Deep Hard X-ray Source Counts from a Uctuation Analysis of Asca Sis Images

An analysis of the spatial uctuations in 15 deep ASCA SIS0 images has been conducted in order to probe the 2-10 keV X-ray source counts down to a ux limit 2 10 ?14 erg cm ?2 s ?1. Special care has been taken in modelling the uctuations in terms of the sensitivity maps of every one of the 16 regions (5:6 5:6 arcmin 2 each) in which the SIS0 has been divided, by means of raytracing simulations with improved optical constants in the X-ray telescope. The very extended`side lobes' (extending up to a couple of degrees) exhibited by these sensitivity maps make our analysis sensitive to both faint on-axis sources and brighter oo-axis ones, the former being dominant. The source counts in the range (2 ? 12) 10 ?14 erg cm ?2 s ?1 are found to be close to a euclidean form which extrapolates well to previous results from higher uxes and in reasonable agreement with some recent ASCA surveys. However, our results disagree with the deep survey counts by Georgantopoulos et al. (1997). The possibility that the source counts atten to a subeuclidean form, as it is observed at soft energies in ROSAT data, is only weakly constrained to happen at a ux < 1:8 10 ?12 erg cm ?2 s ?1 (90 per cent conndence). Down to the sensitivity limit of our analysis, the integrated contribution of the sources whose imprint is seen in the uctuations amounts to 35 13 per cent of the extragalactic 2-10 keV X-ray background.