Discovery of Early Optical Emission from GRB 021211

Abstract

We report our discovery and early time optical, near-infrared, and radio\nwavelength follow-up observations of the afterglow of the gamma-ray burst GRB\n021211. Our optical observations, beginning 21 min after the burst trigger,\ndemonstrate that the early afterglow of this burst is roughly three magnitudes\nfainter than the afterglow of GRB 990123 at similar epochs, and fainter than\nalmost all known afterglows at an epoch of 1d after the GRB. Our near-infrared\nand optical observations indicate that this is not due to extinction. Combining\nour observations with data reported by other groups, we identify the signature\nof a reverse shock. This reverse shock is not detected to a 3-sigma limit of\n110 uJy in an 8.46-GHz VLA observation at t=0.10d, implying either that the\nLorentz factor of the burst gamma <~ 200, or that synchrotron self-absorption\neffects dominate the radio emission at this time. Our early optical\nobservations, near the peak of the optical afterglow (forward shock), allow us\nto characterize the afterglow in detail. Comparing our model to flux upper\nlimits from the VLA at later times, t >~ 1 week, we find that the late-time\nradio flux is suppressed by a factor of two relative to the >~ 80 uJy peak flux\nat optical wavelengths. This suppression is not likely to be due to synchrotron\nself-absorption or an early jet break, and we suggest instead that the burst\nmay have suffered substantial radiative corrections.\n