Analysis of X-ray flares in GRBs

We present a detailed study of the spectral and temporal properties of the X-ray flares emission of several GRBs. We select a sample of GRBs which X-ray light curve exhibits large amplitude variations with several rebrightenings superposed on the underlying three-segment broken powerlaw that is often seen in Swift GRBs. We try to understand the origin of these fluctuations giving some diagnostic in order to discriminate between refreshed shocks and late internal shocks. For some bursts our time-resolved spectral analysis supports the interpretation of a long-lived central engine, with rebrightenings consistent with energy injection in refreshed shocks as slower shells generated in the central engine prompt phase catch up with the afterglow shock at later times.