ESO imaging survey: optical follow-up of 12 selected XMM-Newton fields

This paper presents the data recently released for the XMM-Newton/WFI survey carried out as part of the ESO Imaging Survey (EIS) project. The aim of this survey is to provide optical imaging follow-up data in $\it BVRI$ for identification of serendipitously detected X-ray sources in selected XMM-Newton fields. In this paper, fully calibrated individual and stacked images of 12 fields as well as science-grade catalogs for the 8 fields located at high-galactic latitude are presented. These products were created, calibrated and released using the infrastructure provided by the EIS Data Reduction system and its associated EIS/MVM image processing engine, both of which are briefly described here. The data covers an area of ~3 square degrees for each of the four passbands. The median seeing as measured in the final stacked images is $0\farcs94$, ranging from $0\farcs60$ and $1\farcs51$. The median limiting magnitudes (AB system, 2´´ aperture, $5\sigma$ detection limit) are 25.20, 24.92, 24.66, and 24.39 mag for B-, V-, R-, and I-band, respectively. When only the 8 high-galactic latitude fields are included these become 25.33, 25.05, 25.36, and 24.58 mag, in good agreement with the planned depth of the survey. Visual inspection of images and catalogs, comparison of statistics derived from the present data with those obtained by other authors and model predictions, as well as direct comparison of the results obtained from independent reductions of the same data, demonstrate the science-grade quality of the automatically produced final images and catalogs. These survey products, together with their logs, are available to the community for science exploitation in conjunction with their X-ray counterparts. Preliminary results from the X-ray/optical cross-correlation analysis show that about 61% of the detected X-ray point sources in deep XMM-Newton exposures have at least one optical counterpart within 2´´ radius down to $R \simeq$ 25 mag, 50% of which are so faint as to require VLT observations thereby meeting one of the top requirements of the survey, namely to produce large samples for spectroscopic follow-up with the VLT, whereas only 15% of the objects have counterparts down to the DSS limiting magnitude.