The observed SED of the X-ray/optical/NIR afterglow of GRB 080928 at t = 20 ks after correction for Galactic extinction by dust (Table 3) and Galactic absorption by gas. Left: the joint X-ray/optical SED is almost a pure power law (dashed line) affected by only a small amount of host extinction by dust (Table 4) and 3.5 × 10²¹ cm^-2 of host absorption by the gas. The UV bands are affected by Lyman drop-out. The dotted line represents the SED that follows from the numerical energy injection model for this particular time, which slightly overpredicts the flux in the X-ray band (see Sect. 3.2.3). Residuals refer to the the plot with β_OX = 1.02 (broken line). Right: zoom-in to the optical/NIR SED, and the different dust models used to fit the data, where it is possible to discern the dip resulting from the 2175 Å feature.