Ensemble PSDs of quasars with λ_rest = 3000 Å, log(λ_Edd) = − 1.0 and various M_BH (points with different colours on the plot). Black points indicate the universal PSD of all the quasars, computed when light curves are normalized with the gravitational timescale, T_g. Black dashed and solid lines show the best-fit single and bending power law models to the black points, respectively. Coloured lines show the same best-fit models re-scaled in frequency by 1/T_g, for each of the BH mass listed in the bottom left part of the plot (see Sect. 5.2 for details).