Lomb-Scargle periodograms and distribution of periods in the bootstrap approach of the X-ray and optical data. Upper left panel: Lomb-Scargle periodogram of the X-ray data. The FAP levels of 50 and 80% resulting from the bootstrap analysis described in Sect. 3.3 are marked as blue lines. Upper right panel: distribution of maximum periods determined for all 10⁴ repetitions. Lower panels: results of a similar analysis of the optical data. No FAP levels are displayed here, since none of the bootstrap repetitions yielded a higher Lomb-Scargle power than the unperturbed data.