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Astron. Astrophys. 359, 573-585 (2000)

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3. ASCA binary pulsar candidates

The 1st ASCA Catalogue of X-ray sources in the SMC was compiled by Yokogawa (1999). The sources were classified according to their hardness ratios and Be/X-ray binaries were detected as sources with hard X-ray spectrum in the ASCA energy band. From this classification Yokogawa (1999) proposed eight new Be/X-ray binary candidates (binary pulsar candidates, BPc). We correlated our list of Be/X-ray binary candidates with the ASCA catalogue and find for five of the eight BPc a likely counterpart within 2´ (the maximum ASCA position uncertainty). In addition one out of the nine (probably because of its low flux) unclassified ASCA sources also correlates with a PSPC Be/X-ray binary candidate. In Table 2 the Be/X-ray binary candidates with likely ASCA counterpart are summarized. The first three columns show ASCA source number, classification and observed X-ray luminosity (0.7 - 10 keV, but corrected for the distance of 65 kpc used throughout this paper) from Yokogawa (1999). The next two columns contain PSPC source number and distance between ASCA and PSPC position and the last two columns give the same for HRI sources. The X-ray luminosities observed by ASCA are generally a factor of 1.2 - 2.4 higher than the ROSAT values (Table 1) which may only partly be explained by X-ray variability. Finding a systematically higher luminosity with ASCA is probably caused by the different sensitive energy bands and/or different intrinsic source spectrum. In particular the PSPC count rate to luminosity conversion is very sensitive to the assumed absorption. In the case of ASCA sources 36 (PSPC 279) and 7 (PSPC 468) the ASCA/ROSAT luminosity ratio is much higher than average (4.2 and 17, respectively), indicating strong flux variability.


Table 2. Be/X-ray binaries with likely ASCA counterparts

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© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000