PSDs of PKS 2155-304 in the four sub-band observed from ObsIDs 0124930301 (top), 0124930501 (middle), and 0124930601 (bottom). The black solid circles mark the frequency threshold at which white noise starts to dominate the PSD and the dotted horizontal lines mark the white noise level for each band. 1σ, 2σ, and 3σ significance levels are calculated based on the simulation of 5000 LCs with the Gaussian process (GP) method. The resulting power is a dimensionless quantity that lies in the range 0<P<1.