Red curve: an example of an XRB spectrum produced by our synthesis model for parameters Γ = 1.88, E_C = 230 keV, f_REF = 0.055 and f_CT = 0.1. The points are the observational constraints on the intensity of the XRB at different energies from the studies discussed in the text. Comparison of the model curve with the data yields a χ² of 70.8 for 86 degrees of freedom (90 data points minus four free parameters).