Discovery of Early Optical Emission from GRB021211

We report our discovery and early time optical, near-infrared, and radio wavelength follow-up observations of the afterglow of the gamma-ray burst GRB021211. Our optical observations, beginning 21min after the burst trigger, demonstrate that the early afterglow of this burst is roughly three magnitudes fainter than the afterglow of GRB990123 at similar epochs, and fainter than almost all known afterglows at an epoch of 1 d 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 110μJy in an 8.46-GHz VLA observation at t = 0.10 d, 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 & 1week, we find that the late-time radio flux is suppressed by a factor of two Caltech Optical Observatories 105-24, California Institute of Technology, Pasadena, CA 91125; [email protected], [email protected], [email protected], [email protected], [email protected] Space Radiation Laboratory 220-47, California Institute of Technology, Pasadena, CA 91125; [email protected], [email protected], [email protected] Research School of Astronomy and Astrophysics, Mount Stromlo and Siding Spring Observatories, via Cotter Road, Weston Creek 2611, Australia; [email protected], [email protected] Theoretical Astrophysics 130-33, California Institute of Technology, Pasadena, CA 91125; [email protected] National Radio Astronomy Observatory, Socorro, NM 87801; [email protected] National Astronomical Observatory of Japan, Mitaka, Tokyo, 181-8588, Japan; [email protected] National Research Council of Canada, Joint Astronomy Centre, 660 N. A’ohoku Place, Hilo, HI 96720; [email protected]