An X - ray and optical study of the cluster A 33

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 2.2m 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 1SAXJ0027.21930 as the X-ray counterpart of the A33 cluster. The faint, F2−10 keV ≈ 2.4×10 −13 erg s cm, X-ray source 1SAXJ0027.2-1930, ∼ 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 and Van Den Bergh (1977). The source X-ray luminosity, L2−10 keV = 7.7 × 10 43 erg s, and intracluster gas temperature, T = 2.9 keV, make this cluster interesting for cosmological studies of the cluster LX − 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 emission are blended to form an extended X-ray emission ∼ 4 arcmin north of the A33 cluster. A third possibly point-like X-ray source detected ∼ 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. Send offprint requests to: S. Colafrancesco