In the upper part of the plot we show the power spectrum produced using the MOS2 event list as an example. In the bottom part of the plot we show the curves representing the upper limits to the non-detection of pulsations calculated according to Israel & Stella (1996). The solid, dashed and dotted lines correspond to the different sampling times and event list combinations discussed in the text. The two horizontal dashed lines represent the 20% and 30% upper limit levels on the pulsed fraction upper limits as a function of the frequency. The most stringent upper limits we could provide are at 20–30% for frequencies in the range 0.005–200 Hz.