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

We present the prompt Burst Alert Telescope and afterglow X-ray Telescope data of Swift-discovered GRB 081109A up to similar to 5 x 105 s after the trigger, and the early ground-based optical followups. The temporal and spectral indices of the X-ray afterglow emission change remarkably. We interpret this as the gamma-ray burst jet first traversing the freely expanding supersonic stellar wind of the progenitor with density varying as proportional to r-2. Then, after approximately 300 s 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. The optical afterglow data are generally consistent with such a scenario. Our best numerical model has a wind density parameter A(*) similar to 0.02, a density of the stalled wind n similar to 0.12 cm-3 and a transition radius similar to 4.5 x 1017 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.