Because of its large throughput and good spatial resolution, pointed observations with the XMM-Newton EPIC X-ray cameras reach very faint X-ray flux limits ( a few erg cm s, 0.2-12 keV, in typical observations). At these fluxes, each XMM-Newton EPIC field at both high and low galactic latitudes contains substantial numbers of ``serendipitous'' X-ray sources. Typical EPIC observations yield 70 serendipitous X-ray source detections per field. As XMM-Newton makes of the order 700 observations per year, covering 100 sq.deg. of the sky, after allowing for overlaps in the sky coverage and observations where EPIC was not used, the number of serendipitous unique sources grows at a rate of 30,000 sources per year. The XMM-Newton data thus provide a deep, large area sky survey which represents a major resource for a wide range of programmes. The extended energy range of XMM-Newton, compared with previous imaging X-ray missions such as ROSAT and the Einstein Observatory, means that XMM-Newton detects significant numbers of obscured and hard-spectrum objects (e.g. obscured AGNs) which are absent from earlier studies. Pointed XMM-Newton observations thus provide a ``serendipitous'' X-ray survey (the ``XMM-Newton Serendipitous Sky Survey'', see Watson et al., 2001, A&A, 365, L51 and Watson et al., 2009, A&A, 493, 339) which, like previous surveys, is making a major impact in a number of front-line areas of astrophysics, a fact that provided one of the original motivations for the SSC concept within the XMM-Newton project. The SSC is responsible for two main aspects of the XMM-Newton Serendipitous Survey: creation of the XMM-Newton source catalogues (section A.3) and the XID Follow-up Programme described briefly below.
The SSC's XID follow-up programme, designed to ensure that the potential of the XMM-Newton serendipitous survey can be exploited by the community in the context of a wide range of scientific programmes, involves a substantial series of new observations, primarily in the optical and IR using ground-based facilities. The main elements of the programme are a number of complementary sub-programmes to obtain the identifications for well-defined samples of X-ray sources drawn from selected XMM-Newton fields using optical/IR spectroscopy. The programme is also supported by a substantial imaging programme, needed for the selection of the spectroscopic targets, but which is also an important resource in its own right given the large sky area and hence X-ray source sample it covers. One of the principal objectives of the whole programme is to obtain completely identified samples which can be used to characterise the overall XMM-Newton source population sufficiently well that the basic X-ray and optical parameters can be used to assign a statistical identification for a large fraction of all the sources in the XMM-Newton serendipitous source catalogue.
The entire XID Programme is based on samples of serendipitous X-ray sources drawn from pointed XMM-Newton observations that are in the public domain, or are made available to the SSC by the observation PI.
Most of the XID Programme is now complete. Information about the
background to the project and some publications describing various aspects
of the programme and its results and links to related site are available at
while some further, more up to date information on the status of the project, publications related to the primary source samples, and details of the XID results database are provided at
The XID programme is designed to support the community's access to, and exploitation of, the serendipitous data from XMM-Newton, and as such all the results will be made public. The XID project began releasing the XID data through a dedicated, searchable XID results database at l'Observatoire Astronomique de Strasbourg in 2010 (URL: http://xcatdb.unistra.fr/). Further data from the XID project will be made available through this database once it is finalised.