Left-hand panel: the frequency-dependent lag between 0.3–1 and 1–5舁keV. A positive lag indicates that the hard band lags behind the soft band. We see a hard lag at ν < 3.5 × 10⁻⁵舁Hz, and zero lag above that frequency. Poisson noise begins to dominate the power spectrum at ν ∼ 5 × 10⁻⁴舁Hz. (The blue hash shows the frequency range explored for the lag-energy spectrum in the left-hand panel of Fig. 8.) Right-hand panel: the frequency-dependent lag between the 3–5 and 5–8舁keV bands. The hard lag extends up to 3 × 10⁻⁴舁Hz. (The red hash shows the frequency range for the lag-energy spectrum in the right-hand panel of Fig. 8.)