The XMM survey will be accompanied by a ground-based survey which will cover each field in UBVRI. Therefore, the interest of the OM contribution originates in its ability to observe in the UV. The exposure time per field as imposed by the X-ray survey is 10 ksec. The default configurations for the OM perform five exposures to cover the whole OM field (which is already less than the EPIC field). This means that the maximum available exposure time is, for a default configuration, 2 ksec (1 ksec per filter if one uses UVW1 and UVW2). For the UV bandpasses, this does not yield sufficiently deep exposures. Therefore, we prefer to restrict the extent of the observed field and to use the recovered free time to perform longer exposures (a factor 4 to 5). We managed to design the largest science window available (7.7 by 7.7 arcmin) and we plan to devote all the time to this region.
The UBVRI images from ground-based telescopes (ESO,CFHT and other telescopes) and from the OM in 3 filters (UVW2, UVW1 and B) is required to characterise and and study the evolution of activity in AGN, and galaxies in general. This choice of filters will enable us to achieve a multi-color study of outstanding X-ray sources (QSOs, AGN, ...) detected in the single OM field.
Since star formation may account for a large part of galaxy activity at low as well as at high redshift, we propose to select appropriate filters (i.e. UVW2, UVW1 and B) to detect A-B type stars in galaxies from their UV colors. If present, nuclear activity will also be detected with this combination of filters.
Typically, in order to detect A-B type stars at z = 0 in the UVW2 band with a similar signal-to-noise as in the UVW1 band, an exposure time 2.5 times larger is necessary with the UVW2 filter.
Observations with the B filter are aimed at monitoring possible variability between the ground-based and OM data. Exposures of duration 1 ksec will detect stars of magnitude B = 23.
The following OM observations will be made: