Covariance rms amplitude (in percent, left column) and phase lags (radians, right panel) of the imaginary QPO that cause the coherence drop of Cygnus X-1 as a function of energy. The centroid frequency of the imaginary QPO is indicated in each panel. A cubic polynomial has been fitted in each case to track the corresponding minima or maxima. In each panel, we show cubic model realisations in grey that depict the 1σ confidence range of the fits.