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D. W. Fox (Caltech Optical Observatories), Caltech/NRAO GRB Colloboration Collaboration
We present our discovery of early-time optical emission from two recent gamma-ray bursts, GRB 021004 and GRB 021211. The prompt observations of these bursts are the first such observations since GRB 990123, and in both cases, allow the identification of the reverse-shock component of the afterglow emission. In GRB 021004, the initial decline is unexpectedly slow, which has intriguing implications for the nature of the central engine in this burst. For GRB 021211, the reverse shock is not detected in the radio, to an upper limit of 110 \muJy at 8.46 GHz, implying either that the Lorentz factor of the burst \gamma\lesssim 200, or that synchrotron self-absorption effects dominate the radio emission at this time. With a sample of three early-time GRB afterglows we are beginning to sample the range of behaviors on these timescales, corresponding to ejecta Lorentz factors \gamma\gtrsim 100 and length scales r\lesssim 1018 cm.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.