A transient brightening detected both in 3 mm and 304 Å target 4 data. Row a: characteristic ALMA snapshots with field of view of 35″ × 35″; while row b shows the same snapshots after the subtraction of the average ALMA image. Rows c–f: same as rows a and b but for the 304 Å data (rows c and d), and the 1600 Å data (rows e and f). The white arrows indicate the transient brightening in the ALMA and 304 Å data. Bottom row: time profiles of the event emission at 3 mm (black curves) and 304 Å (blue curves), before (left panel) and after (right panel) our processing; the processed light curves show values above the background level. The 304 Å light curves are in arbitrary units, normalized to fit the vertical extent of the ALMA plots.