X-shooter spectrum of GRB 161023A. Top left panel: acquisition image indicating the position of the slit. Top right panel: flux-calibrated spectrum. each colour represents one of the X-shooter arms. Horizontal gray bars mask the regions affected by strong telluric absorption bands. Below panels: normalised spectrum together with the 2D spectrum. Each of the different absorption systems are marked in a different colour, as indicated at the bottom. At the top of the 1D spectra we mark the telluric features with black lines, their widths being proportional to their intensity, as done in de Ugarte Postigo et al. (2010).