The U.K. Deep and Medium Surveys with ROSAT - N-Log Relation
Abstract
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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- October 1994
- DOI:
- 10.1093/mnras/270.4.947
- Bibcode:
- 1994MNRAS.270..947B