Single-frequency (top panels) and global-frequency (bottom panels) power spectral densities of PKS2155−304. Left panels are related to the 100 MeV–300 GeV Fermi light curve, and right panels to the R-band light curve. Lomb-Scargle spectrum of the input time-series data is given in blue and the best-fit noise spectrum in red. The 95.0% and 99.7% false alarm levels are reported with yellow and green lines. Solid and dashed lines refer to PL and AR1 models, respectively.