onset of the 1 Hz modulation. The average power spectrum shows a fractional amplitude of $(21\pm 1)\%$ rms in the 0.05–10 Hz band. In chunk B we find a 1 Hz QPO clearly present with a fractional amplitude of $(68\pm4)\%$ rms.

We then calculated power spectra from 256 s data segments and determined the fractional rms amplitude in the 0.05-10 Hz band. (see Fig. \[appears\]). Significant power is detected in three segments during the observations of chunk A, meaning that some low level activity is already present in the same frequency range where the QPO will later appear. In chunk B power is nearly always detected; the onset of the 1 Hz QPO must have occurred within the 5000 s gap. At MJD 53540.48 the amplitude shows an abrupt decrease from $\sim 60\%$ rms down to $<30\%$ (98% confidence level) on a time scale of $256$ s.

We also calculated the power in a number of frequency bands in the range 1/128 to 256 Hz for all power spectra of chunk A. The power is always consistent with zero at the three sigma level in all the frequency bands except for the 0.05-10 Hz band in the three segments mentioned 