Author: Eleonora Troja
Abstract: X-ray flares are sudden brightening episodes observed in a large fraction of GRB afterglows. Several models have been proposed to explain their origin. Most of them require a reactivation of the central engine, that therefore has to pour out substantial energy long after the cessation of the prompt gamma-ray emission. In a range of models the low-energy flare observed by Swift is also expected to be accompanied by a high energy counterpart detectable by the Fermi/LAT. We selected a sample of GRBs with bright X-ray flares and simultaneous Swift and Fermi observations, and carried out the first systematic search for the predicted high-energy flare in the Fermi/LAT range. We have performed a broadband study of the X-ray flares from UV/optical to GeV energies and derived the spectral and temporal behavior expected by models involving either internal or external shocks of the relativistic jet. Here we show the results of our multi-wavelength analysis, the comparison with model predictions in the case of late internal shock model, and discuss the implications of our results in the context of a long duration central engine.