Each image in this panel is a different flash from Spot C at t = 11.20, 14.42, 16.80, 19.60, 21.98, 30.52 min from the top left to the bottom right. Log-scale was used. The bottom arrow indicates the disk-centre direction. The centre arrow points to an UCF selected from the top left panel. The times t = 16.80, t = 19.60, and t = 16.80 min were composed using the technique described in Sect. 2.1. The frames without composition are shown in the online movie. Major tickmarks every 1′′.