Publication details for Prof Tim RobertsMiddleton, M.J., Roberts, T.P., Done, C. & Jackson, F.E. (2011). Challenging times: a re-analysis of NGC 5408 X-1. Monthly Notices of the Royal Astronomical Society 411(1): 644-652.
- Publication type: Journal Article
- ISSN/ISBN: 0035-8711, 1365-2966
- DOI: 10.1111/j.1365-2966.2010.17712.x
- Keywords: Accretion discs, Black hole physics, X-rays, Binaries.
- Further publication details on publisher web site
- Durham Research Online (DRO) - may include full text
Author(s) from Durham
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.