Observations of the long-lived emission - or 'afterglow' - of long-duration γ-ray bursts place them at cosmological distances, but the origin of these energetic explosions remains a mystery. Observations of optical emission contemporaneous with the burst of γ-rays should provide insight into the details of the explosion, as well as into the structure of the surrounding environment. One bright optical flash was detected during a burst1, but other efforts2,3 have produced negative results. Here we report the discovery of the optical counterpart of GRB021004 only 193 seconds after the event. The initial decline is unexpectedly slow and requires varying energy content in the γ-ray burst blastwave over the course of the first hour. Further analysis of the X-ray and optical afterglow suggests additional energy variations over the first few days.
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Acknowledgements We thank D. L. Kaplan and C. Trujillo for assistance with supporting observations at Palomar and R. D. Blandford for discussions. The results presented in this paper are based on data obtained using the NASA-JPL CCD camera at the Oschin Telescope, Palomar Observatory, California Institute of Technology (Caltech). The research was performed in part by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration (NASA). g-ray burst research at Caltech is supported in part by funds from NSF and NASA.