PSD calculated as the modulus-squared of the DFT, which is normalized with [rms/mean]². Top panel: PSD of light curve from the periodic segment (XMM2000 data on left and XMM2019 data on right). The red dashed line shows the single-trial 99.78% significance curve, while the black dashed line shows the best-fitted PSD model with a powerlaw plus constant (P(f) = Nf^−α + C). The corresponding data/model ratios are shown in the lower panel. The vertical line marks the QPO frequency found from the LSP and WWZ analyses (see text). Bottom panel: PSD for the full light curve of two XMM observations where no QPO is seen at the frequency of ∼6.7 × 10⁻⁴ Hz.