The assumed PSD shape in case A (continuous line) and case B (dashed line) scenarios discussed in Sect. 4.5. ν_br and ν_br,2 are the characteristic “frequency breaks”, at which the PSD slope changes, while ν_min and ν_max are the minimum and maximum frequencies sampled by the light curve segments used in this work. We assume a second break frequency which is 10 times smaller than ν_br. The regions filled with diagonal-dotted lines (vertical-continuous lines) indicate the area below the PSD shape in case A (case B) scenario which should be approximately equal to the excess variance of the light curves. This area (hence as well) depends on the location of the break frequencies (determined by black hole mass and accretion rate) with respect to ν_max and ν_min (which are fixed according to the values T and Δt of the light curves used in this work).