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[Aims]: Pointed observations with XMM-Newton provide the basis for creating catalogues of X-ray sources detected serendipitously in each field. This paper describes the creation and characteristics of the 2XMM catalogue. [Methods]: The 2XMM catalogue has been compiled from a new processing of the XMM-Newton EPIC camera data. The main features of the processing pipeline are described in detail. [Results]: The catalogue, the largest ever made at X-ray wavelengths, contains 246 897 detections drawn from 3491 public XMM-Newton observations over a 7-year interval, which relate to 191 870 unique sources. The catalogue fields cover a sky area of more than 500 deg2. The non-overlapping sky area is ∼360 deg2 (∼1% of the sky) as many regions of the sky are observed more than once by XMM-Newton. The catalogue probes a large sky area at the flux limit where the bulk of the objects that contribute to the X-ray background lie and provides a major resource for generating large, well-defined X-ray selected source samples, studying the X-ray source population and identifying rare object types. The main characteristics of the catalogue are presented, including its photometric and astrometric properties., M. Ceballos, F. Carrera and X. Barcons acknowledge financial support by the Spanish Ministerio de Educacion y Ciencia under projects ESP2003-00812 and ESP2006-13608-C02-01. The French teams are grateful to CNES for supporting this activity. In Germany the XMM-Newton project is supported by the Bundesministerium für Wirtschaft und Technologie/Deutsches Zentrum für Luft und Raumfahrt e.V. (DLR) and the Max Planck Society. Part of this work was supported by the DLR project numbers 50 OX 0201, 50 OX 0001, and 50 OG 0502. The Italian team acknowledges financial support from the Agenzia Spaziale Italiana (ASI), the Ministero dell’ Istruzione, Università e Ricerca (MIUR) and the Istituto Nazionale di Astrofisica (INAF) over the last years; they are currently supported by the grant PRIN-MIUR 2006-02-5203 and by the ASI grants n.I/088/06/0 and n.I/023/05/0. UK authors thank STFC for financial support.

[Aims]: We present the optical classification and redshift of 348 X-ray selected sources from the XMM-Newton Bright Serendipitous Survey (XBS), which contains a total of 400 objects (identification level=87%). About 240 are new identifications. In particular, we discuss in detail the classification criteria adopted for the active galactic nuclei (AGNs) population. [Methods]: By means of systematic spectroscopic campaigns using various telescopes and through the literature search, we have collected an optical spectrum for the large majority of the sources in the XBS survey and applied a well-defined classification >flow chart>. [Results]: We find that the AGNs represent the most numerous population at the flux limit of the XBS survey (~10-13 erg cm-2s-1) constituting 80% of the XBS sources selected in the 0.5-4.5 keV energy band and 95% of the >hard> (4.5-7.5 keV) selected objects. Galactic sources populate the 0.5-4.5 keV sample significantly (17%) and only marginally (3%) the 4.5-7.5 keV sample. The remaining sources in both samples are clusters/groups of galaxies and normal galaxies (i.e. probably not powered by an AGN). Furthermore, the percentage of type 2 AGNs (i.e. optically absorbed AGNs with 2$]]> mag) dramatically increases going from the 0.5-4.5 keV sample (%) to the 4.5-7.5 keV sample (%). We finally propose two simple diagnostic plots that can be easily used to obtain the spectral classification for relatively low-redshift AGNs even if the quality of the spectrum is not good., A.C., R.D.C., T.M., and P.S. acknowledge financial support from the MIUR, grant PRIN-MUR 2006-02-5203 and from the Italian Space Agency (ASI), grants n. I/088/06/0 and n. I/023/05/0.

[Aims]: We investigate here the X-ray luminosity function (XLF) of absorbed ( between 4 10 and cm) and unabsorbed ( < 4 10 cm) AGN, the fraction of absorbed AGN as a function of (and ), the intrinsic distribution of the AGN population, and the XLF of Compton thick ( > 10 cm) AGN. [Methods]: To carry out this investigation, we have used the XMM-Newton Hard Bright Serendipitous Sample (HBSS), a complete sample of bright X-ray sources ( 7 10 erg cm s) at high galactic latitude (|| > 20°) selected in the 4.5-7.5 keV energy band. The HBSS sample is now almost completely identified (97% spectroscopic identifications) and it can be safely used for a statistical investigation. The HBSS contains 62 AGN out of which 40 are unabsorbed (or marginally absorbed; < 4 10 cm) and 22 are absorbed ( between 4 10 and ~ cm). [Results]: Absorbed and unabsorbed AGN are characterised by two different XLF with the absorbed AGN population being described by a steeper XLF, if compared with the unabsorbed ones, at all luminosities. The intrinsic fraction of absorbed AGN (i.e., the fraction of sources with between 4 10 and cm divided the sources with below cm, corrected for the bias due to the photoelectric absorption) with 3 10 erg s is 0.57 0.11; we find that decreases with the intrinsic luminosity, and probably, increases with the redshift. Our data are consistent with a flat Log distribution for between and cm. Finally, by comparing the results obtained here with those obtained using an optically-selected sample of AGN we derive, in an indirect way, the XLF of Compton thick AGN; the latter is well described by a XLF similar, in shape, to that of absorbed AGN, but having a normalization of about a factor of 2 above. The density ratio between Compton thick AGN ( 10 cm) and Compton thin AGN ( 10 cm) decreases from 1.08 0.44 at ~ erg s to 0.57 0.22 at ~ erg s to 0.23 0.15 at ~ erg s. [Conclusions]: The results presented here on the anti-correlation between and are fully consistent with the hypothesis of a reduction of the covering factor of the gas as a function of the luminosity and are clearly inconsistent with the simplest unified scheme of AGN. These results strongly support the recently proposed radiation-limited clumpy dust torus model although alternative physical models are also consistent with the observations., R.D.C., A.C., T.M., and P.S. acknowledge financial support from the MIUR, grant PRIN-MUR 2006-02-5203 and from the Italian Space Agency (ASI), grants n. I/088/06/0 and n. I/023/05/0. F.J.C. acknowledges financial support from the Spanish Ministry of Education and Science, through project ESP2006-13608-C01-01.

We have performed a detailed analysis of 3 optically normal galaxies extracted from the XMM Bright Serendipitous Source Sample. Thanks to the good statistics of the XMM-Newton data, we have unveiled the presence of an AGN in all of them. In particular, we detect both X-ray obscured (NH > 1022 cm-2) and unobscured (NH < 1022 cm-2) AGNs with intrinsic 2-10 keV luminosities in the range between 1042-1043 erg s-1. We find that the X-ray and optical properties of the sources discussed here could be explained assuming a standard AGN hosted by galaxies with magnitudes MR < M*, taking properly into account the absorption associated with the AGN, the optical faintness of the nuclear emission with respect to the host galaxy, and the inadequate set-up and atmospheric conditions during the optical spectroscopic observations. Our new spectroscopic observations have revealed the expected AGN features also in the optical band. These results clearly show that optical spectroscopy sometimes can be inefficient in revealing the presence of an AGN, which instead is clearly found from an X-ray spectroscopic investigation. This remarks the importance of being careful in proposing the identification of X-ray sources (especially at faint fluxes) when only low quality optical spectra are in hand. This is particularly important for faint surveys (such as those with XMM-Newton and Chandra), in which optically dull but X-ray active objects are being found in sizeable numbers., PS acknowledge financial support by the Italian Consorzio Nazionale per l’Astronomia e l’Astrofisica (CNAA). This work has received partial financial support from ASI (I/R/037/01) and from the Italian Ministry of University and Scientific and Technological Research (MURST) through grant Cofin 00-02-36. XB and FJC acknowledge financial support from the Spanish Ministry of Science and Technology, under grant AYA 2000-1690.

We present the first results on the identifications of a medium sensitivity survey (X-ray flux limit 2 × 10-14 erg cm-2s-1 in the 0.5-4.5 keV band) at high galactic latitude (|b| > 20°) carried out with the XMM-Newton X-ray observatory within the AXIS observing programme. This study is being conducted as part of the XMM-Newton Survey Science Centre activities towards the identification of the sources in the X-ray serendipitous sky survey. The sample contains 29 X-ray sources in a solid angle of 0.26 deg2 (source density 113±21 sources deg-2), out of which 27 (93%) have been identified. The majority of the sources are broad-line AGN (19), followed by narrow emission line X-ray emitting galaxies (6, all of which turn out to be AGN), 1 nearby non-emission line galaxy (NGC 4291) and 1 active coronal star. Among the identified sources we find 2 broad-absorption line QSOs (z ∼ 1.8 and z ∼ 1.9), which constitute ∼10% of the AGN population at this flux level, similar to optically selected samples. Identifications of a further 10 X-ray sources fainter than our survey limit are also presented., XB, FJC, MTC and SM acknowledge financial support by the Spanish MCYT under project AYA2000-1690. RDC and TM acknowledge partial financial support by the Italian Space Agency (ASI) and by the MURST (Cofin00-32-36). This project was supported by the DLR under grants 50 OR 9908 0 (GPS) and 50 OX 9801 3 (HB).