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

doi:10.1051/0004-6361:20030731

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Variable polarization in the optical afterglow of GRB 021004

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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 ~ 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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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. 2003-07-01. Variable polarization in the optical afterglow of GRB 021004. https://doi.org/10.1051/0004-6361%3A20030731

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