2016A&A...585A..89A

NGC 5408 X-1 is one of the few ultraluminous X-ray sources (ULXs) with an extensive monitoring program in X-rays (a temporal baseline of 4.2yr), making it one of the most suitable candidates to study the long-timescale quasi-periodic oscillations (QPOs). Previous timing analysis of the Swift data of NGC 5408 X-1 led to detection of multiple periodicities ranging from 2.6d to 230d. In this paper, we focus on the statistical significance and the temporal evolution of these periodicities. A time-series analysis technique in the time-frequency domain, the weighted wavelet Z-transform (WWZ), was employed to identify the periodicities and trace their variations with time. Three periodic components were detected from the WWZ periodogram, corresponding to periods of 2.65±0.01d, 115.4±14.4d and 189.1±15.2d. All three have statistical significance higher than 99.74% (>3σ). The 2.65-d periodicity is quite stable in the majority of the light curve, whereas it changes to a shorter 2.50d period in the last third of the time interval covered by the light curve. The 115-d periodicity is the most prominent but appears variable. It starts at an initial value of ∼117.8d and reaches 115.4d around MJD 54800; then it shifts into a lower-frequency branch (136d period) between MJD 55200-55750; after that, this periodicity returns to 115d until the end of the monitoring period. The 189-d periodicity is persistent across the whole time coverage. It shows a steadily decreasing trend from the beginning (193d period) to the end (181d period). The long-timescale periodicities in NGC 5408 X-1 are most likely of super-orbital origin, and are probably associated with the precession of a warped accretion disc. The disc may have been broken into two distinct planes with different precessing periods, i.e. the 189-d and 115-d periodicities corresponding to the outer and inner disc, respectively.