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

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1. Introduction

In high mass X-ray binaries (HMXBs) a neutron star or black hole orbits a massive early-type star and accretes matter either via Roche-lobe overflow or from the stellar wind which powers the X-ray emission (for recent reviews see Nagase 1989; White et al. 1995; Bildsten et al. 1997). One divides the class of HMXBs according to the stellar type of the mass donor star into supergiant X-ray binaries with luminosity class I-II OB star and Be/X-ray binaries with luminosity class III-IV Be star companions. Be/X-ray binaries form the larger sub-group of HMXBs. Balmer emission lines in the optical spectrum and a characteristic infrared excess are attributed to the presence of circum-stellar material, probably forming a disk in the equatorial plane of the Be star. Be/X-ray binaries often show transient behaviour with two types of outbursts. X-ray outbursts repeating with the orbital period are most likely associated with the passage of the neutron star through the circum-stellar disk in an eccentric orbit while giant outbursts, often lasting longer than a binary period, probably arise from an expansion of the disk.

Currently about 100 HMXBs and candidates are known. Nearly one third were found in the Magellanic Clouds (MCs) from which the majority is located in the Small Magellanic Cloud (SMC, Coe 1999). Most of the Be/X-ray binaries in the SMC were discovered in recent years by X-ray missions like ASCA, BeppoSAX, ROSAT and RXTE (Nagase 1999). From 20 optically identified HMXBs in the SMC only one is securely associated with a supergiant system (the X-ray pulsar SMC X-1) and from 11 of the 19 Be/X-ray binaries X-ray pulsations were detected. Five additional X-ray pulsars are yet to be identified, but are most likely also Be systems. The location of such a large number of HMXBs at a similar distance makes the SMC ideally suited for statistical and in particular spatial distribution studies of the population of HMXBs in a galaxy as a whole.

Recent surveys to look for [FORMULA] emission-line objects in the SMC were performed by Meyssonier & Azzopardi (1993, hereafter MA93) and Murphy & Bessell (1999, MB99). The survey of MA93 mainly covers the main body and eastern wing of the SMC and their catalogue lists 1898 emission-line stars. The catalogue of MB99 covers nearly all the area where ROSAT PSPC observations of the SMC are available (except the southern half of the most south-east observation) but is less sensitive (372 objects, partially in common with MA93). A main goal of MA93 and MB99 was to identify planetary nebulae in the SMC, however, the catalogues also contain Be stars. MA93 state that all three at the time of publication known B[e] supergiants which were covered by the survey were detected. Very few Be/X-ray binaries were known in the SMC until 1993 and it was not noticed that the Be stars proposed as optical counterparts for SMC X-3, 2E 0050.1-7247 and 2E 0051.1-7304 were listed in MA93 as emission-line stars (LIN 198, AzV 111 and AzV 138, respectively). A correlation of the larger sample of Be/X-ray binaries known today in the SMC shows that most of them are found in the catalogues of MA93 and MB99. In this paper we use the identification of X-ray sources with emission-line stars to propose new very likely candidates for Be/X-ray binaries in the SMC (Sect. 2). X-ray source catalogues of the SMC which we used for our correlations with the emission-line star catalogues were published by Wang & Wu (1992) based on Einstein IPC observations (Seward & Mitchel 1981, Inoue et al. 1983, Bruhweiler et al. 1987) and by Haberl et al. (2000, HFPK00) produced from ROSAT PSPC data. We also used a preliminary version of the ROSAT HRI catalogue of Sasaki et al. (2000b).

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© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000