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

TL;DR

This study confirms a 115.5-day periodicity in the X-ray light curve of ULX NGC 5408 X-1 using advanced analysis methods, supporting the presence of an intermediate-mass black hole.

Contribution

It introduces two new numerical approaches, WWZ and CLEANest, to detect periodicities in ULX light curves, confirming the 115.5-day cycle with high statistical significance.

Findings

Detected a 115.5-day periodicity with >99.98% confidence

Confirmed the periodicity aligns with previous findings

Supports the hypothesis of an intermediate-mass black hole

Abstract

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 $115.5\pm1.5$ 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.