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

We report an earliest detection of an optical afterglow of GRB 030329 at the Tokyo Institute of Technology (Tokyo, Japan). Our observation started 67 minutes after the burst and continued for two succeeding nights. Combining our data with those reported in GCN, we find that the early afterglow light curve of the first half day is described by a broken powerlaw (α t ^-α) function with indices α₁ = 0.88 ± 0.01 (0.047 days < t < t_b1), α₂ = 1.18 ± 0.01 (t_b1 < t < t_b2), and α ₃ = 1.81 ± 0.04 (t_b2 < t < 1.2 days), where t_b1 ∼ 0.26 days and t_b2 ∼ 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".