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We present the prompt Burst Alert Telescope and afterglow X‐ray Telescope data of  Swift ‐discovered GRB 081109A up to  ∼5 × 10 5  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  ρ∝ 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  * ∼ 0.02 , a density of the stalled wind   n ∼ 0.12 cm −3   and a transition radius  ∼4.5 × 10 17   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.

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