Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...275....1h&link_type=abstract
Astronomy and Astrophysics, Vol.275, NO. 1/AUG(I), P. 1, 1993
Astronomy and Astrophysics
Astrophysics
351
Scientific paper
The longest pointed observation (152 ks) with the ROSAT PSPC, in the direction of the absolutely lowest neutral hydrogen column density (5.7 10^19^ cm^-2^) is discussed. In addition 26 shallower fields from the ROSAT medium sensitivity survey (RMSS) are analysed. 1176 X-ray sources have been detected in at least one ROSAT energy band in these fields covering a total solid angle of 9.3 deg^2^; 661 of these sources constitute a statistically complete sample detected in the hard band with 0.5-2 keV fluxes greater than 2.5 10^-15^ erg cm^-2^ s^-1^. At the faintest limiting flux of our survey the surface density of X-ray sources is ~413 deg^-2^. Detailed simulations show that confusion effects and other selection biases are relatively small and can be corrected for in the sample. The differential source counts, corrected for systematic effects on the basis of extensive Monte Carlo simulations, are well fitted with two power laws with slopes ~2.7 and ~1.9 at bright and faint fluxes, respectively. The change in slope, detected with high significance in our data, occurs at a flux of ~2.5 10^-14^ erg cm^-2^ s^- 1^, well above the faintest limiting flux. About 60% of the extragalactic 1-2 keV background is already resolved into discrete sources at our faintest limiting flux; reasonable extrapolations of the source counts to fainter fluxes can saturate the soft X-ray background. From a P(D) analysis in the Lockman field we find a best fit slope of ~1.8 for the extrapolation of the differential X-ray counts below 2.5 10^-15^ erg cm^- 2^ s^-1^. On the basis of this analysis we can set an upper limit of ~25% for a truly diffuse background component in the ROSAT hard band.
Burg Richard
Giacconi Ricardo
Hartner Gisela
Hasinger Guenter
Schmidt Matthias
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