The Space Missions WAXS/WFXT and SWIFT: New Targets for the VLT

At OAB we were, during the year 1998, deeply involved in planning two important space missions for which very large ground based telescopes, VLT in particular, would play a very large and important role in the optical followup. The study of the first mission, Wide Angle X-ray Survey using a Wide Field X-ray Telescope, was coordinated by the Observatory of Brera, involved mainly Italian industries and resulted in a proposal to the Italian Space Agency under the Small Satellite Program. The second mission, SWIFT, has been coordinated and directed by the Goddard Space Flight Center and resulted in the submission of a proposal to NASA under the MIDEX program. The science goal of this mission is the detection and study of the Gamma-ray bursts. 1 Background of the WAXS/WFXT Mission It is very well known that the clue to the understanding the formation and evolution of Clusters of Galaxies resides in the detection and detailed study of clusters at redshifts larger than 0.5 and up to a redshift z∼ 2 assuming they still exist and are recognizable. It is also well known that the detection of distant clusters by optical surveys is very limited because of the confusion of the galaxy background at faint magnitudes and the fading in that of the member galaxies. Indeed that had been clearly discussed some time ago by Cappi et al. (1989) and more recently evidenced by the galaxy surveys carried out by Postman et al. (1996) and by the ESO Survey (EIS), these proceedings. On the other hand while the X-ray cluster detection is a much cleaner method due to the net separation at any redshift between an extended emission and a point like source over a rather faint X-ray background, a good resolution of the optics is a must. Possibly the X-ray telescope should allow a resolution better than 15 arcsec Half Energy Width (HEW). A good sensitivity is also needed since the surface brightness of even bright clusters dims considerably with distance. It has been shown, especially by Bahcall and Soneira (1983) but see also the most recent results obtained with the REFLEX project, Guzzo et al. (1999), that clusters are unique tools in describing the statistical characteristics of the Large Scale Structure. While shortly the Power Spectrum for the local Universe will be quite nicely measured by REFLEX, it remains fundamental to verify if the theoretical prediction are confirmed at redshifts z∼ 1 and larger. Statistical information on much higher redshifts could be