2003AJ....125.2291H

We present UBVR_CI_Cphotometry of the optical afterglow of the gamma-ray burst GRB 021004 taken at the Nordic Optical Telescope between approximately 8 hours and 30 days after the burst. These data are combined with an analysis of the 87 ks Chandra X-ray observations of GRB 021004 taken at a mean epoch of 33 hr after the burst to investigate the nature of this GRB. We find an intrinsic spectral slope at optical wavelengths of β_UH_=0.39±0.12 and an X-ray slope of β_X=0.94±0.03. There is no evidence for color evolution between 8.5 hr and 5.5 days after the burst. The optical decay becomes steeper approximately 5 days after the burst. This appears to be a gradual break due to the onset of sideways expansion in a collimated outflow. Our data suggest that the extragalactic extinction along the line of sight to the burst is between A_V~0.3 and 0.5 and has an extinction law similar to that of the Small Magellanic Cloud. The optical and X-ray data are consistent with a relativistic fireball with the shocked electrons being in the slow cooling regime and having an electron index of p=1.9±0.1. The burst occurred in an ambient medium that is homogeneous on scales larger than approximately 10¹⁸ cm but inhomogeneous on smaller scales. The mean particle density is similar to what is seen for other bursts (0.1 cm^–3≲n≲100 cm^–3). Our results support the idea that the brightening seen at approximately 0.1 days was due to interaction with a clumpy ambient medium within 10¹⁷-10¹⁸ cm of the progenitor. The agreement between the predicted optical decay and that observed approximately 10 minutes after the burst suggests that the physical mechanism controlling the observed flux at t~10 minutes is the same as the one operating at t>0.5 days.