The XMM-Newton serendipitous survey. VIII: The first XMM-Newton serendipitous source catalogue from overlapping observations

XMM-Newton has observed the X-ray sky since the beginning of 2000. The XMM-Newton Survey Science Centre Consortium has been publishing catalogues of X-ray and ultraviolet sources found serendipitously in the individual observations. This series is now augmented by a catalogue dedicated to X-ray sources detected in spatially overlapping XMM-Newton observations. This catalogue aims at exploring repeatedly observed sky regions. It thus makes use of the long(er) effective exposure time per sky area and offers the opportunity to investigate long-term flux variability directly through the source detection process. A new standardised strategy for simultaneous source detection on multiple observations is introduced. It is coded as a new task edetect_stack within the XMM-Newton Science Analysis System and used to compile a catalogue of sources from 434 stacks comprising 1,789 selected overlapping full-frame observations. The first stacked catalogue is called 3XMM-DR7s. It contains 71,951 unique sources with positions and source parameters like fluxes, hardness ratios, quality estimate, and information on inter-observation variability, which are directly derived from a simultaneous fit and calculated for the stack and for each contributing observation. More than 8,000 sources are new with respect to 3XMM-DR7. By stacked source detection, the parameters of repeatedly observed sources can be determined with higher accuracy than in the individual observations. It is more sensitive to faint sources and tends to produce fewer spurious detections. With this first catalogue we demonstrate the feasibility and benefit of the approach. It supplements the large data base of XMM-Newton detections by additional, in particular faint, sources and new variability information. In the future, it will be expanded to larger samples and continued within the series of serendipitous XMM-Newton source catalogues.