A systematic search of highly variable X-ray sources with the XMM-Newton serendipitous survey

We describe an ongoing programme to search for X-ray sources with long-term, large amplitude X-ray variability. The method is to compare the X-ray sources from the XMM-Newton Serendipitous Survey with those from the previous ROSAT pointed observations and the ROSAT All-sky Survey. The work is largely motivated by the recent discovery of a new type of X-ray flares characterised by enormous amplitude flux variability and associated with normal galaxies [Komossa, S., Ludwig Biermann Award lecture: X-ray evidence for supermassive black holes at the centers of nearby, non-active galaxies. Rev. Mod. Astron. 15, 27, 2002]. Other objects of interest are black hole accretion systems, such as X-ray binaries and AGN, GRB afterglows, extremes of stellar flares, and other objects undergoing extremely violent physical processes. In this paper, we describe the programme and present some preliminary results.