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

Rongrong Xue, Yizhong Fan, Daming Wei

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…