Comparison between the amplitude spectrum near f₁ of the data (left) and the amplitude spectrum near f₁ of a simulated data set (right) with the same time distribution. In this test we used only the data up to 2009.0 because in this period it is easier to simulate the behavior of f₁ . For the simulated data we used a single pure sine wave (no noise) with the same frequency and amplitude of f₁ and also with similar long-term frequency and amplitude variations (linear variation of the period with = 1.34 × 10⁻¹², as derived by the O–C analysis, and sinusoidal variation of the amplitude like in Fig. 7upper right panel, green section). Like in the previous figures, the upper left panel is the normalized spectral window (red), while the other panels are the amplitude spectra of data and residuals after one, two, and three prewhitening steps. This simple test shows that up to the secondary peak on the right side of f₁, the data are well reproduced by the simulation, both in terms of frequency and amplitude. See text for more details.