Top panel shows the NICER time-averaged spectrum of Swift J1727.8–1613 (ObsID 6203980105) with a QPO centroid frequency of 0.89 Hz, fitted using the model TBfeo*(diskbb + gaussian + nthComp) together with the residuals of the best-fitting model. The middle panel shows the RMS spectrum of the QPO fitted with the vkdualdk*dilution (red) and vkompthdk*dilution (blue) models with the residuals of the best-fitting model. The bottom panel shows the phase-lag spectrum of at the QPO fitted with the vkdualdk (red) and vkompthdk model (blue) with the residuals of the best-fitting model. The 0.5–10.0 keV energy band is used as the reference for the phase-lag spectra.