  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.
