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Energy input and response from prompt and early optical afterglow emission in γ-ray bursts

Abstract

The taxonomy of optical emission detected during the critical first few minutes after the onset of a γ-ray burst (GRB) defines two broad classes: prompt optical emission correlated with prompt γ-ray emission1, and early optical afterglow emission uncorrelated with the γ-ray emission2. The standard theoretical interpretation attributes prompt emission to internal shocks in the ultra-relativistic outflow generated by the internal engine3,4,5; early afterglow emission is attributed to shocks generated by interaction with the surrounding medium6,7,8. Here we report on observations of a bright GRB that, for the first time, clearly show the temporal relationship and relative strength of the two optical components. The observations indicate that early afterglow emission can be understood as reverberation of the energy input measured by prompt emission. Measurements of the early afterglow reverberations therefore probe the structure of the environment around the burst, whereas the subsequent response to late-time impulsive energy releases reveals how earlier flaring episodes have altered the jet and environment parameters. Many GRBs are generated by the death of massive stars that were born and died before the Universe was ten per cent of its current age9,10, so GRB afterglow reverberations provide clues about the environments around some of the first stars.

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Figure 1: The onset of prompt optical emission from GRB 050820a.
Figure 2: A comparison of the early optical light curve and the γ-ray light curve measured for GRB 050820a.
Figure 3: The decomposition the optical light curve measured for GRB 050820a into primary optical components.
Figure 4: Broadband spectra of prompt emission from GRB 050820a.

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Acknowledgements

The RAPTOR project is supported by the Laboratory Directed Research and Development program at Los Alamos National Laboratory. The Konus-Wind experiment is supported by the Russian Space Agency and the Russian Foundation for Basic Research.

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Correspondence to W. T. Vestrand.

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Vestrand, W., Wren, J., Wozniak, P. et al. Energy input and response from prompt and early optical afterglow emission in γ-ray bursts. Nature 442, 172–175 (2006). https://doi.org/10.1038/nature04913

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