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Astron. Astrophys. 346, 82-86 (1999)

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1. Introduction

The soft X-ray transients (SXTs), sometimes referred to as "X-ray novae", are low-mass X-ray binary systems in which a late-type star loses material via an accretion disc to a neutron star or black hole. They undergo brief outbursts, typically lasting a few months with a recurrence time of the order of decades (see Tanaka & Shibazaki 1996 and van Paradijs & McClintock 1995 for reviews of their X-ray and optical properties).

The X-ray transient V616 Mon (=A0620-00) was discovered in 1975 (Elvis et al., 1975). When the system faded into quiescence the binary nature of the system was established; the orbital period was determined to be 7.75 h and the secondary star was found to be a K dwarf (Oke 1977; McClintock et al. 1983). McClintock & Remillard (1986) then went on to measure the orbital radial velocity curve of the secondary star, thereby obtaining a firm minimum mass for the compact object (i.e. the mass function) of 3.08 [FORMULA]. This result immediately placed V616 Mon amongst the best black hole candidates (see van Paradijs & McClintock 1995).

High resolution optical spectroscopy can provide further constraints on the system masses, as a measurement of the rotational broadening of the secondary star absorption features leads directly to the ratio of the component masses (Marsh et al. 1994 hereafter MRW). But the binary inclination, can only be determined by exploiting the ellipsoidal modulation of the secondary star, i.e. the variations caused by observing the differing aspects of the gravitationally distorted star as it orbits the compact object (see Shahbaz et al. 1993 and references within). In V616 Mon these variations have been observed at both optical (McClintock & Remillard 1986; Haswell et al. 1993) and infrared (IR) wavelengths (Shahbaz et al. 1994; hereafter SNC).

SNC) determined the binary inclination for V616 Mon by modelling the IR ellipsoidal modulation of the secondary star. They obtained [FORMULA], which when combined with the value for the binary mass function and mass ratio resulted in a most probable mass for the compact object of [FORMULA]10 [FORMULA] (5.1-17.1[FORMULA]; 90 percent confidence).

There are several complications in such a determination of the binary inclination; inevitably the optical light from the mass donor is diluted by the flux from the quiescent accretion disc. Also, the ellipsoidal light curve may be distorted by other variable contributions to the observed flux; such as the bright spot, star spots and variable disc emission including the superhump phenomenon. Underestimating the diluting flux leads to an underestimate in the binary inclination. At optical wavelengths it is clear that the accretion disc contributes a significant amount of flux to the optical light curves, which makes any interpretation of these light curves somewhat dubious. This can be seen from the scatter and asymmetric maxima observed in the optical light curves of V616 Mon by McClintock & Remillard (1986). The veiling by the continuum emission from the accretion disc has been measured by MRW. They find that 94 percent or more of the optical flux close to H[FORMULA] comes from the secondary star, a fraction which rises with increasing wavelength, while the veiling ratio falls. Therefore one would expect the veiling by the accretion disc to be very small or even negligible in the K band.

Haswell et al. (1993) attempted to deduce the binary inclination from modelling their optical multi-colour light curves with a combination of an ellipsoidal variation and a grazing eclipse, assuming a large circular disk. The inclination derived from the grazing eclipse constraint is in serious disagreement with that determined from the ellipsoidal variations in the IR light curves (SNC)). However, it is known that in low-mass X-ray binaries and cataclysmic variables the outer, cool regions of the accretion disc can be a strong source of IR radiation (e.g. Beall et al. 1984; Berriman et al. 1985). Consequently, this introduces doubts on the SNC) assumption that the observed IR flux has very little contamination by the accretion disc. This would then imply that any interpretation of the ellipsoidal variations of the secondary star in the IR might be misleading. Nevertheless, for the SXT V404 Cyg we have already shown directly that the IR accretion disc contamination is small; it only affects the implied mass of the black hole by at most 2 [FORMULA] (Shahbaz et al., 1996).

We therefore decided to obtain through IR spectroscopy a direct measurement of any contamination of the IR flux by the accretion disc in V616 Mon. From this we can determine the effect it has on the interpretation of the ellipsoidal variation of the secondary star, and hence on the component masses.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999