Left panel: a segment of the 0.4−10 keV light curve of the pn observation 901 where a strong and sudden increase and decrease in flux is evident. Right panel: four 0.2−10 keV EPIC-pn spectral slices extracted at the beginning (during the first ks) of the high-flux level (black), during the remaining of the high-flux level (red), during the first ks of the decrease in flux (green), and during the remaining decreasing flux level (blue): the time intervals are marked in the top panel with the same colour-code. Error bars have been omitted for clarity.