ASCA observations of deep ROSAT fields -- I. The nature of the X-ray source populations

We present ASCA GIS observations (total exposure ~ 100–200 ks) of three fields which form part of our deep ROSAT survey. We detect 26 sources down to a limiting flux (2&3x2013;10 keV) of ~ 5 × l0−14 erg cm−2 s−1. Sources down to this flux level contribute ~ 30 per cent of the 2–10 keV X-ray background. The number-count distribution, log N-log S, is a factor of 3 above the ROSAT counts, assuming a spectral index of Γ = 2 for the ROSAT sources. This suggests the presence at hard energies of a population other than the broad-line AGN which contribute to the ROSAT counts. This is supported by spectroscopic observations that show a large fraction of sources that are not obvious broad-line AGN. The average 1–10 keV spectral index of these sources is flat with Γ = 0.92 ± 0.16, significantly different from that of the broad-line AGN (Γ = 1.78 ± 0.16). Although some of the narrow-emission-line galaxies which are detected with ROSAT are also detected here, the nature of the flat-spectrum sources remains as yet unclear.