The U.K. Deep and Medium Surveys with ROSAT - N-Log Relation

G. Branduardi Raymont, K. O. Mason, R. S. Warwick, F. J. Carrera, V. G. Graffagnino, J. P. D. Mittaz, E. M. Puchnarewicz, P. J. Smith, C. R. Barber, K. A. Pounds, G. C. Stewart, I. M. McHardy, L. R. Jones, M. R. Merrifield, A. C. Fabian, R. G. McMahon, M. J. Ward, I. M. George, M. H. Jones, A. LawrenceM. Rowan-Robinson

Research output: Contribution to journalArticlepeer-review


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-flux 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+/- _5_^56^ 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.
Original languageEnglish
Pages (from-to)947
JournalMonthly Notices of the Royal Astronomical Society
Publication statusPublished - 1 Oct 1994


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