2. The Observations

Cen X-3 was observed by EXOSAT six times. In this chapter I consider data taken with the gas scintillation proportional counter (GSPC) during two of these observations when the source was in its high state. Four datasets were extracted from these two observations. The telescope is described by Peacock et al. (1981) and in Section 2.1. The GSPC observations of Cen X-3 have already been used to investigate the pulse-phase dependence of the iron line intensity by Takalo et al. (1990). Thus I will concentrate on the shape of the continuum spectrum in this study. Takalo et al. combined GSPC data from three observations including the present two and detected pulsations. They also divided their data into five orbital phase bins. Because of the variable nature of the source I chose to keep the observations separate. The datasets used in this analysis are listed in Table 1.

Table 1: The GSPC Data.

Dataset	Date	Orbital Phase	Exposure (s)	L373	EW4
1	1985 May 21	0.81-0.91	16312	1.6	246$\pm$11
2	1985 May 22	0.95-0.13	31288	0.02	1700$\pm$1200
3	1985 May 22	0.30-0.60	10776	2.1	250$\pm$11
4	1985 August 9	0.61-0.87	45072	2.7	249$\pm$12

The GSPC's overall effective area (150 cm²) is much smaller than that of the medium energy detector (ME; 1600 cm²), but around 7 keV the efficiency of the GSPC becomes greater than that of an ME Ar detector half. In addition, the GSPC's energy resolution is better than that of the ME by a factor of two. Thus the GSPC is more suitable than the ME Ar detectors for iron line spectroscopy. Also, the GSPC's effective area is greater than that of the ME's Ar detectors at high energies. The effective energy range of the GSPC was 2-25 keV for the gain settings of the present data. Previous studies of data from the ME have investigated the orbital and pulse phase dependence of the iron line amplitude and have used the fitted column density as a probe of the structure of the stellar wind (e.g. Day and Stevens (1993)).

Footnotes

...L₃₇³

Unabsorbed luminosity in the 2-20 keV band in units of 10³⁷<#rm#> erg s^-1 assuming a distance of 8 kpc.

...EW⁴

Pulse-phase averaged iron line equivalent width (eV).