XMM-Newton/EPIC is the best available tool to study the variability in the soft X-ray sky. The unprecedented combination of a large field of view, high sensitivity to point sources, and good time resolution yields unrivaled potential to reveal faint and/or rare variable sources. The three EPIC cameras have a FoV as large as the full Moon and the largest effective area of any imaging X-ray instrument. Moreover, the EPIC spatial resolution, although worse than that of the Chandra observatory, is good enough to avoid source confusion and background contamination up to integration times of thousands of seconds.
The combination of large collecting area and good angular resolution makes EPIC very sensitivity to point sources (~5 10-15 erg cm-2 s-1 in 10,000 s). The default XMM-Newton/EPIC time resolutions of 2.6s and 73ms for the MOS and the PN cameras, respectively, are among the best available in currently operating X-ray telescopes. The simultaneous coverage of the same FoV with the three EPIC cameras, provides, in some cases, more robust detections by comparing the results obtained by the different detectors. Finally, in addition to these key instrumental properties, the long time actively spent in orbit (net exposure time of each camera is ~230 millions seconds up to now, with the prospect of more years of observations) guarantees unprecedented sky coverage for an X-ray telescope and the possibility to discovering relatively rare events.
Large efforts are ongoing to explore the serendipitous content of the XMM-Newton database, although the time domain remains largely unexplored.
The XMM serendipitous source catalogue
A catalogue of all the X-ray sources detected in EPIC observations is periodically released (the XMM-Newton Serendipitous Source Catalogue). This effort is led by the XMM-Newton Survey Science Centre, a consortium of ten different international institutions (including the University of Leicester and the Max Planck Institut fuer Extraterrestrische Physik, partners of the EXTraS project). The most recent version of the catalogue, named 3XMM, was released in July 2013. Based on state-of-the-art EPIC data processing, data screening and detection techniques, it is the richest X ray source catalogue ever compiled.
It lists more than 370,000 X-ray sources (more than 66,000 detected in more than one observation) over a sky area of about 800 square degree, with a median flux of 2 10-14 erg cm-2 s-1. For more than 120,000 sources, a light curve (as well as a spectrum) was automatically extracted. Only a simple test for time variability is automatically performed on such light curves, and a variability flag assigned. A FFT is also performed on the light curves to produce a simple power spectrum graphic. Systematic investigations of variability are not carried out by the catalogue team.
The XMM Slew Survey
A second, large effort, aimed at exploiting XMM/EPIC data for serendipitous source science is the XMM slew survey, based on data collected while the telescope moves from one target to the next scheduled one. The XMM-Newton Slew Survey (XSS) covers a significant fraction of the sky (over 60%, currently increasing by approximately 8% per year) in a broad X-ray band-pass. Although shallow by contemporary standards, in the hard 2-10 keV band of X-ray astronomy, the XSS provides significantly better sensitivity (limiting flux; 3 10-12 erg cm-2 s-1) than any currently available all-sky survey. In the soft 0.2-2 keV band, the XSS is almost as sensitive (limiting flux; 6 10-13 erg cm-2 s-1) as the ROSAT All-Sky Survey (RASS), and a large-area RASS-Slew comparison provides, at this present time, the best opportunity for discovering extremely rare, high-variability objects, and a number of such objects (novae, Tidal Disruption events etc) have been seen. 1000's of other objects both known (Galactic transients, AGN, flare stars etc) and unknown are seen, in the XMM-Newton Slew data, to have long-term variability when compared with RASS, but no dedicated study and cataloguing of this variability has yet been performed.