A33 is a medium-distant Abell cluster of galaxies with very few and sparse information in both the X-ray and the optical bands. This cluster was claimed to have been detected by the HEAO1-A1 all sky survey (Johnson et al. 1983; Kowalski et al. 1984) with a count rate of $3.77~\pm~0.47$ counts cm^-2 s^-1 in the 2-6 keV energy band. Its luminosity was estimated, with large uncertainties, to be $L_{2-6~{\rm keV}} \approx 2.34~10^{45}$ erg s^-1.

A33 was also observed with the GINGA LAC detector from December 9 to December 10, 1988 (Arnaud et al. 1991), but no X-ray emission was found at the optical position of the cluster. From such a non-imaging observation, Arnaud et al. (1991) were able to put an upper limit on the luminosity of A33, $L_{2-10~{\rm keV}} < 6~10^{44}$ erg s^-1, assuming a temperature T = 8.4 keV. The value of the X-ray luminosity derived from GINGA data is inconsistent with the one derived from the HEAO1-A1 observation (note, however, that A33 lies at the edge of the error box for the position of the HEAO1 source).

The source 1RXSJ002709.5-192616 in the ROSAT Bright Source Catalog (BSC: Voges et al. 1996), at coordinates $\alpha^x_{2000}= 00^{\rm h}27^{\rm m}09.50^{\rm s}$ and $\delta^x_{2000}=-19^\circ 26'16''$ , has been observed for 317 s with a count rate of $0.062 \pm$ 0.017 cts/s. This source has 19.6 net counts in the 0.1- 2.4 keV energy band corresponding to a flux $F_{0.2-2.4}=(9.3~\pm~2.6)~10^{-13}$ erg s^-1 cm^-2(assuming a nominal conversion factor of 1.5 10^-11 erg cm^-2 s^-1 cts^-1) and does not appear to be extended. This source is unrelated to the cluster and most probably associated with an AGN which is only 5.4'' away (see Table 1, source 1SAXJ0027.1-1926, and Table 2, source A).

In the optical band there is no detailed information except from that derived from the extensive study of Leir & Van Den Bergh (1977), who classified A33 as a distance class D=6, richness R=1, Bautz-Morgan-class-III cluster. In the Abell (1958) catalog, A33 has 69 galaxies which lie within one Abell radius ( $2.7\ z^{-1}$ arcmin) and which are not more than 2 mag fainter than the third brightest galaxy. Its photometrically estimated redshift, z = 0.28, was derived by Leir & Van Den Bergh (1977) from the cluster optical diameter and the magnitude of the brightest and tenth-brightest cluster galaxies.

In this paper we present a new X-ray observation of A33 obtained with Beppo-SAX. This observation enables us to derive detailed information on the X-ray source, on its morphology and thermal properties. The complex appearance of the X-ray emission in the field of A33 prompted us to obtain optical images and spectroscopic information for several objects in the field.

The plan of the paper is the following. In Sect. 2 we present the basic information on the Beppo-SAX observation and data reduction. In Sect. 3 we describe the optical data and in Sect. 4 we discuss the X-ray spectroscopy of the various sources in the A33 field. We summarize our results for A33 and discuss their implications in Sect. 5.

Throughout the paper H₀ = 50 km s^-1 Mpc^-1and $\Omega_0=1$ are used unless otherwise noted.

Table 1: LECS (0.1-2 keV) and MECS (2-10 keV) count rates
Source $\alpha_{2000}$ $\delta_{2000}$ $t_{{\rm exp}}$ Count rate Count rate $R_{{\rm extr}}$

$(^{\rm h}~ ^{\rm m}~ ^{\rm s})$ (^o ' '') (s) ( $10^{-3} {\rm s}^{-1}$ ) ( $10^{-3} {\rm s}^{-1}$ ) arcmin

LECS MECS

1SAXJ0027.1-1926 00 27 08 -19 26 38 77609 $7.6 \pm 0.7$ $7.8 \pm 0.6$ 2

1SAXJ0027.2-1930 00 27 12 -19 30 32 77609 - $1.76 \pm 0.23$ 2

1SAXJ0027.0-1928 00 27 01 -19 28 30 77609 - $1.14 \pm 0.19$ 1

Source	$\alpha_{2000}$	$\delta_{2000}$	$t_{{\rm exp}}$	Count rate	Count rate	$R_{{\rm extr}}$
	$(^{\rm h}~ ^{\rm m}~ ^{\rm s})$	(^o ' '')	(s)	( $10^{-3} {\rm s}^{-1}$ )	( $10^{-3} {\rm s}^{-1}$ )	arcmin
				LECS	MECS
1SAXJ0027.1-1926	00 27 08	-19 26 38	77609	$7.6 \pm 0.7$	$7.8 \pm 0.6$	2
1SAXJ0027.2-1930	00 27 12	-19 30 32	77609	-	$1.76 \pm 0.23$	2
1SAXJ0027.0-1928	00 27 01	-19 28 30	77609	-	$1.14 \pm 0.19$	1

1 Introduction