Spectral evolution and the onset of the X-ray GRB afterglow

TL;DR

This paper models the early-time spectral dip in Swift-XRT GRB data as the transition from prompt emission to afterglow, allowing measurement of the initial Lorentz factor of the fireball.

Contribution

It introduces a model linking the spectral dip to the rise of afterglow emission, enabling estimation of the initial Lorentz factor for GRBs.

Findings

Median Lorentz factor Gamma_0=225 for constant-density medium

Median Lorentz factor Gamma_0=93 for wind-like medium

The dip feature can be explained by the transition from prompt to afterglow

Abstract

Based on light curves from the Swift Burst Analyser, we investigate whether a `dip' feature commonly seen in the early-time hardness ratios of Swift-XRT data could arise from the juxtaposition of the decaying prompt emission and rising afterglow. We are able to model the dip as such a feature, assuming the afterglow rises as predicted by Sari & Piran (1999). Using this model we measure the initial bulk Lorentz factor of the fireball. For a sample of 23 GRBs we find a median value of Gamma_0=225, assuming a constant-density circumburst medium; or Gamma_0=93 if we assume a wind-like medium.