Letter to the Editor

Variable polarization in the optical afterglow of GRB 021004^*

¹ Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, 8000 Århus, Denmark
³ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁴ Instituto de Astrofísica de Andalucía, c/ Camino Bajo de Huétor 24, 18.008 Granada, Spain
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Nordic Optical Telescope, Apartado de Correos, 474, 38700 Santa Cruz de La Palma (Tenerife), Spain
⁷ Real Instituto y Observatorio de la Armada, Sección de Astronomía, 11.110 San Fernando-Naval (Cádiz), Spain
⁸ Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Bologna, CNR, Via Gobetti 101, 40129 Bologna, Italy
⁹ INAF, Astronomical Observatory of Trieste, via GB Tiepolo 11, 34131 Trieste, Italy
¹⁰ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
¹¹ European Southern Obseratory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
¹² NASA MSFC, SD-50 Huntsville, AL 35812, USA
¹³ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany

Corresponding author: E. Rol, evert@science.uva.nl

Received: 18 March 2003
Accepted: 15 May 2003

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  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.