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Astron. Astrophys. 354, 983-986 (2000)

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1. Introduction

Cyg X-3 was discovered as an X-ray emitting object, which was subsequently found to emit radiation at all wavelengths, starting from radio to high energy gamma rays (Bonnet-Bidaud and Chardin 1988). The X-ray emission shows a periodicity of 4.8 hrs, which is attributed to an orbital period of two objects of a binary system, one of which is a compact object. Recently it was shown (van Kerkwijk 1993; Schmutz, Geballe and Schild 1996; van Kerkwijk et al. 1996) that the primary star is a Wolf- Rayet star. Using the radial velocities of infrared lines, Schmutz et al. (1996) obtained a mass function and from this they suggested that the compact object is a black hole.The X-ray emission is suggested to occur when the black hole accretes gas from the strong wind from the Wolf-Rayet star.

Extensive observations of Cyg X-3 were made in the X-ray band (see Bonnet-Bidaud & Chardin 1988; Hermsen et al. 1987; Matz et al.1996). The phase-intensity diagram was obtained both in the soft X-ray band (2-12 keV) and the hard X-ray band (20-200 keV). In the soft X-ray band the light curve is asymmetric and shows a minimum which is designated as phase zero; the maximum occurs at about phase 0.75 (Bonnet-Bidaud & Chardin 1988). In the hard X-ray band, the minimum also occurs at phase zero, but the maximum occurs at a phase 0.5 (Hermsen et al. 1987; Matz et al. 1996).

The infrared emission from Cyg X-3 also shows the 4.8 hr periodicity (see Bonnet-Bidaud and Chardin 1988 for references) and the minimum of the light curve occurs at the X-ray minimum. The early measurement by Mason, Cordova and White (1986) showed a similarity of the X-ray and infrared light curve. The recent photometric measurement of the infrared curve by van Kerkwijk (1993) shows a flat infrared maximum between phase [FORMULA]0.4 and [FORMULA]0.7. The observations of Jones et al.(1994) agree with the result of van Kerkwijk (1993). Infrared spectroscopy was performed on Cyg X-3 by van Kerkwijk (1993), van Kerkwijk et al. (1996), Schmutz et al.(1996) and by Fender, Hanson and Pooley (1999). Line shifts were observed, the maximum blueshift occurring at the time of the infrared minimum and the maximum redshift half an orbit later (van Kerkwijk et al. 1996).

The configuration of the Wolf-Rayet star and the compact object at the time of the X-ray and infrared minima seems to be uncertain. Van Kerkwijk (1993) suggested that the lines which showed the systematic blue shift originate in a region shadowed by the Wolf-Rayet star from the X-radiation of the compact object. This will imply that at the time of the blue shift (X-ray and infrared minimum) the compact star is at the superior conjunction. The phase 0.0 will then correspond to the configuration when the compact object is behind the Wolf-Rayet star along the line of sight. Schmutz et al. (1996) argue that the emission line profiles observed do not show the form predicted by the model of van Kerkwijk (1993).They also point out that at all phases the HeII 1.87 µm line shows the full width of the wind's maximum expansion velocity. Schmutz et al. (1996) further state that this is in "direct conflict, because the HeII 1.87 µm line can only emit over the full width at all phases if it is formed in an accelerating wind in which it remains fully ionised in the region that is in the X-ray shadow". They suggest that the weak lines which show the shifts occur close to the photosphere of the Wolf-Rayet star. The blue shift will then occur when the Wolf-Rayet star is at the descending node and therefore the X-ray and infrared minima, corresponding to phase 0.0 will occur at the ascending node of the compact object. Clearly there is a difference in the interpretations of van Kerkwijk (1993) and Schmutz et al. (1996). This controversy needs to be resolved by further comprehensive study of the origin of the He lines. Pending the resolution of this controversy, in this paper we will use the picture suggested by Schmutz et al. (1996) for two reasons-firstly using their phasing, we have been able to account for the infrared light curve based on emission from a shock produced by the compact object (Apparao 1997), and secondly it is possible to interpret the phase variation of the soft and hard components of X-ray emission as given below. The phases of the minima and maxima of X-ray and infrared emissions are then as shown in Fig. 1.

[FIGURE] Fig. 1. The orbital phase diagram of Hard X-ray, Soft X-ray and infrared maximum and minimum emissions from Cyg X-3. The soft X-ray maximum occurs in a small band around phase 0.75 (shaded), while the infrared maximum is flat and is between [FORMULA]0.4 and [FORMULA]0.7 (cross-hatched)

Several models have been suggested to explain the light curve in the soft X-ray band (see Bonnet-Bidaud and Chardin 1988 for references). In these models, absorption and scattering of the X-radiation, either by the dense wind of WR star or a gas cocoon around the system is suggested to be the cause of the minimum. The absorption is the least at the inferior conjunction, that is at the phase of 0.75 (Fig. 1), and the maximum of intensity should occur at this phase. Similarly the maximum absorption occurs at the superior conjunction and the minimum should occur at phase 0.25. The occurrence of the hard X-ray maximum at phase 0.5 and the occurrence of a minimum at phase zero do not accord with the simple absorption or scattering picture suggested in the earlier models.

We had earlier suggested that infrared emission occurs from the hot post-shock gas produced by the supersonic motion of the secondary through the WR wind (Apparao 1997). This emission causes the observed infrared modulation; the maximum occurs around phase 0.5 when the observer faces the shock, and the minimum occurs at phase zero, when the cooled post-shock gas blocks the infrared radiation from the observer's view.

In order to explain the observed X-ray phase-intensity relation, we suggest that X-ray emission occurs both from the accreting compact object as well as from the hot post-shock gas. Modulation of the X-ray emission occurs due to Compton scattering by electrons inthe wind of the WR star and also by electrons in the hot post-shock gas. In the following we will detail this picture.

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000