SHEEP: The Search for the High‐Energy Extragalactic Population

We present the SHEEP survey for serendipitously-detected hard X-ray sourcesin ASCA GIS images. In a survey area of $\sim 40$ deg$^{2}$, 69 sources weredetected in the 5-10 keV band to a limiting flux of $\sim 10^{-13}$ ergcm$^{-2}$ s$^{-1}$. The number counts agree with those obtained by the similarBeppoSAX HELLAS survey, and both are in close agreement with ASCA and BeppoSAX2-10 keV surveys. Spectral analysis of the SHEEP sample reveals that the 2-10and 5-10 keV surveys do not sample the same populations, however, as we findconsiderably harder spectra, with an average $\Gamma\sim1.0$ assuming noabsorption. The implication is that the agreement in the number counts iscoincidental, with the 5-10 keV surveys gaining approximately as many hardsources as they lose soft ones, when compared to the 2-10 keV surveys. This ishard to reconcile with standard AGN ``population synthesis'' models for theX-ray background, which posit the existence of a large population of absorbedsources. We find no evidence of the population hardening at faint fluxes, withthe exception that the few very brightest objects are anomalously soft. 53 ofthe SHEEP sources have been covered by ROSAT in the pointed phase. Of these 32were detected. An additional 3 were detected in the RASS. As expected thesources detected with ROSAT are systematically softer than those detected withASCA alone, and of the sample as a whole (truncated).Comment: 36 pages, 7 figs, to appear in Ap