GRB 020813: Polarization in the case of a smooth optical decay^*

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, 18080 Granada, Spain
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2463, USA
³ University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁴ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
⁶ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁷ International School for Advanced Studies (SISSA/ISAS), via Beirut 2-4, 34016 Trieste, Italy
⁸ INAF, Osservatorio Astrofisico di Arcetri, Large E. Fermi 5, 50125 Firenze, Italy
⁹ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00044 Monterporzio, Italy
¹⁰ Department of Physics and Astronomy, University of Århus, Ny Munkegade, 8000 Århus C, Denmark
¹¹ CEA/DSM/DAPNIA, L'Orme des Merisiers, Bat. 709, 91191 Gif-sur-Yvette, France
¹² Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany
¹³ Department of Physics, Faculty of Science, Tokyo Institute of Technology 2-12-1 Oookayama, Meguro-ku, Tokyo 152-8551, Japan
¹⁴ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
¹⁵ NASA MSFC, SD-50, Huntsville, AL 35812, USA
¹⁶ Istituto di Astrofisica Spaziale e Fisica Cosmica, CNR, via Gobetti 101, 40129 Bologna, Italy
¹⁷ European Southern Observatory, Karl–Schwarzschild–Straße 2, 85748 Garching, Germany
¹⁸ Università di Padova, Dept. di Astronomia, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
¹⁹ INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
²⁰ Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
²¹ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
²² Scuola Normale Superiore, Piazza dei Cavalieri, 56100 Pisa, Italy
²³ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Corresponding author: J. Gorosabel, jgu@iaa.es

Received: 28 September 2003
Accepted: 23 March 2004

Abstract

We present the results of a VLT polarimetric monitoring campaign of the GRB 020813 optical afterglow carried out in three nights, from 0.88 to 4.20 days after the gamma-ray event. The mean values of the degree of linear polarization (P) and its position angle (θ) resulting from our campaign are and , after correcting for Galactic interstellar polarization. Our VLT data set is most naturally described by a constant degree of linear polarization and position angle, nonetheless a slow θ evolution cannot be entirely ruled out by our data. The VLT monitoring campaign did not reveal either a significant θ rotation respect to the Keck spectropolarimetric observations performed ~0.25 days after the GRB (Barth et al. [CITE]). However, is a factor of two lower than the polarization degree measured from Keck. Additionally, the VLT polarization data allowed us to construct an accurate V-band light curve. The V-band photometric data revealed a smooth light curve with a break located between the last Keck and our first VLT polarimetric measurement, days after the GRB. The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003 mag, 0.010 mag and 0.016 mag for our three observing nights, respectively. We speculate that the stability of θ might be related to the smoothness of the light curve.