(a) Example of TS distributions for the background (injected n_s = 0, with 100 000 generated pseudo-experiments) and various signal hypotheses (injected n_s > 1, with 10 000 pseudo-experiments each) at the location of GRB 120213A, for the single-source search of the flare catalog and assuming spectral index γ = 2. The signal distributions are very different from the background distribution already for n_s = 1, suggesting that few signal events can lead to a high pretrial significance. (b) Background TS distribution, the same as in (a), but with different binning and axis ranges. The bin at TS = 0 corresponds to under-fluctuations: for this GRB, about 96% of the background pseudo-experiments result in under-fluctuations.