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

摘要

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 and Piran [1]. 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) velence 225, assuming a constant-density circumburst medium; or (GAMMA)_(0) velence 93 if we assume a wind-like medium.