2 Beppo-SAX observation

The A33 field was observed with the Narrow Field Instruments (NFI) of the Beppo-SAX satellite from November 23$^{\rm th}$ to 25$^{\rm th}$, 1996. The total effective exposure time is $t_{{\rm exp}}= 3.8417~10^4$ s for the LECS instrument and $t_{{\rm exp}}= 7.7610~10^4$ s for the MECS instrument (see e.g. Boella et al. 1997a and 1997b for a technical description of the Beppo-SAX mission and instrumentation).

Figure 1: The Beppo-SAX image of A33 in the 2-10 keV energy band. The three different components of the emission are labeled according to the text. The circles indicate the extraction area for each X-ray source. Note that 1sSAXJ0027.2-1930 has also a diffuse, low-surface brightness distribution which appears to be extended in the southern part of the image. The image has been deconvolved with a wavelet transform using a smoothing length of 3.5 pixels (1 pixel = 8 arcsecs). North is up and East to the left

Data preparation and linearization was performed using the SAXDAS v.1.3 package under the FTOOLS environment. The imaging analysis was performed using the XIMAGE package (Giommi et al. 1991). The extraction of the source and background spectra was done within the XSELECT package. The spectral analysis was performed using XSPEC v.9.0.

The only previous claimed X-ray detection of A33 was done with the HEAO1 satellite (Johnson et al. 1983; Kowalski et al. 1984). Due to the large error box of the HEAO1 detectors, the coordinates of the X-ray source were associated with the optical coordinates of the A33 cluster. Thus the Beppo-SAX observation was centered on the optical coordinates $\alpha^\circ_{2000}=$ $00^{{\rm h}}26^{{\rm m}}52.7^{{\rm s}}$ and $\delta^\circ_{2000}= -19^\circ 32'29''$. The MECS 2-10 keV X-ray image of the field is shown in Fig. 1, where three different subsystems are evident: a bright and apparently extended source, 1SAXJ0027.1-1926, an extended but smaller source, 1SAXJ0027.2-1930, located to the south of the brightest source and an apparently point-like source, 1SAXJ0027.0-1928, located to the west. Positions, count rates and extraction region radii, $R_{{\rm extr}}$, are listed in Table 1. The sources have sufficient count rates to be detected individually at more than 4 sigma level by the MECS instruments. The poorer spatial resolution of the LECS instead allows only to determine the count rate of the brightest source 1SAXJ0027.1-1926. In the following we describe the spatial structure of each source detected in the A33 field as derived from the MECS data.

The MECS PSF is $\approx 1$ arcmin Half Energy Width, and this spatial resolution allows us to detect the sources 1SAXJ0027.1-1926 and 1SAXJ0027.2-1930 as extended in the MECS image of Fig. 1.

The source 1SAXJ0027.1-1926 has an extension of $\sim 2$ arcmin (radius). As discussed in Sects. 3 and 4, this source is most probably the result of the blending of two point-like sources not resolved by the MECS PSF. The X-ray MECS image contours superposed onto the POSS II image of the field plotted in Fig. 2 show that there is no clear galaxy excess associated to the X-ray source 1SAXJ0027.1-1926.

The source 1SAXJ0027.2-1930, located $\sim 4.5$ arcmin south of the brightest source (see Fig. 1), has an extension of $\ \raise -2.truept\hbox{\rlap{\hbox{$\sim$ }}\raise5.truept \hbox{$>$ }\ }1.5$ arcmin radius. Using a $\beta$-model with values $\beta=0.75$ and $r_{\rm c}=260$ kpc ( $H_0=50, \Omega_0=1$) chosen as representative of such low luminosity objects, and convolved with the MECS PSF we find a central density of $\approx 3.9~10^{-3}$ cm^-3. Moreover, an extended, low surface brightness X-ray emission is visible in the southern part of the image (see Figs. 1 and 2). Such a low surface brightness source extends for a few arcminutes at levels of $\sim 10^{-4}$ cts s^-1 cm^-2 arcmin^-2. The extended source 1SAXJ0027.2-1930 is associated with A33 as shown in the POSS II image of the field (see Fig. 2 and Sect. 3).

The third source 1SAXJ0027.0-1928, located $\sim 4$ arcmin south-west of the brightest source, has a point-like appearance. Two faint objects in the POSS II are positionally consistent with 1SAXJ0027.0-1928.

Figure 2: The optical image of A33 taken from the POSS II plate and the X-ray contours of the Beppo-SAX image obtained with the MECS detector in the 2-10 keV energy band. Contours are taken from the image shown in Fig. 1 and are logarithmically spaced. The image has been deconvolved with a wavelet transform using a smoothing length of 3.5 pixels (1 pixel = 8 arcsecs). The white cross indicates the position of A33 from the Abell catalogue. The first X-ray contour is at $3 \sigma$ from the background level. North is up and East to the left