Simultaneous fitting of the PDS and CS of AT2019wey with a multi-Lorentzian model assuming constant phase lags, using the combined NICER data in zone 5 as an example. Top panels: Best fits (solid black lines) and residuals for the PDS (left panel) in the 0.3 − 2 keV (blue) and 2 − 12 keV (red) bands, and to the real (blue) and imaginary (red) parts of the CS rotated by 45° (right panel). Middle panels: Real (left) and imaginary (right) parts of the non-rotated CS, showing a linear scale on the y-axis. Each model consists of six Lorentzian components; the one corresponding to the imaginary QPO is highlighted with thicker lines. Bottom panels: Phase-lag (left) and coherence-function (right) frequency spectra, along with their residuals with respect to the model derived (solid black curves) from the simultaneous fitting of PDS and non-rotated CS. The dashed black curves show the derived model without the imaginary QPO and without refitting. We rebinned the plots by a factor of 5 to improve the visibility of the dip in the coherence function.