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.Comment: to appear in MNRAS, 12 pages, 7 figure
