Logarithmic distribution of the luminosity ratio R (blue histogram; mean logarithmic value ⟨Log R⟩= − 0.13 ± 0.20). The red histogram is the distribution assuming that R is given by its lower limit (from uncertainties – see Sect. 5.3). The solid blue and red lines are the cumulative functions related to the two histograms. The thick dashed blue line is the KERRBB limit (R = 0.94, for θ_v = 30°) while the maximum SS value (R = 0.43, for θ_v = 30°) is represented with a dashed red line (the thin dashed black line represents the SS case with θ_T = 45° and θ_v = 30°). Sources whose luminosity ratio R is larger than the one shown with a dashed green line have a BH spin a > 0.7 (see Sect. 2). The top x-axis shows the linear value of R.