6. Notes to individual sources
In the following notes X-ray source numbers refer to the catalogues of Wang & Wu (1992) for Einstein IPC, of HFPK00 for ROSAT PSPC and of Sasaki et al. (2000b) for ROSAT HRI. Finding charts with X-ray error circles are shown in Figs. 3, 4, 5, and 6, for SSSs, already identified Be/X-ray binaries, unidentified X-ray pulsars and new Be/X-ray binary candidates, respectively. The order within each group of sources follows Table 1 and for faster access to the table entry the running number is given together with source name.
6.1. Supersoft sources
7) RX J0048.4-7332: The SSS RX J0048.4-7332 was discovered by Kahabka et al. (1994) and identified as the symbiotic M0 star SMC 3 by Morgan (1992). This star is listed in MA93 as object 218 and classified as late type star, consistent with the spectral type determined by Morgan (1992). The accurate HRI position (source 23) confirms the identification of the PSPC source (512).
39) 1E 0056.8-7154: This SSS was discovered in Einstein data (Inoue et al. 1983) and was detected with ROSAT PSPC (source 47) and HRI (79). It coincides in position with the SMC planetary nebula N67 (Aller et al. 1987) which is listed as object 1083 in MA93.
6.2. Optically identified Be/X-ray binaries
1) RX J0032.9-7348: Stevens et al. (1999) identified two Be stars within the PSPC error circle of RX J0032.9-7348, discovered by KP96 as variable source with hard X-ray spectrum. The a factor of 5 smaller error radius obtained from a different PSPC observation by HFPK00 (source 567), however, still contains both Be stars which are very close to each other (Fig. 4). The two stars were not covered by the survey of MA93.
9) AX J0049-729: Yokogawa & Koyama (1998a) reported X-ray pulsations in ASCA data of this source. Kahabka & Pietsch (1998) suggested the highly variable source RX J0049.1-7250 (KP96) as counterpart. Stevens et al. (1999) identified two Be stars, one only 3" from the X-ray position and one just outside the error circle given by KP96. The revised position of PSPC source 351 in HFPK00 makes the more distant Be star further unlikely as counterpart (see Fig. 4). None of the two Be stars turns up in the list of emission-line objects of MA93 with the nearest entry (279) 58" away.
13) AX J0051-733: Yokogawa & Koyama (1998b) discovered X-ray pulsations from this source in ASCA data. The X-ray source was detected in Einstein IPC, ROSAT PSPC and HRI archival data and the 18 year history shows flux variations by at least a factor of 10 (Imanishi et al. 1999). Cowley et al. identified already 1997 a Be star as optical counterpart of the ROSAT HRI source RX J0050.8-7316 (HRI 34) which is located within the ASCA error circle. Cook (1998) reported a 0.708 d period from this star using data from the MACHO collaboration. The source was also detected by the PSPC (source 444) and coincides with object 387 in MA93.
16) AX J0051-722: Corbet et al. (1998b) reported 91 s X-ray pulsations from ASCA observations of this pulsar which was originally confused with the nearby 46 s pulsar XTE J0053-724 in XTE data. AX J0051-722 was not detected by the PSPC. An HRI detection reduced the position uncertainty and Stevens et al. (1999) identified a Be star as likely optical counterpart. The X-ray source is found as source 37 in the HRI catalogue and the star is identical to the only emission-line object from MA93 (413) in the ASCA error circle.
19) RX J0051.9-7311: This X-ray source was detected by Cowley et al. (1997) during ROSAT HRI observations of Einstein IPC source 25 and identified with a Be star by Schmidtke et al. (1999). It is identical to PSPC source 424 and HRI 41. The Be star is found as object 504 in MA93.
20) RX J0051.8-7231: This source was reported as strongly X-ray variable by KP96 and is associated with the X-ray pulsar 2E 0050.1-7247 (Israel et al. 1997). Observed X-ray luminosities range between 5 erg s-1 and 1.4 erg s-1 (Israel et al. 1997). The star AzV 111 (object 511 in MA93) was proposed as counterpart for 2E 0050.1-7247 while Israel et al. (1997) identified another active star within their error circle of RX J0051.8-7231 which is larger than that of KP96. Also the position error given for the corresponding PSPC source 265 by HFPK00 is large. The detection of the source in the PSPC observation 600453p (used by KP96) where the source was bright was rejected by the semi-automatic analysis of HFKP99 because the detection was close to the support structure of the detector entrance window. A careful analysis (and using the latest processed data of 600453p) of the photons of the source in the detector frame shows, however, that it moved nearly parallel to the window support structure and that it was not affected by it. In Table 1 therefore the parameters derived from this PSPC observation are given. They confirm the results of KP96 with small error circle (see Fig. 4). Both AzV 111 and star 1 of Israel et al. (1997) are outside this error circle which, however, contains a Be star identified by Stevens et al. (1999). This star is found as object 506 in MA93 and is the most probable counterpart of RX J0051.8-7231.
22) SMC X-3: This long-known X-ray source was not detected by the ROSAT PSPC but is included in the HRI catalogue as source 43. The Be star counterpart (e.g. Crampton et al. 1978) corresponds to object 531 in MA93.
23) RX J0052.1-7319: Lamb et al. (1999) reported X-ray pulsations from the variable source RX J0052.1-7319 (KP96) found in ROSAT HRI and CGRO BATSE data. Israel et al. (1999) identified a Be star as likely optical counterpart. It is found in MA93 as object 552 and was detected as X-ray source by IPC (29), PSPC (453) and HRI (44). The strong X-ray variability by a factor of 200 between different HRI observations (Kahabka 2000) strongly supports the identification as Be/X-ray binary.
24) 2E 0051.1-7304: For this source, listed as entry 31 in the Einstein IPC source catalogue of Wang & Wu (1992), the Be star AzV 138 (Garmany & Humphreys 1985) was proposed as optical counterpart. AzV 138 corresponds to object 618 in MA93. 2E 0051.1-7304 was not detected in ROSAT observations.
25) RX J0052.9-7158: This source was detected as X-ray transient by Cowley et al. (1997) during ROSAT HRI observations of Einstein IPC source 32 (the largest circle in the finding chart of Fig. 4. Upper limits derived from PSPC observations imply flux variations by at least a factor of 350 (Cowley et al. 1997). The strong variability and the hard X-ray spectrum imply a Be/X-ray transient consistent with the suggested Be star counterpart (Schmidtke et al. 1999). The Be star is identical to object 623 in MA93. The X-ray source was detected by ROSAT (PSPC 94 and HRI 46, the HRI position is most accurate as indicated by the smallest error circle in the finding chart of Fig. 4) and is located near the edge of the error circle of XTE J0054-720. Due to the large position uncertainty of XTE J0054-720 it is, however, not clear if they are identical.
28) SMC X-2: The long known Be/X-ray binary SMC X-2 was caught in outburst with 0.4 cts s-1 by the ROSAT PSPC (source 547, see KP96 and references therein). Another PSPC observation yielded an upper limit indicating X-ray variability of more than a factor of 670. Optical spectra of the Be counterpart were taken by e.g. Crampton et al. (1978). The Be star is located on the rim of the PSPC error circle (Fig. 4) and is not contained in the MA93 catalogue.
31) XTE J0055-724: X-ray pulsations from this source were discovered by RXTE (Marshall & Lochner 1998) and confirmed in a SAX observation (Santangelo et al. 1998). Santangelo et al. (1998) also report pulsations from archival ROSAT data reducing the positional uncertainty. Stevens et al. (1999) identified a Be star as optical counterpart which corresponds to object 810 in MA93 and which is inside the error circle of PSPC source 241 and HRI source 58.
38) RX J0058.2-7231: RX J0058.2-7231 was detected as weak HRI source by Schmitdke et al. (1999) and identified with a Be star. It is contained in the HRI catalogue (source 76) but not found in the PSPC catalogue of HFPK00. The Be star is not detected in the emission-line star surveys of MA93 and MB99.
40) RX J0059.2-7138: This transient X-ray pulsar with peculiar soft component in the X-ray spectrum was discovered by Hughes (1994) during an outburst with a 0.2 - 2.0 keV luminosity of 3.5 erg s-1. The X-ray source was identified with a Be star by Southwell & Charles (1996) as star 1 in their finding chart which is identical to the emission-line object 179 in MB99.
42) RX J0101.0-7206: This source was suggested as X-ray transient by KP96 with a flux variability of at least a factor of 30. Stevens et al. (1999) identified a Be star as optical counterpart. Object 1 in their Fig. 1f corresponds to entry 1240 in MA93.
49) SAX J0103.2-7209: Israel et al. (1998) reported X-ray pulsations from this source consistent in position with the Einstein source 1E 0101.5-7225. They confirm the Be star suggested as counterpart for the Einstein source by Hughes & Smith (1994) as the only object in the SAX error circle showing strong activity. OGLE observations presented by Coe & Orosz (2000) confirm this. The Be star corresponds to object 1367 in MA93 and was also detected by PSPC (source 143) and HRI (101).
58) XTE J0111.2-7317: Chakrabarty et al. (1998a) reported X-ray pulsations found in RXTE data from this source located about 30´ from SMC X-1. Wilson & Finger (1998) confirmed the pulsations from CGRO BATSE data and Chakrabarty et al. (1998b) derived an improved position from ASCA data. Two Be stars were identified by Israel et al. (1999) within or near the ASCA error circle of 30". The closer of the two was concluded as most likely counterpart of XTE J0111.2-7317 by Coe et al. (1999). This Be star has no counterpart in MA93. A week source with existence likelihood of 14.5 is found in the PSPC catalogue (446). The large error circle of 61" overlaps with the ASCA one and includes the position of the Be star. There is an additional MA93 object (1731) within the RXTE error circle and the second Be star found by Israel et al. (1999) is identical to object 1747 in MA93 but both are outside the ASCA and PSPC confidence regions (see Fig. 4).
59) RX J0117.6-7330: Similar to the previous source X-ray pulsations were discovered from the X-ray transient RX J0117.6-7330 (Clark et al. 1997) in ROSAT PSPC and CGRO BATSE data (Macomb et al. 1999). Between two PSPC observations, about 8 months apart, the count rate diminished by a factor of 270. Clark et al. (1997) identified a Be star counterpart which is identical to object 1845 in MA93 and also within the error circle of X-ray source 506 in the SMC PSPC catalogue.
6.3. Optically unidentified X-ray pulsars
10) AX J0049-732: AX J0049-732 was discovered as X-ray pulsar by Imanishi et al. (1998). Filipovi et al. (2000b) reported two hard X-ray point sources from the catalogue of HFPK00 as possible counterparts to the ASCA pulsar. They suggest one of them (PSPC source 427) as the more likely counterpart due to its identification with an emission-line object in MA93 (number 300).
25) XTE J0054-720: The position of this X-ray pulsar could only be determined to an accuracy of 10´ radius (Lochner et al. 1998). There are at least five X-ray sources detected by the HRI within that circle (labeled 1 through 5 in Fig. 5 which correspond to the catalogue sources 55, 50, 62, 59 and 46, respectively). Object 2, 4 and 5 were also detected by the PSPC (104, 157 and 94). The southern of the three (also detected by IPC, 36) is proposed as active galactic nucleus (AGN) by HFPK00 and the northern (PSPC 94, HRI 46, IPC 32?) was identified as Be/X-ray transient RX J0052.9-7158 (see Sect. 6.2.2). The Be star counterpart of RX J0052.9-7158 coincides with object 623 in MA93. It is not clear if this Be/X-ray binary is identical to the RXTE pulsar. A final identification requires the detection of pulsations from RX J0052.9-7158.
27) XTE J0053-724: Corbet et al. (1998a) discovered this pulsar and report a ROSAT source within the error box. The pulse period, originally confused with AX J0051-722, was clarified by Corbet et al. (1998b). HFPK00 give source 242 as likely counterpart of XTE J0053-724. A single emission-line object from MA93 (717) is found inside the intersecting error circles of IPC source 34 and the PSPC source.
35) AX J0058-720: X-ray pulsations from this source were discovered by Yokogawa & Koyama (1998b) in ASCA observations. The source was detected in archival Einstein IPC, ROSAT PSPC and HRI data which span 18 years and showed flux variations by more than a factor of 100 (Tsujimoto et al. 1999). This high variability already strongly suggests a Be/X-ray binary. A single emission-line object from MA93 (1036) is found within the PSPC error circle (source 114) which is also consistent with the HRI position (73). It is not clear whether IPC source 41 originates from the same X-ray source. It may also be associated with another emission-line object (1039 of MA93) closer to the IPC position or completely unrelated.
53) AX J0105-722: Yokogawa & Koyama (1998c) reported AX J0105-722 as X-ray pulsar. From several nearby objects in MA93 number 1517 is closest to the X-ray position of PSPC source 163. This PSPC source was identified as likely counterpart of the ASCA pulsar in an area of complex X-ray emission by Filipovi et al. (2000a) combining the ROSAT X-ray and radio data. The star 1517 in MA93 is the northern and bluer component of a pair of stars close to the error circles of PSPC and HRI detection (110). The nearby IPC source 53, 77" to the north-east is most likely associated to the SNR DEM S128 (Filipovi et al. 2000a).
6.4. New Be/X-ray binary candidates
2) RX J0041.2-7306: HFPK00 classified PSPC source 404 as foreground star based on the hardness ratios. An emission-line object in the error circle is classified as planetary nebula by MA93 indicating a chance positional coincidence. This makes the identification with the bright star just outside the error circle most likely.
3) RX J0045.6-7313: This source (PSPC 436) was detected once in the 0.9 - 2.0 keV band of the PSPC. An emission-line object in the error circle suggests an Be/X-ray binary.
5) RX J0047.3-7239: The PSPC error circle of RX J0047.3-7239 (source 295) overlaps with that of IPC source 19. An emission-line object (168 in MA93 and classified as late type star) and two radio sources from the catalogue of Filipovi et al. (1998) are located in the X-ray confidence region. A point-like radio source as counterpart would favour an AGN identification leaving the nature of RX J0047.3-7239 ambiguous.
6) RX J0047.3-7312: RX J0047.3-7312 (PSPC 434) is most likely identified with the emission-line star 172 in MA93. The fluxes derived from PSPC detections show a factor of nine variations, supporting that the X-ray source is a Be/X-ray binary. RX J0047.3-7312 is probably identical to IPC source 18, which showed an intensity within the range observed by the PSPC. It is also the likely counterpart of ASCA source 2 in Yokogawa (1999; see Sect. 3), an X-ray binary candidate detected with similar intensity.
8) RX J0048.5-7302: The emission-line object 238 in MA93 is the brightest optical object in the error circle of RX J0048.5-7302 (PSPC 392). A Be/X-ray binary is suggested.
11) RX J0049.5-7331: An HRI detection (source 28) with much improved X-ray position compared to the PSPC (source 511) confirms the identification with the emission-line object 302 in MA93. RX J0049.5-7331 is the probable counterpart of ASCA source 6 in Yokogawa (1999; see Sect. 3) further supporting the likely Be/X-ray binary nature.
12) RX J0049.7-7323: This source (PSPC 468) was detected once in the 0.9 - 2.0 keV band of the PSPC. An emission-line object in the error circle suggests an Be/X-ray binary. RX J0049.7-7323 is also the likely counterpart of ASCA source 7 in Yokogawa (1999), classified as X-ray binary candidate (see Sect. 3).
14) RX J0050.7-7332: RX J0050.7-7332 was only once detected by the PSPC (514) and the emission-line object in the error circle suggests a Be/X-ray binary identification.
15) RX J0050.9-7310: HRI (source 36) and PSPC (source 421) detections are consistent with the identification of RX J0050.9-7310 with the emission-line object 414 in MA93, suggesting a Be/X-ray binary.
17) RX J0051.3-7250: Two close emission-line objects suggest RX J0051.3-7250 (PSPC 349) as Be/X-ray binary, but make the identification ambiguous.
18) RX J0051.8-7159: The emission-line object 502 (MA93) found in the error circle of RX J0051.8-7159 (PSPC 99) is classified as late type star in MA93. An active corona of this star may be producing the X-ray emission. The large error circle contains, however, another bright object which could also be responsible for the X-rays. The nature of RX J0051.8-7159 remains therefore unclear.
21) WW 26: Two emission-line objects from MA93 are found near IPC source 26 (hardness ratio 0.51, WW92). Object 521 is located inside the error circle while 487 can not be completely ruled out as counterpart. No ROSAT detection could improve on the position. A Be/X-ray binary nature is suggested.
26) RX J0053.4-7227: A precise HRI position (source 48 at the rim of the error circle of PSPC 246) with the emission-line star 667 (MA93) as brightest object in the error circle makes RX J0053.4-7227 a likely Be/X-ray binary.
29) RX J0054.5-7228: The uncertainty in the position of RX J0054.5-7228 (PSPC 248) is relatively large and six emission-line objects from MA93 are found as possible counterparts to the X-ray source. It is therefore a likely Be/X-ray binary but the optical counterpart remains ambiguous.
30) RX J0054.9-7245: Precise ROSAT X-ray positions (PSPC 324 = HRI 57) include an emission-line star (809 in MA93) with typical Be star magnitudes as brightest object in the error circles. A factor of five X-ray flux variability (the source was bright during a HRI observation) strengthens the identification as Be/X-ray binary.
32) WW 38 = 2E 0054.4-7237: An emission-line object (904 in MA93) is found inside the error circle of IPC source 38 suggesting a Be/X-ray binary. The source was not detected by ROSAT.
33) RX J0057.2-7233: This weak PSPC source (270) was marginally detected once in the hard 0.5 - 2.0 keV band with a likelihood of 10.4. Unlike all other hard sources in Table 1 it was not detected in the 0.9 - 2.0 keV band and therefore is unlikely a Be/X-ray binary.
34) WW 40 = 2E 0055.8-7229: The error circle of IPC source 40 contains two emission-line objects from MA93. Object number 1021 is identified as Be star AzV 111 while 1016, located further north, is of unknown type. ROSAT detected an X-ray source inside the IPC error circle (HRI 71 and PSPC 117 with consistent positions) which, however, is located between the two emission-line objects. The relation between the ROSAT and the Einstein source and the emission-line objects is unclear. IPC, HRI and PSPC count rates are consistent within a factor of two, which may indicate that they come from the same X-ray source. However, the accurate ROSAT positions make an association with one of the nearby objects from MA93 unlikely.
36) RX J0057.8-7207: Again small error circles from ROSAT HRI (source 74) and PSPC (source 136) observations make the identification of RX J0057.8-7207 with an emission-line star (1038 in MA93) very likely. PSPC detections with factor of eight different intensities and an HRI detection during an X-ray bright state which increases the variability to a factor of about 37, make a Be/X-ray binary nature highly probable.
37) RX J0057.9-7156: Be/X-ray binary candidate from positional coincidence of PSPC source 87 with emission-line object 1044 in MA93 which shows typical optical brightness.
43) RX J0101.3-7211: PSPC detections of this source (PSPC 159 = HRI 95) indicate flux variations by at least a factor of 15 and the source was not detected in other observations (upper limit a factor of 100 below the maximum count rate). This high variability and the presence of an emission-line star (1257 in MA93) in the small X-ray error circles likely exclude any other explanation than a Be/X-ray binary. It also is the likely counterpart of ASCA source 27 in Yokogawa (1999; see Sect. 3), an X-ray binary candidate.
44) RX J0101.6-7204: Two accurate positions from HRI (source 96) and PSPC (source 121) observations suggest the identification of RX J0101.6-7204 with object 1277 in MA93. The factor of three variability supports a Be/X-ray binary nature of RX J0101.6-7204 which is probably identical to the IPC source 46 in WW92.
45) RX J0101.8-7223: RX J0101.8-7223 (PSPC 220 = HRI 97) shows X-ray flux variations of a factor of three. The emission-line star 1288 (MA93) exhibits magnitudes typical for a Be star in the SMC and is located near the overlapping area of HRI and PSPC error circles. We suggest RX J0101.8-7223 as Be/X-ray binary as it is also the probable counterpart of ASCA source 28 in Yokogawa (1999; see Sect. 3), an X-ray binary candidate.
46) RX J0102.8-7157: This weak PSPC source (92) was only once marginally detected in the broad 0.1 - 2.4 keV band. The low detection likelihood of 10.5 and the non-detection in the hard bands indicates that it may not be real, or is at least not a hard source. A Be/X-ray binary nature is therefore unlikely.
47) WW 49: The IPC source 49 (WW92 give a hardness ratio of 0.21) contains a faint emission-line object (1357 in MA93) classified as planetary nebula. The spectral hardness of the IPC source is inconsistent with an SSS interpretation. The positional coincidence is likely by chance.
48) RX J0103.1-7151: This source was detected only once by the PSPC (source 77) and the lowest upper limit indicates variability by at least a factor of five, suggesting the detection of a single outburst. The emission-line object near the rim of the PSPC error circle is, however, the optically weakest (see Table 1), unusual in comparison with identified Be/X-ray binaries and candidates derived from this work. We therefore do not regard RX J0103.1-7151 as prime candidate for a Be/X-ray binary.
50) RX J0103.6-7201: Small error circles from HRI (source 105) and PSPC (source 106) observations make the identification with object 1393 in MA93 very likely. RX J0103.6-7201 shows variability by a factor of three between the ROSAT observations, consistent with a Be/X-ray binary. 51) RX J0104.1-7243: Two emission-line objects and a radio source from the catalogue of Filipovi et al. (1998) close to RX J0104.1-7243 (PSPC 317) make the identification somewhat ambiguous. The most likely identification with emission-line star 1440 in MA93 suggests RX J0104.1-7243 as Be/X-ray binary.
52) RX J0104.5-7121: This source was not detected by the PSPC but the accurate HRI position (source 108) includes only the emission-line object 1470 from MA93 as bright object in the error circle. RX J0104.5-7121 is therefore very likely a Be/X-ray binary.
54) RX J0105.7-7226: An emission-line star (1544 in MA93) in the PSPC error circle (737) suggests RX J0105.7-7226 as Be/X-ray binary.
55) RX J0105.9-7203: A single bright object (the emission-line star 1557 in MA93) is found in the small PSPC error circle (source 120), which makes the identification of RX J0105.9-7203 as Be/X-ray very likely.
56) RX J0107.1-7235: The probable PSPC detection (279) of IPC source 56 improves the X-ray position and allows to identify it with the emission-line star 1619 in MA93. The source was a factor of 10 brighter during the Einstein observation and is also the likely counterpart of ASCA source 36 in Yokogawa (1999; see Sect. 3) detected with a factor 4 higher intensity. A Be/X-ray binary nature is likely.
57) RX J0109.0-7229: The emission-line object 1682 in MA93 is classified as planetary nebula. X-ray sources associated with planetary nebulae appear as SSSs which is not compatible with the hard spectrum of RX J0109.0-7229 (PSPC 253). The positional coincidence of RX J0109.0-7229 is therefore by chance and the nature of the X-ray source is unclear.
60) RX J0119.6-7330: This source (PSPC 501) was detected once in the 0.9 - 2.0 keV band of the PSPC. An emission-line object in the error circle suggests a Be/X-ray binary.
© European Southern Observatory (ESO) 2000
Online publication: July 7, 2000