Confirming the 115.5-day periodicity in the X-ray light curve of ULX NGC 5408 X-1

Xu Han, Tao An, Jun-Yi Wang, Ji-Ming Lin, Ming-Jie Xie, Hai-Guang Xu, Xiao-Yu Hong, Sandor Frey


Abstract The Swift/XRT light curve of the ultraluminous X-ray source NGC 5408 X-1 was re-analyzed with two new numerical approaches, the Weighted Wavelet Z- transform and CLEANest, and the results are different from previous studies. Both techniques detected a prominent periodicity with a time scale of 115.5 ± 1.5 days, in excellent agreement with the detection of the same periodicity first reported by Strohmayer . Monte Carlo simulations were employed to test the statisitical confidence of the 115.5-day periodicity, yielding a statistical significance of > 99.98% (or > 3.8σ). The robust detection of the 115.5-day quasi-periodic oscillations, if they are due to the orbital motion of the binary, would infer a mass of a few thousand M⊙ for the central black hole, implying there is an intermediate-mass black hole in NGC 5408 X-1.


Keywords methods: statistical — galaxies: individual (NGC 5408) — stars: oscillations

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