A link between prompt optical and prompt γ-ray emission in γ-ray bursts

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Abstract

The prompt optical emission that arrives with the γ-rays from a cosmic γ-ray burst (GRB) is a signature of the engine powering the burst, the properties of the ultra-relativistic ejecta of the explosion, and the ejecta's interactions with the surroundings1,2,3,4,5. Until now, only GRB 990123 had been detected6 at optical wavelengths during the burst phase. Its prompt optical emission was variable and uncorrelated with the prompt γ-ray emission, suggesting that the optical emission was generated by a reverse shock arising from the ejecta's collision with surrounding material. Here we report prompt optical emission from GRB 041219a. It is variable and correlated with the prompt γ-rays, indicating a common origin for the optical light and the γ-rays. Within the context of the standard fireball model of GRBs, we attribute this new optical component to internal shocks driven into the burst ejecta by variations of the inner engine. The correlated optical emission is a direct probe of the jet isolated from the medium. The timing of the uncorrelated optical emission is strongly dependent on the nature of the medium.

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Figure 1: The prompt optical emission detected from GRB 041219a.
Figure 2: Comparison of the prompt γ-ray and prompt optical light curves measured9 for both GRB 041219a and GRB 990123.
Figure 3: The measured optical light curve and that predicted for GRB 041219a assuming a constant prompt optical to prompt γ-ray flux ratio.
Figure 4: Broad-band spectra of GRB 041219a, here plotted in flux density Fν as a function of observed frequency ν, measured during the period of simultaneous prompt optical and γ-ray emission.

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Acknowledgements

The RAPTOR project is supported by the Laboratory Directed Research and Development programme at Los Alamos National Laboratory.

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Correspondence to W. T. Vestrand.

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The authors declare that they have no competing financial interests.

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Supplementary Discussion

Extra discussion of the methods and results of the study, with additional references. (DOC 24 kb)

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