False-alarm probabilities for the Lomb–Scargle periodogram based on an adaptation of the REFIT code (see text for details) for NGC 5204 X–1 (left) for the 2014−2015 monitoring and for Holmberg II X–1 (right) for the 2009−2010 monitoring under the null-hypothesis of red noise. The coloured 95.5% and 99.7% mark the distribution of the 10 000 synthetic periodograms, whereas the black dashed line marks 2σ and 3σ false-alarm probabilities, which have been determined from the maximum of the peaks of the synthetic periodograms.