The diffuse X-ray background

The deepest observations of the X-ray background approach the surface brightness of the truly diffuse component generated by Thomson scattering of cosmic X-ray photons. Available estimates of the electron density and the X-ray luminosity density of AGNs as a function of cosmological epoch are used to calculate the integral scattered X-ray background component. It is shown that the scattered component constitutes 1.0 - 1.7 % of the total background, depending on the AGN cosmic evolution. Albeit this is a minute fragment of the total flux, it becomes a perceptible fraction of the still unresolved part of the background and should be taken into account in the future rigorous assessments of the X-ray background structure. This diffuse component at energies < 1 keV sums up with the emission by WHIM to 3 - 4 %. Consequently, one should expect that integrated counts of discrete sources account for just 96 - 97 % for soft background and ~99 % at higher energies.