and to the review of @wij06. Here we focus our attention on the 1 Hz modulation and give a brief description of the power spectra observed from the fast decay stage on, in order to provide context. A description of our quantitative analysis of the 1Hz modulation follows in the next sections.

We examined the entire data set of all 5 outbursts for evidence of the 1 Hz modulation. Close to the end of the 2002 and 2005 pulse phase drifts, when the fast decay state is almost over, the X-ray lightcurve clearly shows this strong modulation with a repetition frequency of $\sim1$ Hz (Fig. \[modulation\]). It shows up as a strong quasi periodic oscillation (QPO) peak around 1 Hz in the power spectrum (Fig. \[QPO\], see also @wij04 for a similar plot for the 2002 outburst). Later during the re-flares of these outbursts the modulation occasionally recurs, as discussed in more detail in § \[1Hz:appearence\]. The 1 Hz modulation also appears and disappears sporadically during the re-flares of the 2000 but was not detected in the 1998 and 2008 outbursts (cf. § \[1Hz:intro\]).

We calculated the fractional rms amplitude[^1] of the 1 Hz QPO by integrating the power in 