Spectrogram of the last flip-flop transition shown in Fig. 12. Colours denote the Leahy normalised, Poisson noise subtracted power. The light curve is plotted in green. The time is measured in seconds of observing time since the start of the orbit. We computed the PDS, and the average count rate, for each of the 20 s intervals into which we divided the data. We plotted a smaller frequency range for this figure, due to significant noise at low frequencies, a consequence of the short interval size.