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GRB 121027A is un-usual with its extremely long-lasting, energetic X-rayflares. The total energy release in X-ray flares is about one order ofmagnitude higher than prompt gamma-rays, making it special from most long GRBs.We show that while the prompt gamma-ray emission satisfies the empiricalE_{iso}-E_{p} relation of typical long GRBs, the X-ray flares, whose spectracan be fit with a cutoff-power-law model with well-constrained E_p,significantly deviate from such a relation. Nonetheless, a time-resolvedspectral analysis of X-ray flares suggest that the X-ray emission is consistentwith the L_{iso}-E_{p} relation of long GRBs. We constrain the minimum Lorentzfactor of the X-ray flares to be ~14, which is consistent with theGamma-L_{iso} relation. Our results imply that prompt gamma-ray emission andlate X-ray flares share the similar radiation mechanism, but originate from theoutflows with different Lorentz factors. We search for similar GRBs from theSwift GRB archives, and find that the z=6.29 GRB 050904 is a carbon copy of GRB121027A. The long-lasting, energetic X-ray flares in these GRBs demandsignificant accretion at late times, which point towards a large-radiusprogenitor star.

GRB 121027A: long-lasting, energetic X-ray flares and clues to radiation mechanism and progenitor star