Energy-resolved coherence maps of Cyg X-1 with respect to three different reference bands (written in the top right corner of each panel and indicated via the shaded gray region on the right). The power map for each observation is shown in the top row for reference. Left column: in the hard state, the coherence is generally close to unity in the probed frequency range, except for a drop between 1–2 Hz. Center column: when the PSD is single-humped at Γ ∼ 2.3, the coherence is still close to unity. This behavior is largely independent to the chosen reference band. Right column: in the soft state, the high and low energy variability is incoherent with respect to each other. This can be isolated by choosing a low or high energy reference band. We emphasize that the choice of the reference band has a drastic influence on the derived coherence values for soft state.