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Astron. Astrophys. 329, 606-612 (1998)

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1. Introduction

Soft X-ray Transients (or SXTs) form a small subclass of Low Mass X-ray Binaries (LMXBs) and are composed of a late type low-mass star (usually a dwarf or a subgiant) which loses matter via Roche-lobe overflow onto a massive collapsed primary. The importance of these systems lies in the fact that about 80 [FORMULA] of them are suspected to harbor a black hole. This has been demonstrated in 6 cases: V616 Mon (McClintock & Remillard 1986), GU Mus (Remillard et al. 1992), GRO J1655-40 (Bailyn et al. 1995b), V2107 Oph (Remillard et al. 1996), QZ Vul (Casares et al. 1995) and V404 Cyg (Casares et al. 1992), whose mass functions exceed 3 [FORMULA].

GS 1124-68 (=X-Ray Nova Muscae 1991) was discovered as a bright transient X-ray source on January 8, 1991 by the Ginga (Makino 1991) and the GRANAT (Lund & Brandt 1991) satellites. The analysis of the two-component X-ray spectrum during the outburst (Kitamoto et al. 1992, Grebenev et al. 1992, Ebisawa et al. 1994) allowed the classification of this object as SXT.

Its optical counterpart, GU Mus, was discovered by Della Valle et al. (1991) one week later, on January 15, 1991. GU Mus increased its luminosity from [FORMULA] to [FORMULA], a brightening of [FORMULA] 7 mag which is typical of X-ray novae (van Paradijs & McClintock 1995). The optical spectrum at maximum (Della Valle et al. 1991) showed Balmer, He I, He II, N II and N III emission features superimposed on a blue optical continuum, thus resembling those of outbursting Dwarf Novae (DNe). From these observations a color excess of 0.30 mag was derived, confirmed later on by the analysis of UV spectra (Cheng et al. 1992). In addition, Ball et al. (1995) detected a transient radio emission during maximum, which is another typical feature of outbursting SXTs.

X-ray and optical observations during the decay showed two important behaviours: the presence of an electron-positron annihilation line at 0.511 MeV (Sunyaev et al. 1992, Goldwurm et al. 1992), which is considered a clue for the presence of a black hole (see the review by Tanaka & Lewin 1995), and the appearance of superhumps (Bailyn 1992). The latter phenomenon, first seen in SU UMa-type DNe, takes place only if the mass ratio [FORMULA] is less than 0.25-0.33 (Whitehurst & King 1991). Since the mass of the secondary in SXTs is [FORMULA] 0.5 [FORMULA] (van Paradijs & McClintock 1995, Tanaka & Lewin 1995), the accretor should be a highly-collapsed object. Actually, superhumps in SXTs have been observed in outbursting QZ Vul (Charles et al. 1991), V518 Per (Kato et al. 1995), V2293 Oph (Masetti et al. 1996) and MM Vel (Masetti et al. 1997; Della Valle et al. 1997).

The X-ray lightcurve also showed two secondary maxima at [FORMULA] 70 (Kitamoto et al. 1992) and [FORMULA] 200 days (Ebisawa et al. 1994) after the main X-ray peak. On the contrary, until now there is no indication of optical secondary maxima or minioutbursts.

GU Mus reverted to quiescence about one year after the outburst (Della Valle 1992). In April 1992, when the star was already at minimum, Remillard et al. (1992), found that the mass function of the primary is [FORMULA] 3 [FORMULA], which is already beyond the maximum allowed mass for a neutron star (Rhodes & Ruffini 1974). This value placed GU Mus amongst the galactic black-hole candidates.

In this paper we present the spectrophotometric follow-up of GU Mus, from the discovery to the late decline. Sect. 2 illustrates the observations and the reduction of images and spectra. Sect. 3 shows the analysis of the data, and Sect. 4 discusses the results and draws the conclusions.

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© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997