Discovery of Early Optical Emission from GRB 021211

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Abstract
We report our discovery and early-time optical, near-infrared, and radio wavelength follow-up observations of the afterglow of the gamma-ray burst GRB 021211. Our optical observations, beginning 21 minutes after the burst trigger, demonstrate that the early afterglow of this burst is roughly 3 mag fainter than the afterglow of GRB 990123 at similar epochs, and fainter than almost all known afterglows at an epoch of 1 day after the GRB. Our near-infrared and optical observations indicate that this is not due to extinction. Combining our observations with data reported by other groups, we identify the signature of a reverse shock. This reverse shock is not detected to a 3 σ limit of 110 μJy in an 8.46 GHz Very Large Array (VLA) observation at t = 0.10 days, implying either that the Lorentz factor of the burst γ 200 or that synchrotron self-absorption effects dominate the radio emission at this time. Our early optical observations, near the peak of the optical afterglow (forward shock), allow us to characterize the afterglow in detail. Comparing our model to flux upper limits from the VLA at later times, t 1 week, we find that the late-time radio flux is suppressed by a factor of 2 relative to the 80 μJy peak flux at optical wavelengths. This suppression is not likely to be due to synchrotron self-absorption or an early jet break, and we suggest instead that the burst may have suffered substantial radiative corrections.