Extended ribbon-like feature associated with the burst. The left panel covers the full extent of the burst and is at the same time-step as the left panel in Fig. 4. Three sample ribbon segments are marked by black lines, numbered 1–3. The intensity profiles (solid) and their Gaussian fits (dashed) along the three ribbon segments are shown in the right panels. The FWHMs of the three ribbon segments are quoted in the respective panels. See Sect. 5 for details.