Left panel: Power spectra of the second (top) and third (bottom) EPIC-PN observations of Swift J1727.8−1613 in the 0.3−2 keV (black) and 2−12 keV (orange) energy bands. Right panel: Real (green) and imaginary (pink) parts of the cross spectra of the same two observations using the 0.3−2 keV and 2−12 keV energy bands, rotated by 45° (see Sect. 3.2 for more details). The vertical red line shows the position of the central frequency ν₀ of the imaginary QPO. The total model is shown with the continuous line, and the individual Lorentzians are shown as dotted lines. The residuals defined as Δχ = (data–model)/error are plotted below each fit.