THE UK DEEP AND MEDIUM SURVEYS WITH ROSAT - LOG-N-LOG-S RELATION

We have carried out a soft X-ray survey of the sky using the ROSAT Position Sensitive Proportional Counter (PSPC) in a region of very low Galactic column density (N-H = 6-9 x 10(19) cm(-2)). The data consist of a deep > 70 ks pointing (the Deep Survey) and six pointings at lower sensitivity (13-20 ks; the Medium Survey). We detect a total of 141 sources over the 0.9 deg(2) of sky area used. The faintest source detected has a flux of 3.2 x 10(-15) erg cm(-2) s(-1) (0.5-2.0 keV). We present the source number-nux distribution, and we compare it with the distributions constructed from other ROSAT observations and from surveys carried out with X-ray instruments operating at higher energies. We estimate the contribution of the resolved sources to the soft X-ray background by direct comparison of their integrated spectrum with that of the diffuse background: 44 per cent of the extragalactic X-ray background between 0.5 and 2.0 keV is resolved directly into discrete sources whose average spectrum is steeper than that of the background. Integration of our number-flux relation to infinite flux produces a background resolved fraction of 44 +/- (56)(5) per cent (0.5-2.0 keV). Limits on the slope of the log N-log S curve at very faint fluxes are set by the measured value of the X-ray background.