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

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.