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Astron. Astrophys. 353, 129-134 (2000)

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3. Discussion

In the following we compare radio-continuum, ROSAT PSPC and optical CTIO (H[FORMULA] and [S II ]) images (Figs. 1 and 2). We discuss several features/sources that coincide, among which the most prominent are DEM S128, RX J0105.1-7211 and RX J0105.5-7213. We note that the X-ray sources RX J0105.1-7211 and RX J0105.7-7207 are seen only in X-ray but not at radio wavelengths.

3.1. DEM S128 (RX J0105.4-7209)

The emission nebula DEM S128 (RX J0105.4-7209) shows a prominent steep radio-continuum spectrum of [FORMULA]=-0.48[FORMULA]0.06 (Fig. 3) which is typical for SNRs (Filipovi et al. 1998a). DEM S128 coincides with the extended X-ray source RX J0105.4-7209 which exhibits HR1 and HR2 typical for SNRs (Haberl & Pietsch 1999). No significant X-ray flux variations were found between the two ROSAT PSPC observations, also consistent with an SNR nature. Possibly the most sensitive diagnostic for identifying SNRs is narrow-band optical CCD imaging in the light of H[FORMULA] and [S II ] (Rosado et al. 1994). The ratio of the [S II ] to H[FORMULA] emission in nebulae provides an ideal discriminator between H II regions, which typically show [S II ]/H[FORMULA] of [FORMULA]0.1, and SNRs, which usually have ratios [FORMULA]0.5. In the case of DEM S128 we measure [S II ]/H[FORMULA] ratios of 0.6 to 0.65 (averaged over areas of [FORMULA]30 arcsec2). Therefore, indicators from three different wave-bands independently confirm the SNR nature proposed by Inoue et al. (1983). The diameter of the SNR DEM S128, as measured along an east-west line, is about 150" or 45 pc at the distance of the SMC (65 kpc; Feast 1999). From the radio-continuum images the SNR DEM S128 can be classified as a so-called barrel-shaped (bilateral) remnant (Gaensler 1998). We note that the radio-continuum emission from SNR DEM S128 could be contaminated with a point-like background source which is located at the south limb of the SNR. Further high-resolution observations will clarify the morphology of this SNR.

3.2. RX J0105.6-7211

The X-ray and radio source RX J0105.6-7211 is found about 75" south from DEM S128. In both wave-bands this source shows extent and shell-shape typical for SNRs. However, the radio-continuum spectrum is inconclusive and we fail to detect any optical emission. Therefore, the nature of this source at present remains ambiguous.

3.3. RX J0104.9-7210

The source RX J0104.9-7210 is also not classified despite positive detection at all observed frequencies. Contradictory properties of this object emphasize the complexity of the whole AX J0105-722 region.

3.4. RX J0105.5-7213

The bright X-ray and radio source RX J0105.5-7213 shows a flat radio spectrum with [FORMULA]=-0.15[FORMULA]0.09 (Fig. 3). The X-ray spectrum is hard as indicated by HR2=0.46[FORMULA]0.09. To investigate long term time variations in the flux of the X-ray sources we compared the two PSPC observations which were performed about six months apart. RX J0105.5-7213, of similar intensity to RX J0105.1-7211 during the October 1993 observation was not detected in May 1993 implying an even higher variability than shown by the latter source (see below). However, no optical counterpart is detected in any of our optical images. Therefore, we classify this source to be an optically faint background AGN. This is important as this AGN could serve (together with other background sources in the field of the SMC) as a potential probe of the extragalactic ISM (Crampton et al. 1997).

3.5. RX J0105.1-7211

No radio-continuum emission is detected for the X-ray source RX J0105.1-7211. Between the two PSPC observations RX J0105.1-7211 increased in count rate from (1.5[FORMULA]0.6) 10-3 cts s-1 to (4.1[FORMULA]0.5) 10-3 cts s-1, i.e. by a factor of 1.7 - 5.1 in the 0.5 - 2.0 keV band (note the different energy band to that used in Table 3). From ESO U/B images (Fig. 4) we notice two stars near the position of the X-ray source, with the northern of the two stars showing stronger U-band emission. We identify this northern star with an emission line object in the catalogue of Meyssonier & Azzopardi (1993, MA93). The object number 1517 in MA93 is located only [FORMULA] away from the X-ray position, consistent within the errors. The catalogue of MA93 contains most of the other already identified Be/X-ray binaries in the SMC (Haberl et al., in preparation) and also an MA93 object is proposed as the counterpart of the ASCA pulsar AX J0049-732 (Filipovi et al. 1999). This strongly suggests RX J0105.1-7211 as new Be/X-ray binary which also is most likely the counterpart of the ASCA X-ray pulsar AX J0105-722 originally discovered by Yokogawa & Koyama (1998). Unfortunately, the archival ROSAT PSPC/HRI data cannot confirm the pulsations from the X-ray source due to poor statistics. Further high-resolution optical and X-ray observations are necessary to unambiguously identify RX J0105.1-7211 as a Be/X-ray binary and counterpart of AX J0105-722 .

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© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999