  The ultraluminous X-ray source (ULX), NGC 5408 X-1, is one of only 3 such
objects to show a quasi-periodic oscillation (QPO) in its power spectrum.
Previous analysis of this signal identified it with the well-studied type C
low-frequency QPO (LFQPO) seen in black hole binaries (BHBs), implying an
intermediate mass black hole (IMBH). However, in BHBs this QPO has a centroid
frequency which scales tightly with the position of the low-frequency break in
the broad band power spectrum. We use this relation to predict the frequency of
the power spectral break in NGC 5408 X-1, and show that this is inconsistent
with the break frequencies in both available, archival XMM-Newton observations.
Thus the broad band power spectral shape does not support this identification
of the QPO. The energy spectra also do not support an IMBH interpretation. They
can be fit by a two-component model, best described by soft thermal emission at
low energies, together with low-temperature, optically thick Comptonisation
producing a tail which dominates above 2 keV. The parameters of the tail are
unlike those seen in any of the sub-Eddington BHB spectral states. The energy
dependent variability supports this deconvolution, as it is consistent with the
soft thermal component below 2 keV diluting extreme variability of the high
energy tail. The only objects with similar spectra which have similar amounts
of variability are the BHB, GRS 1915+105, and some extreme NLS1s. This suggests
that NGC 5408 X-1 is in a similar super-Eddington state, placing a natural
limit on the mass of < 100 solar masses. Its QPO could then be similar to the
ultra-LFQPO seen occasionally in GRS 1915+105, consistent with a large stellar
mass black hole. We suggest a model geometry which may explain the spectra and
variability of highly super-Eddington sources.
