Top: ratio of the target 5 “event pixels” with respect to the number of pixels of the whole field of view (expressed in percentage) as a function of the σ multiplication factor that determines the threshold for the detection of transient brightenings. No additional spatial coherence detection criteria have been applied for the production of these results. Bottom: same quantity as top curve but now computed as a function of the minimum number of adjacent pixels on top of the 2.5σ multiplication factor.