On the jet structure and magnetic field configuration of GRB 020813^*

¹ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
² INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
³ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos, 3004, 18080 Granada, Spain
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2463, USA
⁵ University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁶ INAF, Osservatorio Astrofisico di Arcetri, Large E. Fermi 5, 50125 Firenze, Italy
⁷ Department of Physics and Astronomy, University of Århus, Ny Munkegade, 8000 Århus C, Denmark
⁸ CEA/DSM/DAPNIA, L'Orme des Merisiers, Bât. 709, 91191 Gif-sur-Yvette, France
⁹ Niels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
¹⁰ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00044 Monterporzio, Italy
¹¹ Department of Physics, Faculty of Science, Tokyo Institute of Technology 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
¹² International School for Advanced Studies (SISSA/ISAS), via Beirut 2-4, 34014 Trieste, Italy
¹³ Istituto di Astrofisica Spaziale e Fisica Cosmica - Sezione di Bologna, CNR, Via Gobetti 101, 40129 Bologna, Italy
¹⁴ INAF, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
¹⁵ Department of Astronomy, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁶ Research Center for the Early Universe, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁷ European Southern Observatory, Karl–Schwarzschild–Straße 2, 85748 Garching, Germany
¹⁸ Università di Padova, Dip. di Astronomia, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
¹⁹ Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
²⁰ Lawrence Livermore National laboratory, L-038, PO Box 808, Livermore, CA, 94551, USA
²¹ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK

Corresponding author: D. Lazzati, lazzati@ast.cam.ac.uk

Received: 28 December 2003
Accepted: 5 May 2004

Abstract

The polarization curve of GRB 020813 is discussed and compared to different models for the structure, evolution and magnetisation properties of the jet and the interstellar medium onto which the fireball impacts. GRB 020813 is best suited for this kind of analysis for the smoothness of its afterglow light curve, ensuring the applicability of current models. The polarization dataset allows us to rule out the standard GRB jet, in which the energy and Lorentz factor have a well defined value inside the jet opening angle and the magnetic field is generated at the shock front. We explore alternative models finding that a structured jet or a jet with a toroidal component of the magnetic field can fit equally well the polarization curve. Stronger conclusions cannot be drawn due to the incomplete sampling of the polarization curve. A more dense sampling, especially at early times, is required to pin down the structure of the jet and the geometry of its magnetic field.