The X-ray afterglow of GRB 081109A: clue to the wind bubble structure

We present the prompt BAT and afterglow XRT data of Swift-discovered GRB 081109A up to ∼ 5 × 10 sec after the trigger, and the early ground-based optical follow-ups. The temporal and spectral indices of the X-ray afterglow emission change remarkably. We interpret this as the GRB jet first traversing the freely expanding supersonic stellar wind of the progenitor with density varying as ρ ∝ r. Then after approximately 300 seconds the jet traverses into a region of apparent constant density similar to that expected in the stalled-wind region of a stellar wind bubble or the interstellar medium (ISM). The optical afterglow data are generally consistent with such a scenario. Our best numerical model has a wind density parameter A∗ ∼ 0.02, a density of the stalled wind n ∼ 0.12 cm, and a transition radius ∼ 4.5 × 10 cm. Such a transition radius is smaller than that predicted by numerical simulations of the stellar wind bubbles and may be due to a rapidly evolving wind of the progenitor close to the time of its core-collapse.