A New Extragalactic Distance Determination Method Using the Flux-weighted Gravity of Late B and Early A Supergiants

We show that the rebrightening in the GRB 021004 optical afterglow light curve can be explained within the framework of the standard fireball model. The optical light curve of the forward shock emission is expected to rise initially and to decay after the typical synchrotron frequency crosses the optical band. We show that such a rising phase emission was caught for GRB 021004, together with a reverse shock emission. With the standard values of parameters obtained in other afterglow observations, we can construct example cases in which theoretical estimates reasonably fit the broadband observations. Therefore, the early rebrightening might be a common feature in optical afterglows.