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) similar or equal to 7 x 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 similar to71% (similar to90%) 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 (similar to90%) of cases has a magnitude brighter than the POSS II limit (R similar to 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 similar to0.8 (similar to0.64) 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(21) up to few times 10(23) cm(-2). 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(-2) and 0.83 +/- 0.18 deg(-2), 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.