The Optical Afterglow of the Gamma-Ray Burst GRB 011211

We present early-time optical photometry and spectroscopy of the optical afterglow of the gamma-ray burst GRB 011211. The spectrum of the optical afterglow contains several narrow metal lines that are consistent with the burst's occurring at a redshift of 2.140 ± 0.001. The optical afterglow decays as a power law with a slope of α = 0.83 ± 0.04 for the first ≈2 days after the burst, at which time there is evidence of a break. The slope after the break is ≥1.4. There is evidence of rapid variations in the R-band light approximately 0.5 days after the burst. These variations suggest that there are density fluctuations near the gamma-ray burst on spatial scales of approximately 40–125 AU. The magnitude of the break in the light curve, the spectral slope, and the rate of decay in the optical suggest that the burst expanded into an ambient medium that is homogeneous on large scales. We estimate that the local particle density is between approximately 0.1 and 10 cm-3 and that the total gamma-ray energy in the burst was (1.2–1.9) × 1050 ergs. This energy is smaller than, but consistent with, the "standard" value of (5 ± 2) × 1050 ergs. Comparing the observed color of the optical afterglow with predictions of the standard beaming model suggests that the rest-frame V-band extinction in the host galaxy is ≲0.03 mag.