Response to 'Call for ideas for future CFHT imaging surveys' released by CFH AN OPTICAL IMAGING PROGRAMME FOR THE XMM SERENDIPITOUS X-RAY SKY SURVEY Summary: ======== We propose optical imaging of fields drawn from the XMM observing programme using the CFH12K and the future Megaprime cameras at the 3.6 meter CFH telescope in order to provide the crucial optical data needed in realising the full potential of the XMM Serendipitous Survey. The imaging programme, combined with the public XMM data archives and source catalogues provided by the XMM Survey Science Consortium, will provide an X-ray/optical catalogue of X-ray sources which will represent a unique resource for a range of front-line areas of astrophysics. There is no other archival survey available (POSS-II) or accessible (SDSS) which is deep enough to identify a large fraction of the XMM sources. The signatories of this 'response' are members of the XMM Survey Science Consortium (XMM-SSC) and would propose to take the lead in defining the set of XMM fields to be observed and the overall optimization of the imaging programme. We have combined experience in X-ray astronomy and optical observations acquired in conducting successful X-ray surveys. Targets: ======== The launch of XMM is foreseen for December 1999, data from the CAL/PV phase (March/April 2000) become public immediately. The target list will be based entirely on public XMM data which includes CAL/PV fields plus fields that will be made public (through the SSC GT programme and other GT programmes with the express permission of the PIs). We conclude that a large number of XMM fields will be known by the beginning of Megacam science operation. These selected fields will be spread over the entire sky accessible from CFH. Scientific background: The XMM Serendipitous Survey =================================================== Pointed observations with the XMM EPIC X-ray cameras with typical exposures reach flux limits ~1e-15 erg/cmsq/s, i.e. comparable to the deepest ROSAT observations and will contain 100-200 'serendipitous' X-ray sources. The 'serendipitously' discovered X-ray sky will grow at a rate of 50,000 - 100,000 sources per year (500-1000 observations per year covering 100 sq. deg.). The XMM-SSC facilitates the exploitation of this major resource by providing a public archive of data products. Complementary optical data, however, are absolutely essential if the huge investment that has been put into XMM is to be fully realized. One of the task devoted to SSC is to provide the community with identifications either specific of statistical of all serendipitous XMM sources. Science programmes: =================== We propose to perform a wide-angle optical imaging survey in order to exploit the full potential of the XMM Serendipitous Survey. The key step will be the identification of the X-ray sources, i.e. the classification into different object types. To do this at the typical limiting X-ray fluxes one needs as first step optical imaging down to 24-25 mag. This will provide the optical magnitudes, colours and morphology of all potential counterparts to the serendipitous X-ray sources (the X-ray positional accuracy will be of order 5 arcsec or better). The full identification process incorporates a targeted spectroscopic study of a subset of all serendipitous sources and must be based on an imaging programme like the proposed one. The spectroscopically identified samples at different flux levels and galactic latitudes will be used to calibrate the image based identification process which will be applied to the larger sample. The proposed programme should comprise fields at all galactic latitudes and thus will impact galactic stellar as well as extragalactic research. In the following we mention some of the scientific questions which will be addressed using this data resource: Extragalactic: - The evolution of quasars with redshift - The origine of the extragalactic background radiation - The search for high-redshift clusters of galaxies and the evolution of their luminosity function - The nature of density fluctuations in the early universe Galactic: - The coronal activity in stars and its dependance on luminosity, spectral type, stellar rotation and population type - The population characteristics and space density of cataclysmic variables - The identification of deeply buried T Tauri stars and the search for IR excess in field T Tauri stars - The nature of the hard X-ray unresolved galactic emission (e.g. galactic ridge) - Missing links in the evolution of accreting binaries (addresses many aspects: CV evolution, Be/X-ray binaries, sensitive search for OBe+WD binaries, anomalous (breaking) X-ray pulsars) Observations: ============= We propose to obtain photometry, astrometry and morphology of all objects in selected XMM fields down to 25 mag in the optical. The field of view of the XMM EPIC cameras is 30 arcmin and thus matches the FOV of the CFH12K camera. The proposed optical magnitude limit is set by the expected magnitude of the X-ray AGN detections which have median R-magnitudes of 24.5 at f_x = 1e-15 erg/cmsq/s, the limiting X-ray flux of the proposed programme. The individual galaxies in clusters as counterparts of X-ray sources will be typically of the same brightness and the expected magnitudes of distant (5-10 kpc) cataclysmic variables in the galactic plane again are likely to be at the same brightness level. Observations in several optical filters are necessary in order to use the full diagnostic power of optical and X-ray colour-colour diagrams to separate different classes of objects. Patrick Guillout (1), Christian Motch (1), Manfred Pakull (1), Didier Barret (2), Laurent Mirioni (1). (1) Observatoire Astronomique, 11 rue de l'Universite F-67000 Strasbourg (2) CESR, 9 Av du Colonel Roche, F-31028 Toulouse Cedex 04