Simulations based on N = 10 000 synthetic light curves of 1000 s duration with Poisson noise around various given mean count rates to estimate the likelihood of random detections at four levels of confidence. Power spectra of the random light curves were calculated over the period range 25−100 s, testing 500 frequencies. Left panel: Example of 20 counts per second as mean count rate. We show the maximum powers in the order of trial number in grey and sorted by power value with the thick black line. The sorted curve was used to extract the power thresholds given in the legend. Right panel: results for the four confidence levels as a function of mean count rate with connected black bullet points in comparison to the theoretical thresholds according to Eq. (A.2), with N_f = 500, included with the dotted blue lines.