Observations of the filament eruption in the He II line (304 Å) and light curve of the associated solar flare. Panel (a): Main phase of the filament eruption, where the straight line labeled S1 outlines the artificial slit positioned along the filament apex (same as for Hα in Fig. 2). The curved cyan line depicts the filament footpoints separation, while the ‘×’ symbols outline the erupting material. Panel (b): Time-distance diagram extracted from the slit S1. The diagram shows a complex eruption, wherein at around 05:38 UT the erupting material splits into two main branches. The yellow dotted profile is the fitting curve applied to the selected positions along the erupting feature in the plane of sky, while the red dashed plot is the fitting curve applied to Hα data shown in Fig. 3. Panel (c): Running difference time-distance diagram showing the apparent unwinding motion of the erupting filament (see the text). As in Fig. 3, the vertical axis of panels (b) and (c) denote the apparent vertical distance in the plane of sky from the initial position of the filament apex. For context, the soft X-ray light curve of the B-class flare is also plotted in panel (d). The temporal evolution of the profiles in panel (b) suggests that the eruption of the filament started before the soft X-ray flux enhancement.