Optical (red points) and X-ray (blue bars, in 0.3−10 keV band) light curve of GRB 080928 afterglow. The green curve is the best fit of the late-time data, while the grey-shaded region on the left shows the early phase. The latter is also represented in the inset and compared with the prompt-phase emission detected by Fermi-GBM and Swift/BAT. The arrows above the best fit curves on the right represent the epochs of polarimetric observations obtained with ESO-VLT FORS1, both in the imaging polarimetry and spectro-polarimetry mode, and the assumed jet break time t_j is indicated by the purple dashed line (Rossi et al. 2011).