GRB 090510: a short burst from a massive star?

Gamma-ray burst (GRB) afterglow 090510 is (so far) the best-monitored afterglow in the optical, X-ray and above 100MeV, measurements covering two-three 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 proportional to 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.