Quasi‐periodic Variability in NGC 5408 X‐1

Abstract

We report the discovery with XMM-Newton of quasi-periodic variability inthe 0.2-10 keV X-ray flux from the ULX NGC 5408 X-1. The average powerspectrum of all EPIC-pn data reveals a strong 20 mHz QPO with an averageamplitude (rms) of 9% and a coherenceQ{\equiv}{ν}_{0}/{σ}~6. In a 33 ks time interval when the 20mHz QPO is strongest we also find evidence for a second QPO peak at 15mHz, the first indication of a close pair of QPOs in a ULX source. Thefrequency ratio of this QPO pair is inconsistent with 3:2 at the 3{σ} level but is consistent with a 4:3 ratio. A power-law noisecomponent with slope near 1.5 is also present below 0.1 Hz with evidencefor a break to a flatter slope at about 3 mHz. The source showssubstantial broadband variability, with a total amplitude (rms) of about30% in the 0.1-100 mHz frequency band. The power spectrum of hard X-rayphotons ({\gt}2 keV) shows a ``classic'' flat-topped continuum breakingto a power law with index 1.5-2. Both the break and 20 mHz QPO aredetected in the hard band, and the 20 mHz QPO is essentially at thebreak. The QPO is both strong and narrow in this band, having anamplitude (rms) of 15%, and Q~25. Both the timing and spectralproperties of NGC 5408 X-1 are strikingly reminiscent of Galactic blackholes at high inferred accretion rates, but with its characteristicfrequencies (QPO and break frequencies) scaled down by a factor of10-100. We discuss the implications of these findings for the object'smass.