2. X-ray sources and emission-line stars
A cross-correlation of the 517 PSPC X-ray sources in the SMC region (HFPK00) with the catalogue of emission-line stars published by MA93 (1898 entries) yielded 46 possible optical counterparts (distance to account for systematic uncertainties in X-ray and optical positions, where r90 denotes the 90% statistical uncertainty of the X-ray position in arc seconds). An additional object correlating with a PSPC source was found within the catalogue of candidate emission-line objects of MB99. From this sample of 47 objects one coincides with a known supernova remnant, three with supersoft sources and ten with optically identified Be stars proposed as optical counterparts for the X-ray sources. Extending the search by using additional SMC X-ray sources such as from the ROSAT HRI catalogue of Sasaki et al. (2000b), the Einstein IPC catalogue of Wang & Wu (1992) and X-ray pulsars discovered by instruments on ASCA, BeppoSAX and RXTE yields another three emission-line stars identified with known Be stars. Our correlation results are summarized in Table 1 which is sorted in right ascension.
Table 1. X-ray sources correlating with emission-line objects in the catalogues of MA93, MB99 or identified Be-stars.
Table 1. (continued).
Columns 2-4 of Table 1 give the source numbers in the X-ray catalogues for ROSAT PSPC (HFPK00), ROSAT HRI (Sasaki et al. 2000b) and Einstein IPC (Wang & Wu 1992). For sources detected by ROSAT coordinates with statistical 90% error (from HFPK00 when detected by the PSPC or from Sasaki et al. 2000b when detected by HRI only) are given in columns 5-7. For the group of IPC sources which were not detected by ROSAT, the IPC coordinates and a 40" error as published in WW92 is given. The three digits in column 8 denote the number of PSPC detections in the energy bands 0.1 - 0.4 keV, 0.5 - 0.9 keV and 0.9 - 2.0 keV. With a few exceptions most of the sources were detected mainly in the higher energy bands which indicates a hard X-ray spectrum. HFPK00 used count ratios in the different energy bands, the hardness ratios, for a spectral classification of the PSPC sources. However, very hard sources without detection in the lower energy bands were not classified in HFPK00 because of large errors on the hardness ratios. Column 9 lists the maximum observed X-ray luminosity for Be/X-ray binaries and candidates derived in this work. The values are selected from literature or computed from ROSAT count rates using the conversion factor 1.67 erg s-1/cts s-1 (see Kahabka & Pietsch 1996, hereafter KP96), typical for X-ray binaries with hard spectrum at the distance of the SMC. HRI count rates were converted to PSPC rates using a multiplication factor of 3.0 (Sasaki et al. 2000a). Luminosities derived from count rates are indicated with colon. For all given X-ray luminosities we assume a distance of 65 kpc to the SMC.
Column 10 of Table 1 lists the entry number of the nearest object in MA93 (MB99 in one case) and column 11 the MA93 classification type (2 = SNR; 5 = planetary nebula, PN; 9 = late type star). The distance between X-ray and optical position as listed in MA93 (MB99) is listed in column 12. B, V and R magnitudes found in the literature for identified sources are given in columns 13-15. When available B and R obtained from the USNO A2.0 catalogue are listed for the remaining sources. In the last column identifications are given together with references and new proposals are marked with `?' behind the source type.
From the total of 18 high mass X-ray binaries in the SMC with known Be star as proposed counterpart 13 are found in the emission-line catalogues of MA93 and MB99. For completeness the remaining five Be/X-ray binaries are also listed in Table 1.
The identification of most known Be/X-ray binaries with stars in the emission-line catalogues of MA93 and MB99 suggests that un-identified X-ray sources with emission-line star counterparts are most likely also Be/X-ray binary systems. Other objects like SSSs and SNRs can be recognized by their very soft X-ray spectrum (in contrast to the Be/X-ray binaries with hard spectrum) or by their X-ray source extent, respectively. In the following section we summarize the 18 X-ray sources optically identified as Be/X-ray binary. We then propose emission-line stars as likely Be counterparts for the five un-identified pulsars and in addition for 25 hard X-ray sources.
2.1. Supersoft sources
Three supersoft sources detected by ROSAT were identified with emission-line objects in the catalogue of MA93. Two of them are associated with planetary nebulae (PN) while the remaining one is identified with a symbiotic star in the SMC. More detailed information on the individual sources and finding charts with X-ray error circles can be found in Sect. 6.1.
2.2. Optically identified Be/X-ray binaries
For 19 X-ray sources in the SMC nearby Be stars were optically identified and proposed as counterparts, suggesting a Be/X-ray binary nature. Eighteen were covered by ROSAT observations and information on the individual sources is summarized in Sect. 6 where also finding charts are found. The HEAO source 1H 0103-762 was not observed with ROSAT (see KP96 and references therein). Also we do not include the HMXB candidates RX J0106.2-7205 (Hughes & Smith 1994) and EXO 0114.6-7361 in our summary. For RX J0106.2-7205 no optical spectrum from the suggested counterpart is published yet, which would confirm its proposed Be star nature. For EXO 0114.6-7361 Wang & Wu (1992) propose the B0 Ia star AzV 488 as counterpart, however, AzV 477, also a B0 Ia star is even closer to the X-ray position. Both candidates suggest a supergiant type of HMXB. It is remarkable, that together with the only other known supergiant HMXB SMC X-1, EXO 0114.6-7361 is located in the eastern wing of the SMC.
Fourteen of the identified Be/X-ray binaries were detected by the ROSAT PSPC and their X-ray properties can be found in HPFK99. AX J0051-722, SMC X-3 and RX J0058.2-7231 were detected by the ROSAT HRI and are listed in the catalogue of HRI sources in the SMC (Sasaki et al. 2000b). Only 2E 0051.1-7304 was not detected by ROSAT. Thirteen of the proposed Be star counterparts are listed in the catalogues of MA93 and MB99 and only for five X-ray sources the Be counterparts have no entry in MA93 and MB99 (AX J0049-729, SMC X-2, RX J0032.9-7348, RX J0058.2-7231 and XTE J0111.2-7317). RX J0032.9-7348 was not covered by the MA93 survey.
2.3. Optically unidentified X-ray pulsars
Five X-ray pulsars in the SMC were reported for which no optical identifications are published up to day. Four of them were detected by ROSAT PSPC and/or HRI, yielding more accurate positions (HFPK00, Sasaki et al. 2000b) and for the fifth case, XTE J0054-720, several ROSAT sources are found within the large RXTE error circle. In or very close to the ROSAT error circles emission-line objects from MA93 are found and we propose these as optical counterparts. Literature, finding charts and other information on the X-ray binary pulsars is presented in Sect. 6.
Most of the Be stars proposed as optical counterparts for X-ray sources in the SMC, as summarized in the previous section, are found as emission-line objects in the catalogue of MA93. This strongly supports that the unknown emission-line objects within the error circles of the unidentified pulsars are also Be star counterparts of the X-ray pulsars forming Be/X-ray binaries.
2.4. New Be/X-ray binary candidates
From the correlation of X-ray source and emission-line object catalogues 34 hard X-ray sources were found with an emission-line object as possible optical counterpart in the X-ray error circle (see Table 1). The 34 X-ray sources were investigated in detail to obtain more information which can help to identify the nature of the object. Finding charts and notes to the individual sources are compiled in Sect. 6.
Many sources were observed more than once by ROSAT and we looked for long-term time variability. In the case of the PSPC we used the 0.9 - 2.0 keV band because of higher sensitivity for hard sources like Be/X-ray binaries. To combine detections from the different instruments we convert HRI to PSPC count rates by multiplying with 3.0 and IPC to PSPC count rates by multiplying with 1.1 (appropriate for a 5 keV Bremsstrahlung spectrum with 4.3 cm-2 absorption column density). Given the uncertainties in the count rate conversions, variability is only treated as significant above a factor of 3. None of the sources was observed with sufficient counting statistics in order to perform a detailed temporal analysis on shorter time scales (within an observation) and to detect X-ray pulsations.
We discuss all un-identified X-ray sources with emission-line object in or close to the error circle in the following and indicate very promising candidates for Be/X-ray binaries with "Be/X?" in the remark column of Table 1.
2.5. Chance coincidences
To estimate the number of false identifications of X-ray sources with emission-line objects in MA93 we shifted the X-ray positions of the sources in an arbitrary direction and cross-correlated again with the MA93 catalogue. To get statistically more reliable results this was repeated with different distances between 1 - 10 arc minutes. For this purpose we used the PSPC catalogue which is most complete. After application of our selection criteria for accepting Be/X-ray binary candidates (likelihood of existence for the X-ray source 13, no other identification, distance ) we find on average about seven expected chance coincidences between PSPC sources and emission-line objects in MA93. We emphasize that these are mainly caused by the PSPC sources with the largest position uncertainties. The PSPC sources in Table 1 with the largest errors on the X-ray position and therefore most likely chance coincidences are 99, 248, 295 and 404. Indeed three of them were rejected as Be/X-ray binary candidates due to the presence of other likely counterparts. Similarly PSPC sources 77 and 253 were disregarded. Other X-ray sources with large position errors in Table 1 are the IPC sources which were not securely detected with ROSAT. Also here two were not regarded as Be/X-ray binary candidates. For the 25 new Be/X-ray binary candidates we therefore estimate that about two to three may be misidentifications, most likely among those with position error r90 15".
© European Southern Observatory (ESO) 2000
Online publication: July 7, 2000