The Swift/XRT light curve of the ultraluminous X-ray ( ULX ) source NGC 5408 X-1 was re-analyzed with two new numerical approaches , Weighted Wavelet Z -transform ( WWZ ) and CLEANest , that are different from previous studies .
Both techniques detected a prominent periodicity with a time scale of 115.5 \pm 1.5 days , in excellent agreement with the detection of the same periodicity first reported by Strohmayer ( 2009 ) .
Monte Carlo simulation was employed to test the statisiticak confidence of the 115.5-day periodicity , yielding a statistical significance of > 99.98 \% ( or > 3.8 \sigma ) .
The robust detection of the 115.5-day quasi-periodic oscillations ( QPOs ) , if it is due to the orbital motion of the binary , would infer a mass of a few thousand M _ { \odot } for the central black hole , implying an intermediate-mass black hole in NGC 5408 X-1 .