1 Introduction

The XMM-Newton observatory, the second cornerstone of the Horizon 2000 science programme of the European Space Agency has been carrying out science operations since early 2000. Thanks to its high collecting area, large field of view and moderate angular and spectral resolution XMM-Newton is the most powerful observatory in hard X-rays (photon energy >2 keV), opening an almost unexplored window to the Universe (Jansen et al. 2001). The sensitivity to hard X-rays (not attained by previous missions like Einstein and ROSAT) allows the detection and study of the most energetic objects in the Universe (Active Galactic Nuclei - AGN), most of which are believed to be deeply hidden inside large amounts of absorbing gas and inconspicuous at virtually all other wavelengths.

During science observations (with exposure time over $\sim$10 ks) with the EPIC cameras operating in "Full Frame'' mode (Turner et al. 2001; Strüder et al. 2001) XMM-Newton is discovering $\sim$30-150 new X-ray sources, which add to the XMM-Newton serendipitous survey at an expected rate of $\sim$$50\,000$ new sources per year. The XMM-Newton Survey Science Centre (SSC) was appointed by ESA to exploit scientifically the XMM-Newton serendipitous survey for the benefit of the scientific community and as a major legacy of XMM-Newton to future generations. This is being tackled by the SSC consortium in terms of a mostly ground-based optical follow-up and identification (XID) programme.

The XID programme has been described in detail in Watson et al. (2001). Briefly, its implementation has been divided into two parts: a core programme which will identify - spectroscopically - significant samples of sources at X-ray flux limits around $\sim$ $10^{-13}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$(bright sample), $\sim$ $10^{-14}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$ (medium sample) and $\sim$ $10^{-15}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$ (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-Newton fields to facilitate statistical identifications of the serendipitous sources.

AXIS ("An XMM-Newton International Survey") forms the backbone of the XID programme by providing the ground-based resources that are essential for the exploitation of the XMM-Newton serendipitous sky survey. 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. AXIS has been awarded a total of 85 observing nights spread over the period April 2000-April 2002 on the 4 larger telescopes of the Observatorio del Roque de Los Muchachos: the 2.5 m Isaac Newton Telescope (INT), the 2.5 m Nordic Optical Telescope (NOT), the 3.5 m Telescopio Nazionale Galileo (TNG) and the 4.2 m William Herschel Telescope (WHT).

In this paper we report on the first results obtained in the AXIS medium sensitivity survey at high galactic latitude. X-ray sources serendipitously found in two XMM-Newton observations (field names G133-69 Pos_2 and Mkn 205) have been studied and followed up down to a 0.5-4.5 keV flux of $2\times 10^{-14}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$. The survey reaches a source density in excess of 100 sources $\deg^{-2}$, which is appropriate for spectrocopic follow up using fibre spectroscopy. Our survey is therefore shallower than the XMM-Newton Lockman Hole survey (Hasinger et al. 2001) and the Chandra deep surveys (Mushotzky et al. 2000; Barger et al. 2001; Giacconi et al. 2001). The surface density reached is, however, similar to that of the so-called Rosat Deep Survey (Boyle et al. 1994) and deeper than the RIXOS survey (Mason et al. 2000).

Although we are still dealing with a small number of sources (29) the sample presented here provides a flavour of the dominant X-ray source populations at high galactic latitude down to that flux level. The paper also describes the observational techniques that we are following in the AXIS project to build up larger source catalogues at various flux levels and galactic latitudes.