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 distribution, 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 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 Gamma = 0.92 +/- 0.16, significantly different from that of the broad-line AGN (Gamma = 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.