Left: lag energy spectrum. It is obtained by computing time lags in Fourier frequency space, between the energy band 0.3 − 10 keV and a number of adjacent energy channels, averaging the resulting lags over the frequency interval 10^-5 − 10^-4 Hz. Right: coherence between 0.3−1 and 1−10 keV energy bands as a function of Fourier frequency once a correction for the Poisson noise effects is applied (Vaughan & Nowak 1997).