The ULTraS project: Understanding the X-ray variable and transient sky

Time variability is ubiquitous in the X-ray sky and is a unique probe of a wide diversity of astrophysical processes. The aim of the ULTraS (Understanding the Xray variable and transient sky) project is to exploit the scientific results and products from the EU/FP7-funded project EXTraS (a systematic characterisation of time variability in all XMM-Newton sources) and from its more recent developments. In particular, we have addressed a series of science cases, from the search for rare phenomena to the characterization of the properties of source classes, on a large variety of astrophysical objects, ranging from nearby, ultracool stars to exploding supernovae in distant galaxies. Results described here prove the effectiveness and the potential for discovery of our systematic temporal characterization. A full exploitation of the temporal information in the XMM-Newton/EXTraS archive requires a different approach, based on artificial intelligence - this seems definitely the perspective we should adopt to cope with the high data volume expected from future X-ray facilities.