2009A&A...498..671X

We intend to determine the type of circumburst medium and directly measure the initial Lorentz factor Γ₀ of GRB outflows. If the early X-ray afterglow light curve has a peak and the whole profile across the peak is consistent with the standard external shock model, the early rise profile of light curves can be used to differentiate whether the burst was born in the interstellar medium (ISM) or in a stellar wind. In the thin shell case related to a subrelativistic reverse shock, the peak time occurring after the end of the prompt emission can be used to derive an accurate Γ₀, especially for the ISM case. The afterglow light curves for a flat electron spectrum 1<p<2 have been derived analytically. In our GRB sample, we obtained Γ₀∼300 for the bursts born in ISM. We did not find any good case for bursts born in stellar wind and behaving as a thin shell that can be used to constrain Γ₀ reliably.