Panel a: dynamic spectrum of B20 normalised by the blank-sky variances. The inset in the bottom left displays an interferometric image indicating that the emission originates from a point source. The horizontal white bands are frequency channels masked due to the presence of RFI. Panel b: dynamic spectrum of B20, with the highest likelihood drift rate (i.e. the overall drift rate) shown as a dashed red line. Panel c: frequency-integrated burst brightness as a function of the modelled drift delay. The intersection of the dotted red lines indicates the location of the highest S/N at ~1 minute; the horizontal line represents zero drift and corresponds to the original time series.