X-ray and optical counterparts of hard X-ray selected sources from the SHEEP survey: first results

Abstract

We present followup observations of five hard X-ray sources from the ASCA 5-10 keV SHEEP survey, which has a limiting flux of $\sim 10^{-13}$ erg cm$^{-2}$ s$^{-1}$. Chandra data have been obtained to improve the X-ray positions from a few arcmin to $<1&#39;&#39;$, which allows unambiguous optical identification. While the objects almost certainly house AGN 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 followup work. We find a variety of classifications and properties, including a type II 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&#39;&#39; category. They are both identified with broad line QSOs at z$\sim 1$. Clearly this survey - which is relatively unbiased against obscured objects - is revealing a set of remarkable objects quite different to the familiar classes of AGN found in previous optical and soft X-ray surveys.