2. The Light Curve

The GIS light curve is shown in Figure 3.2. The intensity increases systematically through the observation except for a flare at orbital phase 0.4 during which the intensity increases by a factor of 2.5 and decays on a timescale of (3.6$\pm$0.4) x 10³ s. The flare is accompanied by a slight softening of the light curve, as shown in Figure 3.2. The general trend is for the hardness ratio to decrease as the intensity increases, indicating that the X-ray spectrum is luminosity dependent.

Figure 1: Light curve and hardness (4.5-11 keV / 0.7-4.5 keV) plotted against orbital phase. Data from GIS2 and GIS3 have been combined. The gradual X-ray eclipse covers orbital phases $\sim$0.8-0.2.

Figure 2: Hardness plotted against intensity. Two distinct regions are evident. The points on the right hand side (intensity > 20 cts s^-1) are due to the flare at orbital phase 0.4 where the intensity increases with little change in the hardness. In the left hand region the spectrum becomes softer as the intensity increases.

In the Fourier power spectrum of the pre-eclipse data there is a possible QPO. The centroid frequency is 47.4$\pm$0.2 mHz (see Figure 3.2). There is also a possible but marginal QPO in the post-flare data (see Figure 3.2).

Figure 3: Possible QPO in pre-eclipse data.

Figure 4: Possible QPO in post-flare data.