Exploring the X-ray sky with the XMM-Newton bright serendipitous survey

We present here ``The XMM-Newton Bright Serendipitous Survey'', composed of two flux-limited samples: the XMM-Newton Bright Source Sample (BSS, hereafter) and the XMM-Newton ``Hard'' Bright Source Sample (HBSS, hereafter) having a flux limit of f_x≃ 7 × 10^-14 erg cm^-2 s^-1 in the 0.5-4.5 keV and 4.5-7.5 keV energy band, respectively. After discussing the main goals of this project and the survey strategy, we present the basic data on a complete sample of 400 X-ray sources (389 of them belong to the BSS, 67 to the HBSS with 56 X-ray sources in common) derived from the analysis of 237 suitable XMM-Newton fields (211 for the HBSS). At the flux limit of the survey we cover a survey area of 28.10 (25.17 for the HBSS) sq. deg. The extragalactic number-flux relationships (in the 0.5-4.5 keV and in the 4.5-7.5 keV energy bands) are in good agreement with previous and new results making us confident about the correctness of data selection and analysis. Up to now ∼ 71% (∼ 90%) of the sources have been spectroscopically identified making the BSS (HBSS) the sample with the highest number of identified XMM-Newton sources published so far. At the X-ray flux limits of the sources studied here we found that: a) the optical counterpart in the majority (∼ 90%) of cases has a magnitude brighter than the POSS II limit (R ∼ 21^mag); b) the majority of the objects identified so far are broad line AGN both in the BSS and in the HBSS. No obvious trend of the source spectra (as deduced from the Hardness Ratios analysis) as a function of the count rate is measured and the average spectra of the ``extragalactic'' population corresponds to a (0.5-4.5 keV) energy spectral index of ∼ 0.8 (∼ 0.64) for the BSS (HBSS) sample. Based on the hardness ratios we infer that about 13% (40%) of the sources in the BSS (HBSS) sample are described by an energy spectral index flatter than that of the cosmic X-ray background. Based on previous X-ray spectral results on a small subsample of objects we speculate that all these sources are indeed absorbed AGN with the N_H ranging from a few times 10<sup>21</sup> up to few times 10<sup>23</sup> cm<sup>-2</sup>. We do not find strong evidence that the 4.5-7.5 keV survey is sampling a completely different source population if compared with the 0.5-4.5 keV survey; rather we find that, as expected from the CXB synthesis models, the hard survey is simply picking up a larger fraction of absorbed AGN. At the flux limit of the HBSS sample we measure surface densities of optically type 1 and type 2 AGN of 1.63± 0.25 deg<sup>-2</sup> and 0.83± 0.18 deg<sup>-2</sup>, respectively; optically type 2 AGN represent 34± 9% of the total AGN population. Finally, we have found a clear separation, in the hardness ratio diagram and in the (hardness ratio) vs. (X-ray to optical flux ratio) diagram, between Galactic ``coronal emitting'' stars and extragalactic sources. The information and ``calibration'' reported in this paper will make the existing and incoming XMM-Newton catalogs a unique resource for astrophysical studies.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). The majority of the new

optical spectroscopy data used here have been obtained using the

facilities of the Italian ``Telescopio Nazionale Galileo'' (TNG) and of the European Southern Observatory (ESO).

Tables \ref{t2}-\ref{t4} and Appendices are only available in electronic form at http://www.edpsciences.org