Sub-GeV flashes in γ-ray burst afterglows as probes of underlying bright far-ultraviolet flares

Bright optical and X-ray flares have been observed in many gamma-ray burst (GRB) afterglows. These flares have been attributed to late activity of the central engine. In most cases the peak energy is not known and it is possible and even likely that there is a significant far-ultraviolet component. These far-ultraviolet photons escape our detection because they are absorbed by the neutral hydrogen before reaching Earth. However, these photons cross the blast wave produced by the ejecta that has powered the initial GRB. They can be inverse Compton upscattered by hot electrons within this blast wave. This process will produce a strong sub-GeV flare that follows the high-energy (soft X-ray) tail of the far-ultraviolet flare but lasts much longer and can be detected by the upcoming Gamma-ray Large Area Space Telescope (GLAST) satellite. This signature can be used to probe the spectrum of the underlying far-ultraviolet flare. The extra cooling produced by this inverse Compton process can lower the X-ray emissivity of the forward shock and explain the unexpected low, early X-ray flux seen in many GRBs.