Optical observations of the bright long duration peculiar GRB 021004 afterglow

Abstract

The CCD magnitudes in Johnson B,V and Cousins R and I photometric passbands are determined for the bright long duration GRB 021004 afterglow from 2002 October 4 to 16 starting ~ 3 hours after the γ-ray burst. Light curves of the afterglow emission in B,V,R and I passbands are obtained by combining these measurements with other published data. The earliest optical emission appears to originate in a revese shock. Flux decay of the afterglow shows a very uncommon variation relative to other well-observed GRBs. Rapid light variations, especially during early times (Δt < 2 days) is superposed on an underlying broken power law decay typical of a jetted afterglow. The flux decay constants at early and late times derived from least square fits to the light curve are 0.99 ± 0.05 and 2.0 ± 0.2 respectively, with a jet break at around 7 day. Comparison with a standard fireball model indicates a total extinction of E(B-V)=0.20 mag in the direction of the burst. Our low-resolution spectra corrected for this extinction provide a spectral slope β = 0.6 ± 0.02. This value and the flux decay constants agree well with the electron energy index p ~ 2.27 used in the model. The derived jet opening angle of about 7 ° implies a total emitted gamma-ray energy E_γ = 3.5×10⁵⁰erg at a cosmological distance of about 20 Gpc. Multiwavelength observations indicate association of this GRB with a star forming region, supporting the case for collapsar origin of long duration GRBs.