We report the discovery with XMM-Newton of quasiperiodic variability in the 0.2 - 10 keV X-ray flux from the ultraluminous X-ray source 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 \equiv \nu _ { 0 } / \sigma \approx 6 .
In a 33 ksec time interval when the 20 mHz QPO is strongest we also find evidence for a 2nd QPO peak at 15 mHz , the first indication for a close pair of QPOs in a ULX source .
Interestingly , the frequency ratio of this QPO pair is inconsistent with 3:2 at the 3 \sigma level , but is consistent with a 4:3 ratio .
A powerlaw 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 , and there is strong energy dependence to the variability .
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 \approx 25 .
The energy spectrum is well fit by three components , a “ cool ” disk with kT = 0.15 keV , a steep power law with index 2.56 , and a thermal plasma at kT = 0.87 keV .
The disk , power law , and thermal plasma components contribute 35 , 60 , and 5 % of the 0.3 - 10 keV flux , respectively .
Both the timing and spectral properties of NGC 5408 X-1 are strikingly reminiscent of Galactic black hole systems 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 in the context of models for ULXs , and their implications for the object ’ s mass .