Phase lags in radians (colour points) and intrinsic coherence (gray points) of each region of the HID of Cygnus X-1 restricted to frequencies between 0.02 Hz and 30 Hz. The vertical stripe indicates the minimum of the coherence drop and the maximum of the cliff of the phase lags. The phase lags and coherence are computed between 0.3−2 keV and 2−12 keV energy bands, with the latter used as a reference band.