LS periodograms calculated on randomly generated LCs of pure red noise. For plotting purposes, we show 10² (gray solid lines) out of the total 10⁵ realizations, superimposed on the LS periodogram of the real PG 1553+113 X-ray data (blue dot-dashed line) and the corresponding 5σ significance level (black dashed line; see Sect. 4). The relevant frequency interval for our analysis of (2 − 3)×10⁻⁸ Hz (black dotted lines) is also highlighted.