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 nuc leus is heavily obscured. The infrared colours of the majority of the targe ts observed are consistent with absorbed elliptical host galaxies z = 1-2. It is likely that we are observing some of the brighter members of the impo rtant new class of X-ray type II quasars.