the 9.95 Hz wide band $0.05-10\rm\, Hz$. This frequency band was chosen because it is here that the power is observed in the large majority of the observations. Some power above 10 Hz is observed in some cases as an extended tail of the 1 Hz QPO (Fig. \[QPO\]), but selecting an upper frequency of $20\rm\, Hz$ does not change the results significantly. Clearly, using this method we sum up all the power from the different components in the power spectrum: power of the 1 Hz QPO, its harmonics, and the broad band noise. So everything in the range 0.05–10 Hz is included, and therefore this method is not optimal to measure the power generated by one single component. However, the rms calculated in this way is empirical and independent from any model used to fit the data. The fractional rms amplitude in the 0.05–10 Hz band was in the range 10$-$125% in all observations where the modulation was detected.

The QPO is usually quite broad, and its shape cannot be satisfactorily fitted by single or multiple Lorentzians, as it has a sharp fall-off in power at lower frequencies. In some observations a second harmonic peak is visible (Fig. \[QPO\]). 