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±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 >3.8σ). 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⊙​ for the central black hole, implying an intermediate-mass black hole
in NGC 5408 X-1.Comment: 6 pages, 2 figures, submitted to Research in Astronomy and
Astrophysics (RAA