The Interpretation and Implication of the Afterglow of GRB 060218

The nearby GRB 060216/SN 2006aj was an extremely long, weak and very soft GRB. While it was peculiar in many aspects its late (> 10 sec) X-ray afterglow showed a canonical power law decay. Assuming that this component arises due to a relativistic blast wave decelerated by a circumburst matter we infer that the blast wave’s kinetic energy was rather high, 5 × 10 erg, close to what is seen in other GRBs. The lack of a “jet break” implies that the outflow was wide θj ∼ 1. The rather weak early optical emission rules out a dense circumburst wind profile. It also constrains the initial Lorentz factor to be significantly lower than usual, Γini ∼ 15. The observed afterglow suggests that the medium surrounding a massive star progenitor (up to distances of ∼ 10 − 10 cm) is not the expected dense stellar wind (a similar result was seen in many other bursts and in particular in GRB 030329). This implies that the progenitor’s wind was weak during the last 100-1000 years before the burst. This interpretation requires a different source for the thermal emission seen in the early Xray and late optical/UV. We speculate that this emission arises from the interaction of the relativistic ejecta with the stellar envelope.