2011MNRAS.411..644M

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.