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Astron. Astrophys. 345, 117-120 (1999)

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1. Introduction

SMC X-1 was detected during a rocket flight (Price et al. 1971). The discovery of eclipses with the Uhuru satellite with a period of 3.89 days (Schreier et al. 1972) established the binary nature of the source. The optical counterpart Sk 160 has been identified as a B0 I supergiant (Webster et al. 1972; Liller 1973). Optical photometry indicated the presence of an accretion disk influencing the optical light curve (van Paradijs & Zuiderwijk 1977). In X-rays both low- and high-intensity states have been observed with an X-ray luminosity [FORMULA] varying from [FORMULA] to [FORMULA] (Schreier et al. 1972; Tuohy & Rapley 1975; Seward & Mitchell 1981; Bonnet-Bidaud & van der Klis 1981). Angelini et al. (1991) discovered an X-ray burst from SMC X-1 probably from type II like in the Rapid Burster generated by an instability in the accretion flow. A [FORMULA] day quasi-periodicity was suggested by Gruber & Rothschild (1984) from HEAO 1 (A4) data, and was confirmed by more recent RXTE observations (Levine et al. 1996; Wojdowski et al. 1998). A pulse period of [FORMULA] sec (Lucke et al. 1976), neutron star mass [FORMULA], companion mass [FORMULA] and companion radius [FORMULA] (Primini, Rappaport & Joss 1977) are well established. A decay in the orbital period [FORMULA] was found (Levine et al. 1993), probably due to tidal interaction between the orbit and the rotation of the companion star, which is supposed to be in the hydrogen shell burning phase. Li & van den Heuvel (1997) argued that the magnetic moment in SMC X-1 may be low like that of the bursting pulsar GRO J1744-28, i.e. [FORMULA].

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999