The afterglow of GRB 021211: Another case of reverse shock emission

GRB 021211 was first detected by HETE II and its early afterglow has been observed. There is a break in its afterglow light curve at about 12 min after the bursts, before the break the optical flux decays with a power-law index of about -1.6, while at late time the power-law slope is about -1 (Chornock et al. 2002). Here we will show that the afterglow light curve of GRB 021211 can be explained within the framework of the standard fireball model. We show that the afterglow emission before the break time is the contribution of the emission from both the reverse shock and the forward shock, while the afterglow emission after the break time is mainly due to the forward shock emission. From the fitting we can give constraints on the parameters: the initial Lorentz factor 250less than or equal togamma(0)less than or equal to900, and the surrounding medium density ngreater than or equal to1.6x10(-3) atoms cm(-3). We propose that since the values of epsilon(B) and epsilon(e) are somewhat smaller for GRB 021211, so the peak energy of the reverse shock emission is well below the optical band, and thus it is substantially fainter than 990123 at similar epochs. Also we suggest that such a break might be a common feature in early optical afterglows.