2004MNRAS.347L..41N

We present follow-up observations of five hard X-ray sources from the ASCA 5-10 keV Search for the High Energy Extragalactic Population (SHEEP) survey, which has a limiting flux of ∼ 10^–13 erg/cm2/s. Chandra data have been obtained to improve the X-ray positions from a few arcmin to <1 arcsec, which allows unambiguous optical identification. While the objects almost certainly house active galactic nuclei (AGNs) based on their X-ray luminosity, optical spectroscopy reveals a variety of properties. The identifications indicate that the SHEEP survey samples the same populations as deeper surveys which probe the origin of the X-ray background, but because the SHEEP sources are far brighter, they are more amenable to detailed follow-up work. We find a variety of classifications and properties, including a type II quasi-stellar object (QSO), a galaxy undergoing star formation and a broad-line AGN which has a very hard X-ray spectrum, indicating substantial absorption in the X-ray but none in the optical. Two objects have X-ray/optical flux ratios which, were they at an X-ray flux level typical of objects in Chandra deep surveys, would place them in the `optically faint' category. They are both identified with broad line QSOs at z ∼ 1. Clearly, this survey - which is relatively unbiased against obscured objects - is revealing a set of remarkable objects quite different to the familiar classes of AGNs found in previous optical and soft X-ray surveys.