2004A&A...422..113G

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 <P≥1.18±0.10% and <θ≥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 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., 2003ApJ...584L..47B). 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<t_break,V<0.88 days after the GRB. The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003mag, 0.010mag and 0.016mag for our three observing nights, respectively. We speculate that the stability of θ might be related to the smoothness of the light curve.