QPO on the X-ray flux using a similar plot, and we found no clear dependence.

We then calculated the upper limits (quoted at the 98% confidence level) for all the observations after the end of the fast decay where the QPO was not detected. The upper limits were calculated per observation. In the 1998 data two complications occur: the observations ended immediately after the beginning of the re-flares, and the fast decay reached very faint fluxes ($< 1$ mCrab) at its minimum. Only in one observation J1808 was detected; then the upper limit was 55$\%$ rms. A 1 Hz modulation therefore cannot be completely excluded for this outburst. In 2000, 2002, 2005 and 2008 the most constraining upper limits are 15$\%$, 9$\%$, 19% and 19% rms, respectively.

Across the 2000, 2002 and 2005 outbursts, the 1 Hz QPO was observed in a rather narrow range of luminosities during the re-flares ($2$–$15$ mCrab, 2–10 keV), in contrast with the larger range of luminosities covered by the outbursts ($\sim 1$–$80$ mCrab). The [[*Swift*]{}-XRT]{} and [[*XMM*]{}-Newton]{} observations had insufficient time resolution or an insufficient number of counts to probe the presence of the 1 Hz QPO below 1 mCrab.

The appearance of the 