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Issue A&A
Volume 405, Number 3, July III 2003
Page(s) L23 - L27
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20030731



A&A 405, L23-L27 (2003)
DOI: 10.1051/0004-6361:20030731

Letter

Variable polarization in the optical afterglow of GRB 021004

E. Rol1, R. A. M. J. Wijers1, J. P. U. Fynbo2, 3, J. Hjorth3, J. Gorosabel4, 5, M. P. Egholm6, 2, J. M. Castro Cerón7, A. J. Castro-Tirado4, L. Kaper1, N. Masetti8, E. Palazzi8, E. Pian8, 9, N. Tanvir10, P. Vreeswijk11, C. Kouveliotou12, P. Møller13, H. Pedersen3, A. S. Fruchter5, J. Rhoads5, I. Burud5, I. Salamanca1 and E. P. J. Van den Heuvel1

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

(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 $t_{\rm b} \approx 1$ 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.


Key words: gamma rays: bursts -- polarization -- radiation mechanisms: non-thermal

Offprint request: E. Rol, evert@science.uva.nl

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