Panel a: dynamic spectrum of B21 convolved with a kernel designed to follow the burst’s drift pattern. The drift of the burst is overplotted as a dotted red line. Panel b: raw dynamic spectrum of B21. The horizontal white bands are frequency channels masked due to the presence of RFI. The inset in the bottom left shows an interferometric image reconstructed along the dotted line in panel a, indicating that the emission originates from a point source. Panel c: frequency-integrated burst brightness as a function of the modelled drift delay. The dotted cross indicates the location of the highest S/N at a drift of 13 minutes, where the burst aligns vertically and thus appears offset from the horizontal zero drift line, which corresponds to the original time series.