  Observations of gamma ray bursts (GRBs) with Swift produced the initially
surprising result that many bursts have large X-ray flares superimposed on the
underlying afterglow. These flares were sometimes intense, rapid, and late
relative to the nominal prompt phase. The most intense of these flares was
observed by XRT with a flux >500 the afterglow. This burst then surprised
observers by flaring again after >10000 s. The intense flare can be most easily
understood within the context of the standard fireball model, if the internal
engine that powers the prompt GRB emission is still active at late times.
Recent observations indicate that X-ray flares are detected in ~1/3 of XRT
detected afterglows. By studying the properties of the varieties of flares
(such as rise/fall time, onset time, spectral variability, etc.) and relating
them to overall burst properties, models of flare production and the GRB
internal engine can be constrained.
