Maps of the Si IV 140.3 nm velocity for three time steps: from left to right, 08:09:05, 08:10:08 and 08:12:13 UT. The time steps have been chosen around (before, during, and after) the maximum intensity peak of the burst. The time of maximum intensity can be found in Fig. 2 slightly after 08:10 UT. A single Gaussian has been fitted to the Si IV 140.3 nm profiles (but see text for additional explanations). The crosses in the central panel mark the positions of the pixels whose profiles are shown in Fig. 5. The black contours delimit the boundaries of the UV burst for the different time steps by imposing a threshold to the Si IV intensity.