Rapid UBVRI Follow-up of the Highly Collimated Optical Afterglow of GRB 010222

We present the earliest optical observations of the optical counterpart to the gamma-ray burst (GRB) 010222, obtained with the Fred L. Whipple Observatory 1.2 m telescope in UBVRI passbands, starting 3.64 hr after the burst (0.4 hr after public notification of the burst localization). We also present late R-band observations of the afterglow obtained with the 1.8 m Vatican Advanced Technology Telescope ~25 days after the burst. The temporal analysis of our data joined with published data indicates a steepening decay, independent of wavelength, asymptotically approaching Fν ∝ t-0.80±0.05 at early times (t ≪ 1 day) and Fν ∝ t-1.30±0.05 at late times, with a sharp break at tb = 0.72 ± 0.10 days. This is the second earliest observed break of any afterglow (after GRB 980519), which clearly indicates the importance of rapid multiband follow-up for GRB afterglow research. The optical spectral energy distribution, corrected for small Galactic reddening, can be fitted fairly well by a single power law with Fν ∝ ν-1.07±0.09. However, when we fit using our BVRI data only, we obtain a shallower slope of -0.88 ± 0.10, in excellent agreement with the slope derived from our low-resolution spectrum (-0.89 ± 0.03). The spectral slope and light-curve decay slopes we derive are not consistent with a jet model despite the presence of a temporal break. Significant host dust extinction with a starburst reddening law would flatten the spectral index to match jet predictions and still be consistent with the observed spectral energy distribution. We derive an opening angle of 2.°1, smaller than any listed in the recent compilation of Frail et al. The total beamed energy corrected for the jet geometry is 4 × 1050 ergs, very close to the "standard" value of 5 × 1050 ergs found by Frail et al. for a number of other bursts with light-curve breaks.