Top panel: power spectrum obtained by averaging 242 intervals, each  ≃ 124 s long, and logarithmically re-binning the spectrum with a factor of 1.1. The white noise level of 1.9880(4), evaluated by modelling the spectrum between 0.5 and 2 kHz with a constant, is subtracted from each power estimate. Bottom panel: residuals in units of σ of the power spectrum with respect to the best-fitting model composed of three lorentzians centred at zero frequency.