The early light curve of the optical afterglow of GRB 021211

The Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory observed gamma-ray burst GRB 021211 starting 105 s after the burst, among the earliest observations of an optical afterglow to date. Our well-sampled light curve, consisting of 18 points in the first 10 minutes and a total of 28 useful observations in the first 2.5 hr after the GRB, directly shows for the first time a break at t approximate to 10 minutes. The light curve can be represented as a sum of two components with power-law decay indices of -1.82 +/- 0.02 and -0.82 +/- 0.11. We hypothesize that the data before the break were dominated by emission from the reverse shock, as was previously suggested for GRB 990123. Our data suggest that either GRB 021211 underwent a dramatic color change at early times or there are small-scale variations superposed on the power-law decay of the reverse-shock emission. The faintness of GRB 021211, coupled with the fast decline typical of optical afterglows, suggests that some of the "dark bursts" were not detected because optical observations commenced too late after the GRB.