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

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

2011-01-31

arxiv.org/abs/1101.5923v1

Astronomy and Astrophysics

Astrophysics

High Energy Astrophysical Phenomena

4 pages, 3 figures. To appear in the proceedings of GRB 2010, Annapolis November 2010. (AIP Conference proceedings)

Scientific paper

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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