Troja, E.E.TrojaPIRO, LUIGILUIGIPIROVasileiou, V.V.VasileiouOmodei, N.N.OmodeiBurgess, J. M.J. M.BurgessCutini, S.S.CutiniConnaughton, V.V.ConnaughtonMcEnery, J. E.J. E.McEnery2020-09-032020-09-0320150004-637Xhttp://hdl.handle.net/20.500.12386/27083Simultaneous Swift and Fermi observations of gamma-ray bursts (GRBs) offer a unique broadband view of their afterglow emission, spanning more than 10 decades in energy. We present the sample of X-ray flares observed by both Swift and Fermi during the first three years of Fermi operations. While bright in the X-ray band, X-ray flares are often undetected at lower (optical), and higher (MeV to GeV) energies. We show that this disfavors synchrotron self-Compton processes as the origin of the observed X-ray emission. We compare the broadband properties of X-ray flares with the standard late internal shock model, and find that in this scenario, X-ray flares can be produced by a late-time relativistic (Γ > 50) outflow at radii R ~ 10<SUP>13</SUP>-10<SUP>14</SUP> cm. This conclusion holds only if the variability timescale is significantly shorter than the observed flare duration, and implies that X-ray flares can directly probe the activity of the GRB central engine.STAMPAenArticle10.1088/0004-637X/803/1/102-s2.0-84927591081000353015400010https://iopscience.iop.org/article/10.1088/0004-637X/803/1/102015ApJ...803...10TFIS/05 - ASTRONOMIA E ASTROFISICA