Left: confidence intervals for the model parameters Γ, E_C, f_REF and f_CT used to synthesise the XRB spectrum. The (blue) thick continuous line corresponds to the 68 per cent confidence level (Δχ² = 4.72 for four parameters). The (red) thin dotted line marks the 95th percentile (Δχ² = 9.70 for four parameters). Right: same as right panel but including to our XRB synthesis results the contribution of blazars, as modelled by Draper & Ballantyne (2009).