Temporal evolution of the optical (composite light curve with all data shifted to the R_c band) and X-ray afterglow (0.3 to 10 keV) of GRB 080928 (optical: red circles, X-ray: blue error bars). The upper limits are not shown here to avoid confusion. The zoom-in shows the early phase (also highlighted in gray in the big figure) where it is compared with the BAT-GBM prompt emission. The dashed vertical lines indicate the peak times of the two X-ray flares. The curve represents the best fit of the late-time data.