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

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 (è) resulting from our campaign are < P>=1.18 ± 0.10 % and < è> = 148.7° ± 2.3°, 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 è evolution cannot be entirely ruled out by our data. The VLT monitoring campaign did not reveal either a significant è rotation respect to the Keck spectropolarimetric observations performed ~0.25 days after the GRB (Barth et al. cite{Bart03a}). However, < P> 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 < tbreak, 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 è might be related to the smoothness of the light curve. Based on observations collected at the European Southern Observatory, Cerro Paranal (Chile), ESO programmes 69.D-0461(A) and 69.D-0701(A).