Energy-dependent variability functions. The red points show the energy–lag spectrum, and the black points show the covariance spectrum. The gray points represent the time-averaged pn spectrum, while the best-fitting model with the diskbb component removed is shown as a gray curve. The orange-shaded region marks the energies at which the soft band lags the reference band, corresponding closely to the energy range where the diskbb component contributes significantly in the time-averaged spectrum. All statistical errors correspond to the 90% confidence level.