ASCA observations of deep ROSAT fields - II. The 2-10keV AGN luminosity function

We present additional optical spectroscopic identifications of sources identified in three deep ASCA GIS fields which also form part of a deep ROSAT survey. In total, 26 ASCA sources have been detected down to a 2-10keV flux limit of S_2-10=5x10^-14ergcm^-2s^-1. Low Dispersion Survey Spectrograph observations have increased the spectroscopic completeness of the survey to between 65 and 85 per cent, with identifications for up to 13 QSOs with broad emission lines and six objects with narrow emission lines. Combining these objects with the AGN identified in the HEAO A-1 sample by Grossan, we find evidence for significant cosmological evolution in the 2-10keV band (<V_e/V_a>=0.63+/-0.03), fitted by a pure luminosity evolution model: L~(1+z)^k, k=2.04^+0.18_-0.22. The present-epoch 2-10keV AGN X-ray luminosity function, Phi(L_X), is best represented by a two-power-law function: Phi(L_X)~L^-3.0, L^*>10^44.1ergs^-1 Phi(L_X~L^-1.9, L^*<10^44.1ergs^-1. Depending on the extent to which we extrapolate the z=0 AGN luminosity function, we predict a total contribution to the 2-10keV X-ray background from QSOs and narrow-emission-line galaxies which ranges from 48 per cent (for AGN with L_X>10^42ergs^-1) to 80 per cent (L_X>10^39 ergs^-1).