DEEP HARD X-RAY SOURCE COUNTS FROM A FLUCTUATION ANALYSIS OF ASCA SISIMAGES

An analysis of the spatial fluctuations in 15 deep ASCA SISO images ha s been conducted in order to probe the 2-10 keV X-ray source counts do wn to a flux limit similar to 2 x 10(-14) erg cm-2 s(-1). Special care has been taken in modelling the fluctuations in terms of the sensitiv ity maps of every one of the 16 regions (5.6 x 5.6 arcmin(2) each) int o which the SISO has been divided, by means of ray-tracing simulations with improved optical constants in the X-ray telescope. The very exte nded 'sidelobes' (extending up to a couple of degrees) exhibited by th ese sensitivity maps make our analysis sensitive to both faint on-axis sources and brighter off-axis ones, the former being dominant. The so urce counts in the range (2-12) x 10(-14) erg cm(-2) s(-1) are found t o be close to a Euclidean form which extrapolates well to previous res ults from higher fluxes and are in reasonable agreement with some rece nt ASCA surveys. However, our results disagree with the deep survey co unts by Georgantopoulos et al, The possibility that the source counts flatten to a sub-Euclidean form, as is observed at soft energies in RO SAT data, is only weakly constrained to happen at a flux < 1.8 x 10(-1 2) erg cm(-2) s(-1) (90 per cent confidence). Down to the sensitivity limit of our analysis, the integrated contribution of the sources the imprint of which is seen in the fluctuations amounts to similar to 35 +/- 13 per cent of the extragalactic 2-10 keV X-ray background.