EDP Sciences
Free access
Volume 422, Number 1, July IV 2004
Page(s) 113 - 119
Section Extragalactic astronomy
DOI http://dx.doi.org/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. Gorosabel1, 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)

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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