GRB 020813: Polarization in the case of a smooth optical decay J. Gorosabel A. J. Castro-Tirado J. M. C. Cerón A. S. Fruchter J. Rhoads E. Rol L. Kaper Van Den Heuvel E R. A. M. J. Wijers S. Covino G. Ghisellini P. Saracco G. Tagliaferri F. M. Zerbi D. Lazzati J. Hjorth D. Malesani Della Valle M Di Serego Alighieri S F. Fiore G. L. Israel L. Stella J. P. U. Fynbo P. Goldoni Le Floc'h E F. Mirabel J. Greiner N. Kawai S. Klose C. Kouveliotou N. Masetti E. Palazzi P. Møller S. Ortolani E. Pian G. Ricker N. Tanvir M. Vietri P. M. Vreeswijk 2381/25898 https://figshare.le.ac.uk/articles/journal_contribution/GRB_020813_Polarization_in_the_case_of_a_smooth_optical_decay/10118477 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. 2012-10-24 09:21:05 IR content