Infrared observations of serendipitous hard Chandra X-ray sources
  We present observations of a sample of optically-faint, hard X-ray sources of the kind likely to be responsible for much of the hard X-ray background. We confirm that such sources are easily detected in the near-infrared, and find that they have a featureless continuum suggesting that the active nucleus is heavily obscured. The infrared colours of the majority of the targets observed are consistent with absorbed elliptical host galaxies at z=1-2. It is likely that we are observing some of the brighter members of the important new class of X-ray Type II quasars. 
C. S. Crawford, A. C. Fabian, P. Gandhi, R. J. Wilman and R. M.   Johnstone