GRB 090510: a short burst from a massive star ?

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.