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 ~10^23 - 10^24 cm^-2. 
M.A. Worsley, A.C. Fabian, F.E. Bauer, D.M. Alexander, G. Hasinger, S.   Mateos, H. Brunner, W.N. Brandt and D.P. Schneider