The Cosmic X-Ray Background and the Population of the Most Heavily Obscured AGNs

We report on an accurate measurement of the CXB in the 15-50 keV range performed with the Phoswich Detection System (PDS) instrument aboard the BeppoSAX satellite. We establish that the most likely CXB intensity level at its emission peak (26-28 keV) is ≈40 keV cm^-2 s^-1 sr^-1, a value consistent with that derived from the best available CXB measurement obtained over 25 years ago with the first High Energy Astronomical Observatory (HEAO-1) satellite mission (Gruber et al.), whose intensity, lying well below the extrapolation of some lower energy measurements performed with focusing telescopes, was questioned in recent years. We find that 90% of the acceptable solutions of our best-fit model to the PDS data give a 20-50 keV CXB flux lower than 6.5 × 10^-8 ergs cm^-2 s^-1 sr^-1, which is 12% higher than that quoted by Gruber et al. when we use our best calibration scale. In combination with the CXB synthesis models we infer that about 25% of the intensity at ~30 keV arises from extremely obscured, Compton-thick AGNs (absorbing column density N_H > 10²⁴ cm^-2), while a much larger population would be implied by the highest intensity estimates. We also infer a mass density of supermassive black holes of ~3 × 10⁵ M_☉ Mpc^-3. The summed contribution of resolved sources (Moretti et al.) in the 2-10 keV band exceeds our best-fit CXB intensity extrapolated to lower energies, but it is within our upper limit, so that any significant contribution to the CXB from sources other than AGNs, such as star-forming galaxies and diffuse warm-hot intergalactic medium (WHIM), is expected to be mainly confined below a few keV.