GRB afterglow 090510 is (so far) the best-monitored afterglow in the optical,
X-ray, and above 100 MeV, measurements covering 2-3 decades in time at each
frequency. Owing to its power-law temporal decay and power-law spectrum, it
seems very likely that the highest energy emission is from the forward-shock
energizing the ambient medium (the standard blast-wave model for GRB
afterglows), the GeV flux and its decay rate being consistent with that model's
expectations. However, the synchrotron emission from a collimated outflow (the
standard jet model) has difficulties in accounting for the lower-energy
afterglow emission, where a simultaneous break occurs at 2 ks in the optical
and X-ray light-curves, but with the optical flux decay (before and after the
break) being much slower than in the X-rays (at same time). The measured X-ray
and GeV fluxes are incompatible with the higher-energy afterglow emission being
from same spectral component as the lower-energy afterglow emission, which
suggests a synchrotron self-Compton model for this afterglow. Cessation of
energy injection in the blast-wave and an ambient medium with a wind-like n ~
r^{-2} density can explain all features of the optical and X-ray light-curves
of GRB afterglow 090510. Such an ambient medium radial structure is
incompatible with this short-GRB originating from the merger of two compact
stars.Comment: 12 pages, to appear in MNRA
