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

TL;DR

This paper uses early X-ray afterglow data from SWIFT GRBs to determine the circumburst medium type and measure the initial Lorentz factor, finding typical values around 300 for ISM environments.

Contribution

It introduces a method to infer the initial Lorentz factor and circumburst medium type from early X-ray lightcurves, especially in the thin shell case, with analytical derivations for flat electron spectra.

Findings

Initial Lorentz factors around 300 for ISM GRBs

Early X-ray lightcurves can distinguish circumburst medium types

No reliable Lorentz factor estimates for stellar wind cases

Abstract

We intend to determine the type of circumburst medium and measure directly the initial Lorentz factor $\Gamma_0$ of GRB outflows. If the early X-ray afterglow lightcurve 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 interstellar medium (ISM) or in stellar wind. In the thin shell case, related to a sub-relativistic reverse shock, the peak time occurring after the end of the prompt emission, can be used to derive an accurate $\Gamma_0$, especially for the ISM case. The afterglow lightcurves for a flat electron spectrum $1<p<2$ have been derived analytically. In our GRB sample, we obtain $\Gamma_0 \sim 300$ 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 $\Gamma_0$ reliably.