Distribution of X-ray luminosity from BH+OB star binaries for three values of the viscous dissipation parameter δ = 0.001 (top panel), 0.1 (middle panel), 0.5 (bottom panel). Light blue, dark blue, and red colours denote the contributions from binary models with different initial donor mass ranges of 10–20 M_⊙, 20–40 M_⊙, and 40–90 M_⊙, respectively. The coloured histograms are stacked on each other, and the sum of their ordinate values equals unity (see Eq. (33)). The black step histogram shows the distribution of X-ray luminosity arising from the shocks in the winds of the OB star companion and is separately normalised to unity.