Inverse Compton X-ray flare from gamma-ray burst reverse shock

We study synchrotron self-inverse Compton radiation from a reverse shock fireball. If the inverse Compton process dominates the cooling of shocked electrons, an X-ray flare produced by the first-order Compton scattering would emerge in the very early afterglow phase, with the bulk of the shock energy radiated in the second-order scattering component at 10-100 MeV. The dominance of inverse Compton cooling leads to the lack of prompt optical flashes. We show that for plausible parameters this scattering process can produce an X-ray flare with a relative amplitude change of a factor of several. Flares with a larger amplitude and multiple X-ray flares in a single event are likely to be produced by another mechanism (e.g., internal shocks).