ASCA OBSERVATIONS OF DEEP ROSAT FIELDS .1. THE NATURE OF THE X-RAY SOURCE POPULATIONS

We present ASCA GIS observations (total exposure similar to 100-200 ks ) of three fields which form part of our deep ROSAT survey. We detect 26 sources down to a limiting flux (2-10 keV) of similar to 5 X 10(-14 ) cm(-2) s(-1). Sources down to this flux level contribute similar to 30 per cent of the 2-10 keV X-ray background. The number-count distrib ution, log N-log S, is a factor of 3 above the ROSAT counts, assuming a spectral index of Gamma=2 for the ROSAT sources. This suggests the p resence at hard energies of a population other than the broad-line AGN which contribute to the ROSAT counts. This is supported by spectrosco pic observations that show a large fraction of sources that are not ob vious broad-line AGN. The average 1-10 keV spectral index of these sou rces is flat with Gamma = 0.92 +/- 0.16, significantly different from that of the broad-line AGN (Gamma = 1.78 +/- 0.16). Although some of t he narrow-emission-line galaxies which are detected with ROSAT are als o detected here, the nature of the flat-spectrum sources remains as ye t unclear.