The unresolved hard X-ray background: the missing source population implied by the Chandra and XMM-Newton deep fields

We extend our earlier work on X-ray source stacking in the deep XMM-Newton observation of the Lockman Hole, to the 2-Ms Chandra Deep Field North (CDF-N) and the 1-Ms Chandra Deep Field South (CDF-S). The XMM-Newton work showed the resolved fraction of the X-ray background (XRB) to be ∼80-100 per cent at ≤ 2 keV but this decreased to only ∼50 per cent above ∼8 keV. The CDF-N and CDF-S probe deeper, and are able to fill in some of the missing fraction in the 4-6 keV range, but the resolved fraction in the 6-8 keV band remains only ∼60 per cent, confirming the trend seen with XMM-Newton. The missing XRB component has a spectral shape that is consistent with a population of highly obscured active galactic nuclei (AGN) at redshifts ∼0.5-1.5 and with absorption column densities of ∼10 23 -10 24 cm -2 .