2001ApJ...563..592S

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 2°.1, smaller than any listed in the recent compilation of Frail et al. The total beamed energy corrected for the jet geometry is 4x10⁵⁰ ergs, very close to the ``standard'' value of 5x10⁵⁰ ergs found by Frail et al. for a number of other bursts with light-curve breaks.