Multi-band optical follow-up observations of GRB 020813 at KISO and Bisei observatories

Observations were made of the optical afterglow of GRB020813 (Fox, Blake & Price, 2002) with the KISO observatory 1.05 m Schmidt telescope and the Bisei astronomical observatory 1.01 m telescope. Four-band ($B, V, R$, and $I$) photometric data points were obtained from 2002, August 13 10:52 to 16:46 UT, or 0.346$-$0.516 days after the burst. In order to investigate the early-time ($<$1 day) evolution of the afterglow, four-band light curves were produced by analyzing the data taken at these two astronomical observatories, as well as publicly released data taken by the Magellan Baade telescope (Gladders and Hall, 2002c). The light curves can be approximated by a broken power law, of which the indices are approximately 0.46 and 1.33 before and after a break at $\sim$0.2 days, respectively. The optical spectral index stayed approximately constant at $\sim$0.9 over 0.17 $\sim$ 4.07 days after the burst. Since the temporal decay index after the break and the spectral index measured at that time are both consistent with those predicted by a spherical expansion model, the early break is unlikely to be a jet break, but likely to represent the end of an early bump in the light curve as was observed in the optical afterglow of GRB021004.