Because of its large throughput and good angular resolution, pointed observations with the XMM EPIC X-ray cameras will reach very faint X-ray flux limits ( a few erg cm-2 s-1, 0.1-10 keV, in typical observations). At these fluxes, each XMM EPIC field at both high and low galactic latitudes will contain substantial numbers of ``serendipitous'' X-ray sources. Pointed XMM observations will thus provide a ``serendipitous'' X-ray survey (the ``XMM Serendipitous Sky Survey'', see Watson, 1998, Astr. Nach., 319, 117).
The role of the SSC in this area is to coordinate the follow-up of the serendipitous content of XMM observations in order to ensure that this valuable resource can be exploited effectively by the community. The key initial step will be the ``identification'' of the X-ray sources, i.e., classification into different object types. Literally identifying every XMM serendipitous source is not a realistic task, nor a sensible approach. Instead, the emphasis of the SSC programme will be on the characterisation of the XMM source population through the detailed follow-up of well-defined, small subsamples (as outlined below). The SSC ``follow-up'' programme aims to bring together the XMM data themselves, existing catalogue and archival material and new ground-based observational data in an integrated fashion.
The overall programme strategy as currently conceived involves two main components:
Data from the XMM OM, depending on instrumental mode chosen by the observer, can also provide relatively deep optical and UV imaging over most of the EPIC field. The OM sensitivity will not, however, be sufficient to meet all of the Imaging Programme needs, in particular because of the lack of R & I band coverage. The OM UV data will however represent a unique resource.
The entire programme will be based on samples of serendipitous X-ray sources drawn from pointed XMM observations that are in the public domain, or are made available to the SSC by the observation PI. The programme is designed to support the community's access to, and exploitation of, the serendipitous data from XMM, and as such all the results will be made public through the XMM science archive.