Various source classes with different origin of the X-ray emission are found in the MC regions: Foreground stars in the Galaxy, supernova remnants (SNRs), SSSs, XBs, and background objects like galaxies and clusters. The X-ray sources detected by the HRI can be assigned to the aforementioned classes if they are identified with known objects from literature (Sect. 3.1) or if their distinguished X-ray properties allow this. Seventy five out of 121 HRI catalogue sources were also detected by the PSPC. We studied the X-ray properties of the identified sources to obtain tools for classifying detections by the HRI (Sect. 3.2).

3.1 Source identification

56 HRI sources were identified with known SMC objects, foreground stars, or AGN based on their accurate positions from HRI observations and the X-ray properties like extent or PSPC hardness ratios (HFPK00) as it is shown in Table 3. The accurate HRI positions make it possible to identify likely optical counterparts. For optical sources within the error circle of the X-ray position, the B and R magnitudes were determined from the USNO-A2.0 catalogue produced by the United States Naval Observatory (Monet 1998). The flux ratio was computed from the B magnitude and the X-ray count rates applying the equation

$\displaystyle {\rm log}(f_{x}/f_{{\rm opt}})$	=	$\displaystyle {\rm log}(3\cdot{\rm HRI\ counts/s}\cdot 10^{-11})$
		+0.4B+5.37	(3)

3.1.1 Foreground stars

$\begin{figure}\par {\psfig{figure=ds1873f04.ps,angle=270,width=8cm} } \end{figure}$

Figure 4: Flux ratio log( $f_{x}/f_{{\rm opt}}$ ) over hardness ratio 1: Asterisks indicate SSSs, stars SNRs, open squares XBs, open lozenges stars, and open triangles AGN, which are all known objects from literature. Already known candidates are marked with crossed symbols and new classifications with filled symbols

In addition to four foreground stars, two known candidates (sources Nos. 18 and 86) were detected by the HRI. Nos. 18 and 86 were detected by the PSPC and classified as foreground stars by KPFH99 and FHP00. The HRI observations give positions more accurate than the PSPC (total positional error of 5 $.\!\!^{\prime\prime}$ 6 and 7 $.\!\!^{\prime\prime}$ 2, respectively). At the HRI positions an optical source was found for each case, with R = 11.3, log( $f_{x}/f_{{\rm opt}}$ ) = -3.51 for No. 18 and R = 13.1, log( $f_{x}/f_{{\rm opt}}$ ) = -0.67 for No. 86 confirming the proposed classification.

3.1.2 Supernova remnants

Six SNRs were detected by the HRI. For these SNRs extents larger than 5 $.\!\!^{\prime\prime}$ 8 were measured by the HRI with a likelihood higher than 35 (see Fig. 5), except for SNR 0056-72.5 which has no significant extent.

Source No. 22 is identified with a PSPC source which was suggested as SNR candidate by KPFH99. The extent of the HRI source calculated in the maximum likelihood algorithm is only 3 $.\!\!^{\prime\prime}$ 6. However, the HRI image shows that the source is only the brightest knot in an extended emission, confirming the classification as a SNR candidate.

$\begin{figure}\par {\psfig{figure=ds1873f05.ps,angle=270,width=9cm} } \par\end{figure}$

Figure 5: Source extent is plotted over extent likelihood as small dots for all SMC sources, for which the HRI measured an extent higher than zero. SNRs are additionally marked with squares and known point sources with triangles. Open symbols are used for identified sources, crossed symbols for candidates known from literature, and filled symbols for new classifications

3.1.3 Supersoft sources

Four SSSs were found in the HRI observations which were also detected in PSPC observations. For three of the sources (Nos. 7, 23, and 79) flux variability could be verified by the HRI observations (see Sect. 2.3).

3.1.4 X-ray binaries

10 known HMXBs in the SMC were detected both by the PSPC and by the HRI. Two additional HMXBs, AX J0051-722 and SMC X-3, were observed by the HRI only.

Sources Nos. 51 and 73 are HMXB candidates which were observed by other X-ray satellites and are well known from literature. Sources Nos. 39, 81, and 88 were suggested as X-ray binaries by KPFH99. KPFH99 additionally classified the sources Nos. 12 and 83 either as AGN or as XB candidates. For both there is a likely optical counterpart within the HRI error circle with B = 16.8 and 17.3, respectively. The hardness ratios are for No. 12 HR1 = 1.00 $\pm$ 0.61, HR2 = 0.43 $\pm$ 0.16 and for No. 83 HR1 = 1.00 $\pm$ 1.30, HR2 = 1.00 $\pm$ 1.93. 10 HRI sources associated with emission line stars (Meyssonnier & Azzopardi 1993) in the SMC were classified as new HMXB candidates forming Be/X-ray binaries by Haberl & Sasaki (2000, hereafter HS00).

3.1.5 AGN

Source No. 8 is a known AGN with the redshift z = 0.922 (Tinney et al. 1997). The hardness ratios are HR1 = 1.00 $\pm$ 1.09, HR2 = -0.06 $\pm$ 0.12. The optical counterpart seen on DSS2 at the HRI position, which is the same as reported by Tinney et al. (1997), has B = 18.6 (log( $f_{x}/f_{{\rm opt}}$ ) = -0.49).

Source No. 59, showing both radio (SMC B0053-7227 (FHW98)) and X-ray emission, was classified as an AGN candidate by KPFH99. A faint optical source (B = 18.6) can be found at the HRI position resulting in log( $f_{x}/f_{{\rm opt}}$ ) = -0.30. For the source No. 112, also an AGN candidate (FHP00), no optical counterpart was found. The PSPC pointing shows a hard source (HR1 = 1.00 $\pm$ 0.37, HR2 = 0.46 $\pm$ 0.09). Seven HRI sources were classified as AGN candidates by KPFH99. For them likely optical counterparts exist with brightness 17 < B < 19. The computed log( $f_{x}/f_{{\rm opt}}$ ) are around -0.50.

3.2 New classifications

In addition to the identification of HRI sources with known objects we classified new SMC X-ray sources with the help of their properties obtained from HRI observations. First we looked for extended sources which could suggest new SNR candidates, but without any result. No new source with significant extent was detected.

By comparing positions of X-ray sources with those of stars measured in the optical and near-infrared one can conclude if the X-ray source is in coincidence with a Galactic foreground star. Therefore the HRI source catalogue was cross-correlated not only with the USNO-A2.0 catalogue, but also with the DENIS catalogue, looking for stars within the error circle of the X-ray detection. Hardness ratios determined by PSPC are additional parameters for classifying hard X-ray sources not belonging to the Galaxy as candidates for XBs or AGN.

Table 4 summarizes the new classifications of this work, and in Fig. 8 DSS2 images around these sources are presented with X-ray position and positional error.

3.2.1 Sources classified as Galactic foreground stars

$\begin{figure}\par {\psfig{figure=ds1873f06.ps,angle=270,width=8cm} } \end{figure}$

Figure 6: Color-magnitude diagram for HRI sources correlating with entries in the USNO-A2.0 catalogue. Small dots are used for all sources. Extra asterisks indicate SSSs, stars SNRs, open squares XBs, open lozenges stars, and open triangles AGN. Already known candidates are marked with crossed symbols, so an overlay of open triangle, open square, and cross marks hard sources as candidates either for AGN or XB. Filledsymbols are new classifications

$\begin{figure}\par {\psfig{figure=ds1873f07a.ps,angle=270,width=8cm} } \par {\psfig{figure=ds1873f07b.ps,angle=270,width=8cm} } \end{figure}$

Figure 7: Color-color and color-magnitude diagram for HRI sources correlating with objects in the DENIS catalogue. Open lozenges for foreground stars, crossed for candidates, and filled for new classifications. The filled triangle mark the source classified as AGN. Stars signify AGN observed by Maiolino et al. (2000)

In comparison to SMC members or background objects, foreground stars are brighter in R as shown in the color-magnitude diagram (Fig. 6). For the near-infrared CLH00 estimated the number of foreground stars and Magellanic Cloud members in the DENIS catalogue by studying color-color and color-magnitude diagrams for the DENIS data. They showed that sources with $J - K_{{\rm s}}$ lower than $\sim 1.0$ are most likely foreground stars with increasing probability for smaller $J - K_{{\rm s}}$ and smaller I - J that such a classification is correct. In our catalogue there are in total 14 sources correlating with entries in the DENIS catalogue. Apart from one, $J - K_{{\rm s}} < 1.1$ was measured for all of them. Since sources observable both in near-infrared and in X-rays are mainly galactic stars or AGN in the background, seven out of these 13 sources with $J - K_{{\rm s}} < 1.1$ (Nos. 16, 26, 35, 68, 80, 89, 121) were classified as foreground stars. In Fig. 7a they show a good correlation between $J - K_{{\rm s}}$ and I - J which is also shown by CLH00. For most of them we got R < 14.0 from the USNO-A2.0 catalogue and likely optical counterparts can be seen in the DSS2 images within the error circles (Fig. 8). In addition the source No. 25 was classified as a foreground star since an optical counterpart with R = 10.8 was found.

$\begin{figure}\par\begin{tabular}{cc} \psfig{figure=ds1873f08a.ps,angle=0,width=... ...\psfig{figure=ds1873f08f.ps,angle=0,width=69mm}\\ \end{tabular}\par\end{figure}$

Figure 8: Positions of newly classified HRI sources on DSS2 images. The comments in brackets < > give the classifications. The X-ray source position is in the center of the image, circles mark the 90% confidence error

3.2.2 Candidate for AGN

In the color-color diagram in Fig. 7 there is an outlying source (No. 64). Looking at the color-magnitude diagram (Fig. 7b) it is the one with the highest $J - K_{{\rm s}}$ and highest J. For comparison magnitudes and colors for a sample of AGN determined by Maiolino et al. (2000) are also plotted. The near-infrared and optical brightness of the HRI source No. 68 correspond well with those typical for AGN (B = 19.2, R = 16.4). So it was classified as an AGN.

3.2.3 Candidates for AGN or X-ray binaries

For the source No. 71 no optical counterpart can be found on the DSS2 frame. In the USNO-A2.0 catalogue there is a faint object (B = 20.1, R = 17.6) within the error circle, and with log( $f_{x}/f_{{\rm opt}}$ ) = 0.40 the HRI source is bright in X-rays. The PSPC spectrum of this source is hard (HR1 = 1.00 $\pm$ 0.13, HR2 = 0.38 $\pm$ 0.09) favoring its classification as XB or AGN.

$\begin{figure} {\psfig{figure=ds1873f09.ps,angle=0,width=125mm} } \end{figure}$

Figure 9: Distribution of identified HRI sources in the SMC region plotted on a grey scale image (0.1 - 2.4 keV) obtained from the PSPC data. Squares are used for SNRs, double squares for SSSs, crossed squares for XBs, crossed circles for AGN, circles for foreground stars. Candidates from literature are included for each source class

$\begin{figure}{\psfig{figure=ds1873f10.ps,angle=0,width=125mm} } \end{figure}$

Figure 10: The distribution of unidentified HRI sources and new source classifications is shown. Unidentified HRI sources are plotted as small squares, circles are classifications as foreground star, crossed circle as AGN, and double circles as AGN or XB candidates

Another five HRI sources (Nos. 5, 53, 72, 78, 85) are associated with hard PSPC sources with HR1 = 1.00 and HR2 $\geq$ 0.25. For all of them large errors were determined for HR1, but not for HR2 (HR2 - error_HR2 > 0.00), resulting from the fact that the source is so hard that the photon statistics were low in the softer bands (for definition of the hardness ratios see HFPK00). In the USNO-A2.0 catalogue only faint or no optical source is found at the HRI position within the error circle. This can be also verified on the DSS2 images (see Fig. 8). Finally these HRI sources were classified as XB or AGN.

3 Source classes