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The ultraluminous X‐ray source, NGC 5408 X‐1, is one of only three 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 Comptonization 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 Narrow‐Line Seyfert 1s. This suggests that NGC 5408 X‐1 is in a similar super‐Eddington state, placing a natural limit on the mass of  ≤ 100 M  ⊙  . 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.

Challenging times: a re‐analysis of NGC 5408 X‐1