Does NGC 5408 X-1 really contain an intermediate-mass black hole?

NGC, 5408 X-1 is one of only a handful of ultraluminous X-ray sources (ULXs) to demonstrate a strong quasi-periodic oscillation (QPO) in its lightcurve. This has been interpreted as being analogous to the type C low-frequency QPO seen in black hole binaries (BHBs) when accreting at sub-Eddington rates, implying an intermediate-mass black hole (IMBH). However, in BHBs this QPO has a centroid frequency that scales tightly with the position of the low-frequency break in the broad band power spectrum. In recent work we re-assessed the data and found that the position of the break does not comply with the observed relationship in either of the available, archival XMM-Newton observations. Thus the broad band power spectral shape does not support this identification of the QPO. We also found that the X-ray spectra do not support a sub-Eddington identification, instead favouring a two component model of soft thermal emission at low energies, together with low-temperature, optically thick Comptonisation producing a tail which dominates above 2 keV. The energy dependent variability supports this deconvolution and indicates a large amount of variability present in the hard component. Such variability is only met by the BHB, GRS 1915+105, and some extreme NLS1s. This suggests that NGC5408 X-1 is in a similar super-Eddington state, placing a natural limit on the mass of <= 100M(circle dot). Here we compare in greater detail our analysis to that of previous work on this source's variability characteristics. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim