3 Optical imaging and spectroscopy

 

 

Table 2: Optical results

Name	$\alpha_{2000}$	$\delta_{2000}$	z	Identification and Comments
	$(^{\rm h}~ ^{\rm m}~ ^{\rm s})$	(o ' '')
A	00 27 09.8	-19 26 12.6	$0.2274 \pm 0.0006$	AGN ([OII], [OIII], [Ne III], broad Balmer)
B	00 27 07.3	-19 26 36.4		M star
C	00 27 00.5	-19 28 56.5	$0.2420 \pm 0.0005$	galaxy (G-band, H$\beta$, MgIb, NaId)
D	00 26 59.5	-19 28 18.6	$0.2863 \pm 0.0015$	galaxy (H+K, G-band, H$\beta$, MgIb)
g1	00 27 12.3	-19 30 45.5	$0.2406 \pm 0.0008$	galaxy (H+K, G-band, H$\beta$, MgIb, NaId)
g2	00 27 12.6	-19 30 43.7	$0.2380 \pm 0.0012$	galaxy (CaII-break, G-band, H$\beta$, MgIb, NaId)
g3	00 27 12.5	-19 30 40.1	$0.2395 \pm 0.0017$	galaxy (CaII-break, G-band, H$\beta$, MgIb, NaId)
g4	00 27 13.1	-19 30 29.4	$0.2445 \pm 0.0004$	galaxy (H+K, G-band, H$\beta$, MgIb, NaId)
g5	00 27 13.0	-19 30 25.6	$0.2406 \pm 0.0005$	galaxy (H+K, G-band, H$\beta$, MgIb, NaId)

Figure 3: The two images are 1024 $\times$ 1024 (3.7 $\times$ 3.7 arcmin) subarrays extracted from two B-band exposures taken at the Keck II telescope. The image to the top shows the field around 1SAXJ0027.1-1926 and 1SAXJ0027.0-1928 and the image to the botton shows the field around 1SAXJ0027.2-1930. North is up and East to the left

Figure 4: The combined LECS and MECS spectrum of the source 1SAXJ0027.1-1926 extracted from a 2 arcmin radius regions. The spectrum shown in figure has been rebinned so that the significance of each bin is at least 3$\sigma$. The best fit model is a MEKAL thermal model (see text for details). The spectrum has been further rebinned using XSPEC for graphical purposes

Figure 5: The SAX MECS spectra of 1SAXJ0027.2-1930 fitted with a thermal MEKAL model (upper panel) and with an absorbed power-law model (lower panel). Details of the spectral analysis are given in Table 4. The spectrum has been further rebinned using XSPEC for graphical purposes

Figure 6: The SAX MECS spectrum of 1SAXJ0027.0-1928 fitted with a non-thermal power-law model (see Table 4 for details). The spectrum has been further rebinned using XSPEC for graphical purposes

Due to the lack of detailed optical information in the literature for A33, we took I and B images of the cluster region on November 23 and 24 1997 at the Keck II telescope. The images were obtained using the Low-Resolution and Imaging Spectrograph (LRIS) (Oke et al. 1995) in imaging mode, resulting in a scale of 0.215 $^{\prime\prime}$ pixel^-1 and a field of view of 6$^\prime$$\times$ 7 $.\mkern-4mu^\prime$3. The I (B) images were taken in 0.4 $^{\prime\prime}$- 0.5 $^{\prime\prime}$ seeing on the first night and consist of 3 $\times$ 300 s (4 $\times$ 120 s) dithered exposures centered at $\alpha$=00 $^{{\rm h}}$27 $^{{\rm m}}$10 $\hbox{$.\!\!^{\rm s}$ }$5 and $\delta=-19\hbox{$^\circ$ }29\hbox{$^\prime$ }18^{\prime\prime}$ (J2000), the southern region of the X-ray emission complex. On the second night (0.8 $^{\prime\prime}$ seeing) we took 2 $\times$ 120 s I (2 $\times$ 300 s B) exposures centered at $\alpha$=00 $^{{\rm h}}$27 $^{{\rm m}}$09 $\hbox{$.\!\!^{\rm s}$ }$8 and $\delta=-19\hbox{$^\circ$ }26\hbox{$^\prime$ }12\hbox{$.\!\!^{\prime\prime}$ }4$ (J2000), the northern region of the X-ray emission system. The optical position of A33 (Fig. 2) is close to an open stellar cluster. Figure 3 shows the B images for both North (Fig. 3a) and South (Fig. 3b) regions. No excess of galaxies is present in the northern region at the position of 1SAXJ0027.1-1926 (Fig. 3a), while Fig. 3b reveals an overdensity of galaxies in the region of the X-ray source 1SAXJ0027.2-1930.

Spectroscopic observations for several objects in the field were carried out on August 16, 17 and 19, 1998, with the Wide Field Grism Spectrograph and the Tek2048 $\times$ 2048 CCD attached to the University of Hawaii 2.2 m telescope on Mauna Kea. We used the 420 l/mm grating which provided a $\sim 3990-9900$ Å coverage and a pixel size of 3.6 Å/pix, and a long-slit of 2.4'' which gives a low spectral resolution of about 24 Å. For the reduction of the data we have used the IRAF package (Tody 1993). In the region of the northern X-ray emission we identified 2 objects labeled as A and B in Fig. 3a.

In the region to the west, where the X-ray source 1SAXJ0027.0-1928 is present, we found two galaxies labeled C and D in the above mentioned figure. In the region of the southern X-ray emission we obtained spectra for five galaxies which turned out to be members of the cluster. These galaxies are labeled g1 through g5 in Fig. 3b. Table 2 gives the results of the observations:

Based on our imaging and spectroscopic results, we conclude that a blend of the AGN (A) and M-type star (B) X-ray emissions contribute to the extended source 1SAXJ0027.1-1926 to the north. The Abell cluster A33 is the source of the southern X-ray emission 1SAXJ0027.2-1930, while the identification of the source of the western X-ray emission, 1SAXJ0027.0-1928, remains unknown. The two galaxies for which we measured the spectra, and which are the two brightest optical sources in the region, might be responsible for part of the emission of 1SAXJ0027.0-1928, but we need spectroscopic data for more objects to help in the identification. One of the sources (C) is consistent with being part of A33. From the six cluster members listed in Table 2 we obtain for A33 an average <z> = 0.2409 $\pm$ 0.0009, and a very tentative velocity dispersion, given the few cluster galaxies, $\sigma_{{\rm los}}= 472^{+295}_{-148}$ km s^-1. This estimate includes the 1+z correction.