PSD from six different epochs labeled in Fig. 1 (bottom panel). The PSDs are fitted with a model consisting of a zero-frequency Lorentzian modeling the broadband noise, one or two narrow Lorentzians modeling the QPO (fundamental and first upper harmonic components), an additional power-law component needed for PSD in panels a, d–f, and Poisson noise as a constant power component (removed from the PSDs shown here).