Author/Authors :
KAPER، T. J. نويسنده , , Masetti، Enrico نويسنده , , Tanvir، Nial R. نويسنده , , A. S. Fruchter، نويسنده , , C. Kouveliotou، نويسنده , , J. Gorosabel، نويسنده , , J. Hjorth، نويسنده , , I. Burud، نويسنده , , Rhoads، J. E. نويسنده , , J. M. Castro Ceron، نويسنده , , R. A. M. J. Wijers، نويسنده , , E. Rol، نويسنده , , J. P. U. Fynbo، نويسنده , , M. P. Egholm، نويسنده , , A. J. Castro-Tirado، نويسنده , , E. Palazzi، نويسنده , , E. Pian، نويسنده , , P. Vreeswijk، نويسنده , , P. Moller، نويسنده , , H. Pedersen، نويسنده , , I. Salamanca، نويسنده , , E. P. J. Van den Heuvel، نويسنده ,
Abstract :
We present polarimetric observations of the afterglow of gamma-ray burst (GRB) 021004, obtained with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT) between 8 and 17 hours after the burst. Comparison among the observations shows a 45 degree change in the position angle from 9 hours after the burst to 16 hours after the burst, and comparison with published data from later epochs even shows a 90 degree change between 9 and 89 hours after the burst. The degree of linear polarization shows a marginal change, but is also consistent with being constant in time. In the context of currently available models for changes in the polarization of GRBs, a homogeneous jet with an early break time of t(b) ~~ day provides a good explanation of our data. The break time is a factor 2 to 6 earlier than has been found from the analysis of the optical light curve. The change in the position angle of the polarization rules out a structured jet model for the GRB.
