The Obscured X-ray Background and Evolution of AGN
  The X-ray Background has been resolved in the 0.5-5keV band and found to consist mostly of both unabsorbed and absorbed AGN with column densities <1e23. This contrasts with the local AGN population where the column density range extends to Compton-thick objects and beyond (>1e24). Stacking analysis of the integrated emission of sources detected by XMM-Newton in the Lockman Hole, and by Chandra in the CDF-N and S reveals that the resolved fraction of the X-ray Background drops above 6keV and is about 50% above 8keV. The missing flux has the spectrum of highly absorbed AGN, making it likely that the range of column density at redshift one is similar to that locally, and that many AGN are as yet undetected in well-studied fields. 
A.C. Fabian and M.A. Worsley (IoA Cambridge, UK)