  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 of Q~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. This is 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 3sigma 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. We discuss the implications of these findings in the context of
models for ULXs, and their implications for the object's mass.
