The X-ray background 2–50 kev: A superposition of known sources,or evidence of a new component?

The contribution of the canonical Quasars to the XRB 2–50 keV is not likely to exceed 50%. Nay, consideration of X-ray selection effects results in a further reduction of the value of sensible estimators for the intensity ratio L_X/L_O; in addition, recent counts of radioloud and radio-quiet Quasars definitely limit the number of faint sources. A missing component with definite spectral properties should comprise a sizeable fraction of the XRB.