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 ∼1023–1024 cm−2.