Quasi-periodic Variability in NGC 5408 X-1

We report the discovery with XMM-Newton of quasi-periodic variability in the 0.2-10 keV X-ray flux from the ULX NGC 5408 X-1. The average power spectrum of all EPIC-pn data reveals a strong 20 mHz QPO with an average amplitude (rms) of 9% and a coherence Q≡ν₀/σ~6. In a 33 ks time interval when the 20 mHz QPO is strongest we also find evidence for a second QPO peak at 15 mHz, the first indication of a close pair of QPOs in a ULX source. The frequency 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 noise component with slope near 1.5 is also present below 0.1 Hz with evidence for a break to a flatter slope at about 3 mHz. The source shows substantial broadband variability, with a total amplitude (rms) of about 30% in the 0.1-100 mHz frequency band. The power spectrum of hard X-ray photons (>2 keV) shows a ``classic'' flat-topped continuum breaking to a power law with index 1.5-2. Both the break and 20 mHz QPO are detected in the hard band, and the 20 mHz QPO is essentially at the break. The QPO is both strong and narrow in this band, having an amplitude (rms) of 15%, and Q~25. Both the timing and spectral properties of NGC 5408 X-1 are strikingly reminiscent of Galactic black holes at high inferred accretion rates, but with its characteristic frequencies (QPO and break frequencies) scaled down by a factor of 10-100. We discuss the implications of these findings for the object's mass.