1. Background subtraction

To investigate the effect of the galactic ridge on the HEXTE spectrum we assumed a thin thermal bremsstrahlung spectrum with kT = 7 keV and a 2-11 keV flux of 8 x 10^-8 erg cm^-2 s^-1 sr^-1 (Koyama 1989). We represented the hard tail component by a power law with photon index -2.65 Gehrels and Tueller (1993) and assumed a flux at 100 keV of 4.5 x 10^-7 photons cm^-2 s^-1 keV^-1 in HEXTE's field of view. Due to the brightness of Cen X-3 and the fact that the galactic ridge spectrum falls off much faster with energy, emission from the galactic ridge does not significantly affect the data. The HEXTE background spectra are accumulated by dwelling on blank sky fields on either side of the source. These were checked to make sure they agreed. Any difference would have indicated the presence of an X-ray source in one of the background fields. Because of uncertainties in the deadtime corrections and collimator response the normalization of the HEXTE background was varied until the count rate between 200 and 250 keV was zero. This renormalization of the background was not required by the PCA data from orbit #1 due to the high source count rate.

The light curve of the observations is shown in Figure 2.1. Each point is accumulated on a 256 s interval. There is considerable short-term variability. The source appears to have made a transition from its high to its low state.

Figure 1: Light curve of the observations accumulated in 256 s intervals.