X-ray flares, plateaus, and chromatic breaks of GRB afterglows from up-scattered forward-shock emission

Scattering of the forward-shock synchrotron emission by a relativistic outflow located behind the leading blast-wave may produce an X-ray emission brighter than that coming directly from the forward-shock and may explain four features displayed by Swift X-ray afterglows: flares, plateaus (slow decays), chromatic light-curve breaks, and fast post-plateau decays. For a cold scattering outflow, the reflected flux overshines the primary one if the scattering outflow is nearly baryon-free and highly relativistic. These two requirements can be relaxed if the scattering outflow is energized by weak internal shocks, so that the incident forward-shock photons are also inverse-Compton scattered, in addition to bulk-scattering. Sweeping-up of the photons left behind by the forward shock naturally yields short X-ray flares. Owing to the boost in photon energy produced by bulk-scattering scattering, the reflected emission is more likely to overshine that coming directly from the forward shock at higher photon energies, yielding light-curve plateaus and breaks that appear only in the X-ray. The brightness, shape, and decay of the X-ray light-curve plateau depend on the radial distribution of the scatterer’s Lorentz factor and mass-flux. Chromatic X-ray light-curve breaks and sharp post-plateau decays cannot be accommodated by the direct forward-shock emission and argue in favour of the scattering-outflow model proposed here. On the other hand, the X-ray afterglows without plateaus, those with achromatic breaks, and those with very long-lived power-law decays are more naturally accommodated by the standard forward-shock model. Thus the diversity of X-ray light-curves arises from the interplay of the scattered and direct forward-shock emissions.