  We investigate the effect that the absorption of high-energy (above 100 MeV)
photons produced in GRB afterglow shocks has on the light-curves and spectra of
Fermi-LAT afterglows. Afterglows produced by the interaction of a relativistic
outflow with a wind-like medium peak when the blast-wave deceleration sets in,
and the afterglow spectrum could be hardening before that peak, as the optical
thickness to pair-formation is decreasing. In contrast, in afterglows produced
in the interaction with a homogeneous medium, the optical thickness to
pair-formation should increase and yield a light-curve peak when it reaches
unity, followed by a fast light-curve decay, accompanied by a spectral
softening. If energy is injected in the blast-wave, then the accelerated
increase of the optical thickness yields a convex afterglow light-curve. Other
features, such as a double-peak light-curve or a broad hump, can arise from the
evolution of the optical thickness to photon-photon absorption. Fast decays and
convex light-curves are seen in a few LAT afterglows, but the expected spectral
softening is rarely seen in (and difficult to measure with) LAT observations.
Furthermore, for the effects of photon-photon attenuation to shape the
high-energy afterglow light-curve without attenuating it too much, the ejecta
initial Lorentz factor must be in a relatively narrow range (50-200), which
reduces the chance of observing those effects.
