The initial Lorentz factors of fireballs inferred from the early X-ray data of SWIFT GRBs

¹ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China e-mail: yizhong@nbi.dk
² Graduate School, Chinese Academy of Sciences, Beijing, 100012, PR China
³ Niels Bohr International Academy, Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100 Copenhagen, Denmark
⁴ Joint Center for Particle Nuclear Physics and Cosmology of Purple Mountain Observatory – Nanjing University, Nanjing 210008, PR China

Received: 17 November 2008
Accepted: 10 February 2009

Abstract

Aims. We intend to determine the type of circumburst medium and directly measure the initial Lorentz factor  of GRB outflows.

Methods. 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 , especially for the ISM case. The afterglow light curves for a flat electron spectrum have been derived analytically.

Results. In our GRB sample, we obtained 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 reliably.