Optical follow-up of the GRB 010222 afterglow by Subaru Telescope

The optical afterglow of a gamma ray burst, GRB 010222, which was discovere d by the BeppoSAX Team as the brightest one ever detected, was observed by Suprime-Cam of the Subaru Telescope at Mauna Kea. The obtained R-band magni tude was 18.59 +/- 0.04 at February 22.51 UT, 18.99 +/- 0.04 at February 22 .65 UT, and 21.76 +/- 0.03 at February 25.64 UT. The light curve made from all data reported so far shows the decay of the brightness with a broken po wer law, of which the indices are -0.92 +/- 0.01 before and -1.27 +/- 0.01 after the break. The break point of the light curve is 0.73 +/- 0.02 d afte r the burst, which is the earliest example ever observed. A brief summary o f our follow-up observations, together with the importance of the optical f ollow-up capable by the Subaru telescope. is described in this letter.