During a typical science observation, XMM will discover ca. 50-250 new X-ray sources and add to the XMM serendipitous survey at an expected rate of 50000-100000 new sources per year. The XMM Survey Science Centre (SSC) was appointed by ESA to exploit scientifically the XMM serendipitous survey for the benefit of the scientific community and as a major legacy of XMM to future generations. This is being tackled by the SSC consortium in terms of an (mostly ground-based) X-ray follow-up and identification (XID) programme.

The goal of the XID programme is to exploit fully the XMM serendipitous sky survey. Its implementation has been divided into two parts: a core programme which will identify -spectroscopically- significant samples of sources at X-ray flux limits of 1 E-13 erg/cm2/s (bright sample), 1 E-14 erg/cm2/s (medium sample) and 1 E-15 erg/cm2/s (faint sample) covering a range of galactic latitudes, and an imaging programme aiming at providing deep optical/infrared images in several colours of a large number of XMM fields to facilitate statistical identifications of the serendipitous sources.

AXIS (An XMM International Survey) aims at forming the backbone of the SSC XID programme by providing the ground-based resources that are essential for the exploitation of the XMM data. Besides making a first and major contribution to the XID programme, AXIS will define the quality standard and will guide future steps in the implementation of the XID programme. AXIS has been conceived and designed to make optimal use of the available instrumentation on the telescopes of the Observatorio del Roque de los Muchachos. The observational aims of AXIS are:

• Wide-field multi-colour optical and near-infrared imaging of XMM fields to be used as spectroscopic targets in the medium and faint samples, and for the imaging programme.

• Deep optical and/or near infrared imaging of fields around the positions of X-ray sources not showing obvious optical or infrared counterparts, as primary candidates for heavily absorbed X-ray sources.

• Spectroscopic identification of several hundreds of sources of the medium sample, including significant numbers of absorbed sources.

• Spectroscopic identification of up to 100 X-ray sources in the bright sample.

We anticipate that during this first AXIS round, the major scientific output will be the discovery of many absorbed X-ray sources powered by accretion onto black holes (mostly Active Galactic Nuclei) at significant redshifts, of which only a handful of examples are known at present. These objects, which are thought to provide over 80% of the accretion power in the Universe, are the crucial ingredient of all models for the hard (>2 keV) X-ray background. Studying their distribution in luminosity, intrinsic absorbing column, and redshift evolution are the main scientific goals of AXIS.

AXIS observations will also achieve a number of secondary science goals: finding cluster candidates (for later spectroscopic identification) to study the redshift evolution of clusters of various luminosities; measuring the distribution of cataclysmic variables (and other accreting binaries) as a function of galactic latitude, leading to new determinations of their space density; measuring coronal activity in stars and its dependence on luminosity, stellar type, rotation, etc.; and last, but not least, to set the starting point for a statistical identification method which combines X-ray and optical data to produce likely identifications of a much larger set of X-ray sources without optical spectroscopy being necessary. This is necessary to exploit fully the XMM serendipitous sky survey.