Astronomy & Astrophysics

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Issue		A&A Volume 422, Number 1, July IV 2004
Page(s)		113 - 119
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20034409

A&A 422, 113-119 (2004)
DOI: 10.1051/0004-6361:20034409

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

J. Gorosabel^{1, 2}, E. Rol³, S. Covino⁴, A. J. Castro-Tirado¹, J. M. Castro Cerón², D. Lazzati⁵, J. Hjorth⁶, D. Malesani⁷, M. Della Valle⁸, S. di Serego Alighieri⁸, F. Fiore⁹, A. S. Fruchter², J. P. U. Fynbo¹⁰, G. Ghisellini⁴, P. Goldoni¹¹, J. Greiner¹², G. L. Israel⁹, L. Kaper³, N. Kawai¹³, S. Klose¹⁴, C. Kouveliotou¹⁵, E. Le Floc'h¹¹, N. Masetti¹⁶, F. Mirabel¹¹, P. Møller¹⁷, S. Ortolani¹⁸, E. Palazzi¹⁶, E. Pian¹⁹, J. Rhoads², G. Ricker²⁰, P. Saracco⁴, L. Stella⁹, G. Tagliaferri⁴, N. Tanvir²¹, E. van den Heuvel³, M. Vietri²², P. M. Vreeswijk²³, R. A. M. J. Wijers³ and F. M. Zerbi⁴

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

(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 ( $\theta$ ) resulting from our campaign are $\langle P\rangle=1.18 \pm 0.10 \%$ and $\langle \theta\rangle = 148.7^{\circ} \pm 2.3^{\circ}$ , 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 $\theta$ evolution cannot be entirely ruled out by our data. The VLT monitoring campaign did not reveal either a significant $\theta$ rotation respect to the Keck spectropolarimetric observations performed ~0.25 days after the GRB (Barth et al. 2003). However, $\langle P\rangle$ 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, $0.33 < t_{{\rm break}, V} < 0.88$ 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 $\theta$ might be related to the smoothness of the light curve.

Key words: gamma rays: bursts -- techniques: photometric -- techniques: polarimetric

Offprint request: J. Gorosabel, jgu@iaa.es

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