We report the earliest detection of an extremely bright optical afterglow of the gamma-ray burst ( GRB ) 030329 using a 30cm-telescope at Tokyo Institute of Technology ( Tokyo , JAPAN ) .
Our observation started 67 minutes after the burst , and continued for succeeding two 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 ( \propto t ^ { - \alpha } ) function with indices \alpha _ { 1 } = 0.88 \pm 0.01 ( 0.047 < t < t _ { b 1 } days ) , \alpha _ { 2 } = 1.18 \pm 0.01 ( t _ { b 1 } < t < t _ { b 2 } days ) , and \alpha _ { 3 } = 1.81 \pm 0.04 ( t _ { b 2 } < t < 1.2 days ) , where t _ { b 1 } \sim 0.26 days and t _ { b 2 } \sim 0.54 days , respectively .
The change of the power-law index at the first break at t \sim 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 ISM environment .