Left panel: PBD image with arc sector over-plotted. Right panel: intensity profile across arc sector. The intensity profile at a given distance corresponds to the mean intensity across the arc sector at that distance. This is then fitted using a Gaussian model and repeated for the next image. The vertical lines in the right-hand plot and arc lines in the left-hand plot correspond to the 2σ limits of the Gaussian fit. The associated movie (15879_mov.mpeg) shows the procedure used to build these diagrams.