster $L_{\rm X}-T$ relation at intermediate redshifts. Two other X-ray sources in the A33 field are identified. An AGN at z=0.2274, and an M-type star, whose emissions are blended to form an extended X-ray emission $\sim 4$ arcmin north of the A33 cluster. A third possibly point-like X-ray source detected $\sim 3$ arcmin north-west of A33 lies close to a spiral galaxy at z=0.2863 and to an elliptical galaxy at the same redshift as the cluster.

Key words: galaxies clusters: individual: A33 -- Cosmology: observations -- galaxies: active observations: X-rays -- intergalactic medium

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