Earliest detection of the optical afterglow of GRB 030329 and its variability

We report the earliest detection of an extremely bright optical afterglow of the gamma-ray burst GRB 030329 using a 30 cm telescope at the Tokyo Institute of Technology (Tokyo, Japan). Our observation started 67 minutes after the burst and continued for two succeeding nights until the afterglow faded below the sensitivity limit of the telescope (approximately 18 mag). Combining our data with, those reported in GCN Circulars, we find that the early afterglow light curve of the first half-day is described by a broken power-law (∝t^-α) function with indices α ₁ = 0.88 ± 0.01 (0.047 days < t < t_{b, 1}), α₂ = 1.18 ± 0.01 (t_{b, 1} < t < t_{b, 2}), and α₃ = 1.81 ± 0.04 (t_{b, 2} < t < 1.2 days), where t_b,1 ∼ 0.26 days and t_{b, 2} ∼ 0.54 days, respectively. The change of the power-law index at the first break at t ∼ 0.26 days is consistent with that expected from a "cooling break" when the cooling frequency crossed the optical band. If the interpretation is correct, the decay index before the cooling break implies a uniform interstellar medium environment.