Simultaneous fit of the power and cross spectra of Cyg X-1 for NICER observation 2636010101. The data can be modeled with one zero-centered Lorentzian (dashed line), two broad Lorentzians (dotted and dash-dot-dotted lines), and one narrow Lorentzian at 1.57 Hz (dash-dotted lines). The total fit model is shown as a black line. The fit model with and without the narrow component evaluated on the lags and coherence is shown as a solid and dash-dotted green line, respectively. (a) Power spectral density and fit residuals with and without the narrow component at 1.6 Hz. (b)–(c) Real and imaginary part of the cross spectrum (soft band: 0.5–1 keV, hard band: 2–4 keV) and the fit residuals with and without the narrow component. The additional Lorentzian is very significant in the imaginary part. (d)–(e) Model evaluated (not fitted) on the phase and time lag spectra. The model without the narrow component is shown as green dash-dotted line. (f) The coherence spectrum changes only slightly when including the narrow component. The dip at the overlap of L₁ and L₂ emerges because the Lorentzians are assumed to be incoherent with respect to each other.