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Astron. Astrophys. 321, 776-790 (1997)

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1. Introduction

The X-ray transient GS 2023+338 was discovered with Ginga (Makino et al. 1989) during an outburst in 1989. The optical counterpart was identified as the old Nova Cygni 1938 (V404 Cygni, Wagner et al. 1989). From optical spectroscopy Casares, Charles & Taylor (1992) found that the mass function is 6.26 [FORMULA] 0.31 [FORMULA] which makes it the most convincing stellar-mass Black-Hole Candidate (BHC) currently known. Casares and Charles (1994) further refined the mass function to be: 6.08 [FORMULA] 0.06 [FORMULA]. Modelling of the ellipsodial variations of the source by Shahbaz et al. (1994) led to a most probable mass of the compact object of 12 [FORMULA] (see however, Hasswell 1992, who argued, for A 0620-00, that superhump variations with a period different from the orbital period may lead to distortions of the orbital lightcurve and incorrect parameter estimates)

Observations of GS 2023+338 with Ginga (All Sky Monitor and Large-Area Counters) have been reported by Kitamoto et al. (1989). Spectral variations were studied by In 't Zand et al. (1992) using data from Kvant. Miyamoto et al. (1992, 1993a) studied the time variations of GS 2023+338 and compared it to the behaviour of other BHCs in the low-intensity state 1. Long-term variability was discussed by Terada et al. (1994). The source has also been detected at radio wavelengths (Hjellming & Han 1989), with a flux of 1.6 Jy, which is high for X-ray transients (see Hjellming 1995).

In this paper we investigate the relation between the spectral changes of GS 2023+338 and the rapid variability behaviour of its X-ray flux. Application of this kind of analysis to low-mass X-ray binaries (LMXB) in which the accretor is a neutron star has led to a distinction of two groups of accreting low magnetic field neutron stars in LMXBs: viz. atoll- and Z-sources (Hasinger & van der Klis 1989).

Analysis of the fast-timing behaviour has been applied to other sources such as pulsars (Takeshima 1992), BHC (Miyamoto et al. 1991, 1992, 1993a, b, 1994a, b) and Cir X-1 (Oosterbroek et al. 1995). The general expectation underlying these analyses is that the character of the compact object is reflected in the characteristics of the fast-timing behaviour and thus the fast-timing behaviour can be used as a tool to probe the character of the compact object (e.g., Miyamoto 1993, 1996).

From these analyses a global picture is emerging where the main parameters which determine the behaviour are the mass accretion rate ([FORMULA]) and the strength of the magnetic field of the compact object (see e.g. Van der Klis 1994a, 1994b, 1995a). The character of the accreting object (black hole vs. neutron star) may play a smaller role in determining the fast variability behaviour, and also the spectral behaviour, than the magnetic field strength. A classical example of a source whose fast timing behaviour is reminiscent of "typical" black-hole behaviour is the neutron star accretor Cir X-1 (Oosterbroek et al. 1995). Cir X-1 also shows spectral characteristics which are resembling those of BHCs (e.g., a hard spectrum, see Dower, Hale & Morgan 1982).

In this paper we use Colour-Colour Diagrams (CDs) and Hardness-Intensity Diagrams (HIDs) to study spectral variations, and Fourier transformations to study the power spectra and phase delay spectra. We make a detailed comparison between the properties of this source and the data which are available of other BHCs and to a lesser extent neutron stars.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998