Variable polarization in the optical afterglow of GRB 021004
E. Rol, R. A. M. J. Wijers, J. P. U. Fynbo, J. Hjorth, J. Gorosabel, M. P. Egholm, J. M. Castro Cerón, A. J. Castro-Tirado, L. Kaper, N. Masetti, E. Palazzi, E. Pian, N. Tanvir, P. Vreeswijk, C. Kouveliotou, P. Moller, H. Pedersen, A. S. Fruchter, J. Rhoads, I. Burud, I. Salamanca, E. P. J. Van den Heuvel
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 tb ~ 1 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.
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