Onset time of 85–105 keV electrons as observed by STEREO- A / SEPT found at 19:19:33 by traditional Poisson-CUSUM on the SEP event of April 17, 2021 with the background window set to 08:00–16:00 when using three-minute time-averaging. The onset time is marked with a vertical red line and the background window with light red. The mean of the background, µ, and µ + 2σ are illustrated with dashed and dotted red lines, respectively.