X-ray variability of serendipitous AGNs discovered in Swift GRB Fields

Active Galactic Nuclei are variable in all wave bands and in a wide range of time scales. In the X-ray band, short time scale (10^3-10^5 s) variability provides evidence that the emission comes from a compact region around the central supermassive black hole. In the optical bands the ensemble analysis on large optical samples was made possible by the Sloan Digital Sky Survey, and provided a characterization of the dependence of optical variability on luminosity, redshift, wavelength and time delay. A similar analysis can be performed in the X-rays, taking advantage of the catalogue of serendipitous sources extracted from ~230 SWIFT GRB fields, which contains about 7000 objects down to ~10^-15 erg/cm^2/s flux in the 0.5-10 keV band and ~800 objects down to 5x10^-14 erg/cm^2/s. The temporal sampling of Swift observations is designed to follow, in the X-ray band, the GRB evolution with typical intervals of a few days for a duration of weeks to months, and simultaneous UV-optical monitoring of the field. I will present preliminary results on the ensemble analysis of variability for a subsample of serendipitous sources with known redshift extracted from the Swift archive.