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arXiv · 1101.5923

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

P. A. Evans·J. P. Osborne·R. Willingale·P. T. O'Brien
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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.

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P. A. Evans, J. P. Osborne, R. Willingale, P. T. O'Brien. 2011-01-31. Spectral evolution and the onset of the X-ray GRB afterglow. https://doi.org/10.1063/1.3621751

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