Temporal evolution of long-timescale periodicities in ULX NGC 5408 X-1

• Published in: arXiv.org 2015-11
• Main Authors: An, Tao, Xiang-Long, Lu, Jun-Yi, Wang
• Format: Article
• Language: English
• Subjects: Deposition; Evolution; Light curve; Periodic variations; Quasi-Periodic Oscillations; Statistical significance; Time series; Wavelet analysis; X ray sources; Z transforms
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1510.01414

Context. NGC 5408 X-1 is one of the few ultraluminous X-ray sources with an extensive monitoring program in X-rays (a temporal baseline of 4.2 yr), making it one of the most suitable candidates to study the long-timescale quasi-periodic oscillations (QPOs). Aims. Previous timing analysis of the Swift data of NGC 5408 X-1 led to detection of multiple periodicities ranging from 2.6 d to 230 d. In this paper, we focus on the statistical significance and the temporal evolution of these periodicities. Methods. 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. Results. Three periodic components were detected from the WWZ periodogram, corresponding to periods of 2.65\(\pm\)0.01 d, 115.4\(\pm\)14.4 d and 189.1\(\pm\)15.2 d. All three have statistical significance higher than 99.74%. The 2.65-d periodicity is quite stable in the majority of the light curve. The 115-d periodicity is the most prominent but appears variable. The 189-d periodicity is persistent across the whole time coverage. It shows a steadily decreasing trend from the beginning (193 d period) to the end (181 d period). Conclusions. 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.