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 and the 1 Ms Chandra Deep Field South. The XMM-Newton work showed the resolved fraction of the X-ray background 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 X-ray background component has a spectral shape that is consistent with a population of highly obscured AGN at redshifts ∼ 0.5–1.5 and with absorption column densities of ∼ 10–10 cm.