X-ray flares from late internal and late external shocks

We analyze several recently detected gamma-ray bursts (GRBs) with late X-ray flares in the context of late internal shock and late external shock models. We find that the X-ray flares in GRB 050421 and GRB 050502B originate from late internal shocks, while the main X-ray flares in GRB 050406 and GRB 050607 may arise from late external shocks. We also constrain the lower limit on the Lorentz factor of X-ray flares in the context of the late internal shock model.