Energy-resolved PSDs for three representative observations of Cyg X-1 at different hardness. The blue, green, and red thick lines in the side panels show the relative contribution from the accretion disk (that is, the quantity F(diskbb)/F(simpl ⊗ diskbb)) for the hard (Γ ∼ 1.8), intermediate (Γ ∼ 2.3), and soft state (Γ ∼ 3.1), respectively. The gray lines show the curves of the other states for comparison. Left: in the hard state, the power spectrum peaks at ∼2 keV. It is double-humped at energies ≳1.5 keV while the second hump reduces in amplitude at lower energies. Center: at Γ ∼ 2.3, the PSD has a single-humped structure that peaks at ∼4 keV. The variability peak shifts to higher frequencies compared to the hard state. Right: in the soft state, the PSD is red noise-like and is strongest at the highest energies covered by NICER.