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 F_ν ∝ t^-0.80±0.05 at early times (t ≪ 1 day) and F_ν ∝ t^-1.30±0.05 at late times, with a sharp break at t_b = 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 F_ν ∝ 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 21, 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.