1999A&A...346..731Z

We present the X-ray data and the optical identifications for a deep ROSAT PSPC observation in the ``Marano field''. In the inner region of the ROSAT field (15' radius) we detected 50 X-ray sources with S<sub>x</sub>≥3.7x10<sup>–15</sup>erg/cm<sup>2</sup>/s. When corrected for the different sensitivity over the field, the estimated observed surface density at S<sub>x</sub>≥4x10<sup>–15</sup>erg/cm<sup>2</sup>/s is 272±40 sources/sq.deg. Four X-ray sources, corresponding to 8% of the total sample, have been detected in radio images with a flux limit of about 0.2mJy. Careful statistical analysis of multi-colour CCD data in the error boxes of the 50 X-ray sources has led to the identification of 42 sources, corresponding to 84% of the X-ray sample. These 42 reliable identifications are 33 AGNs (including two radio galaxies and one BL Lac candidate; 79% of the identified sources), 2 galaxies, 3 groups or clusters of galaxies and 4 stars. If we divide our sample into two equally populated sub-samples as a function of flux, at S<sub>x</sub>=6.5x10<sup>–15</sup>erg/cm<sup>2</sup>/s, we find that the percentage of identifications remains approximately constant (88% and 80% in the high and low flux sub-samples, respectively). AGNs are the dominant class of objects in both sub-samples (90% of the optical identifications in the high flux sub-sample and 65% in the low flux sub-sample), while the few identifications with clusters and galaxies are all in the low flux sub-sample. We also show that it is likely that a few of the 8 unidentified sources are such because the derived X-ray positions may be offset with respect to the real ones due to confusion effects. The unidentified sources have a large ratio of X-ray to optical fluxes and most of them have harder than average X-ray spectra. Since most of the identified objects with these characteristics in our field and in the Lockman field are AGNs, we conclude that also most of these sources are likely to be AGNs. Finally, comparing the optically and X-ray selected samples of AGNs in this field, we estimate that the ``efficiency'' of AGN selection with X-ray exposures reaching about 4x10^–15erg/cm²/s is ∼65% and ∼20% in the magnitude ranges m_B<22.5 and 22.5<m_B<23.5, respectively. On the other hand, a not negligible fraction of the X-ray selected AGNs would have not been easily selected as AGN candidates on the basis of purely optical criteria, either because of colours similar to those of normal stars or because of morphological classification not consistent with that of point-like sources.