2003A&A...408L..21P

We determine Johnson B,V and Cousins R,I photometric CCD magnitudes for the afterglow of GRB 021211 during the first night after the GRB trigger. The afterglow was very faint and would have been probably missed if no prompt observations had been conducted. A fraction of the so-called ``dark'' GRBs may thus be just ``optically dim'' and require very deep imaging to be detected. The early-time optical light curve reported by other observers shows a prompt emission with properties similar to that of GRB 990123. Following this, the afterglow emission from ∼11-min to ∼35 days after the burst is characterized by an overall power-law decay with a slope 1.1±0.01 in the R passband. We derive the value of spectral index in the optical to near-IR region to be 0.6±0.2 during 0.13 to 0.8 day after the burst. The flux decay constant and the spectral slope indicate that during the first day after the burst, the optical band lies between the cooling frequency and the synchrotron maximum frequency of the afterglow.