X-ray time lags for PG 1448 (top), IRAS 13224 (middle), and PG 1211 (bottom). The “low-/high-flux w/wo UFO” observations are marked in black and royal blue, respectively. The first column shows the unfolded stacked flux energy spectrum with respect to a power law of Γ = 2 of each AGN. The second column shows the lag-frequency spectrum between soft and hard energy bands. The arrows separately refer to the frequencies, at which the PSD of the highest-energy bin (in the lag-energy spectra) becomes dominated by Poisson noise, for the corresponding stacked observations. The third and fourth columns present the low- (third) and high-frequency (fourth) lag-energy spectra separately at particular frequencies, marked in the plots. The shaded blue regions in the low-frequency lag-energy spectra show the 1σ spread in lags from MC simulations (see details in Sect. 3.2).