Phase lags in radian (top) and the coherence function (bottom) for the second (left) and third (right) EPIC-PN observation of Swift J1727.8−1613. The continuous lines show the predicted model from the fits of the PS and CS represented in Fig. 1. The bottom panels show the residuals defined as (data-model)/error. The dotted vertical red line indicates the central frequency of the imaginary QPO for each observation.