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One of the most impressive recent discovery of SWIFT is the evidence thatX-ray flares occurring during the GRB afterglows are quite common, beingobserved in roughly 50% of the afterglows. These X-ray flares range fluencescomparable with the GRB prompt emission and could be also repetitive. Severalpictures have been proposed on their origin and among them the most acceptedregards the internal shock scenario, interpreting the X-ray flares as late timeactivity of the GRB central engine. We propose to describe the spectral shapeof the X-ray flares adopting the same physical model recently used to interpretthe GRB prompt emission: the log-parabolic function. In particular, we showthat their spectral energy distribution (SED) is remarkably curved, while nosignificant curvature appears in the underlying X-ray afterglow emission. Inaddition, the log-parabolic function is statistically favored with respect toother proposed spectral models. By using a time resolved spectral analysis, weshow the evolution of the peak energy and the curvature parameters of the SEDduring the X-ray flares in two of the brightest GRBs afterglows observed bySWIFT. We found that in the X-ray flares there is an anti-correlation betweenthe peak energy and the curvature, as expected in a stochastic accelerationscenario.

Spectral curvature behavior during X-ray flares in GRB afterglow emission