Left panel: EPIC pn power spectrum of 3XMM J004301.4+413016 (Obs. Id. 0650560301) unbinned events, the first accreting NS found in the nearby galaxy M31 (Esposito et al. 2016; Zolotukhin et al. 2017; see also Sect. 4.6). One peak is above the 3.5σ detection threshold (solid red line) and corresponds to the 1.2 s period. Right panel: EPIC pn power spectrum of the same source for an earlier pointing (Obs. Id. 0112570101) during which the signal is not detected: the 3σ upper limits (red dots) are obtained. Upper limit units are in %/100 (e.g., 0.1 stands for 10%). The increasing values towards high frequencies are due to the x/sin x term in the relation between the signal amplitude (pulsed fraction) and FFT powers (Leahy et al. 1983; see also Sect. 4.7).