The 3XMM-DR4 catalogue is the third generation catalog of serendipitous X-ray sources from the European Space Agency's (ESA) XMM-Newton observatory, and has been created by the XMM-Newton Survey Science Centre (SSC) on behalf of ESA. The catalog has 2474 more observations and about 178,000 (50%) more detections than the preceding 2XMMi-DR3 catalog, which was made public in April 2010. The history of the versions can be summarized as: ---------------------------------------------------- Name DR# Designation Year Cat. #Sources ---------------------------------------------------- 2XMMp 0 2XMMp-DR0 2006 2XMM 1 2XMM-DR1 2007 IX/39 191870 2XMMi 2 2XMMi-DR2 2008 IX/40 221012 2XMMi-DR3 3 2XMMi-DR3 2010 IX/41 262902 3XMM-DR4 4 3XMM 2013 IX/44 372728 ---------------------------------------------------- The production and content of the 3XMM catalogue is described in the the 3XMM-DR4 User Guide at http://xmmssc-www.star.le.ac.uk/Catalogue/3XMM-DR4/UserGuide_xmmcat.html The "slim" version of the catalogue (file "xmm3r4s.dat") contains one row per unique source, while the the main catalogue has one row per detection. This slim version includes 44 columns, essentially those containing information about the unique sources, while the full catalogue (file "xmm3r4.fit") describes all detections with 318 columns. The slim version also contains a column with links to the LEDAS summary pages. In the case of sources with multiple detections the summary page of the best detection is selected (i.e., the detection with the largest exposure time, summed over all cameras), and the summary page gives cross-links to the other detections. A separate file "summary.dat" contains the key details about the observations used in the construction of the 3XMM-DR4 catalogue.