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

Middleton, M.J.; Roberts, T.P.; Done, C.; Jackson, F.E.

Challenging times: a re-analysis of NGC 5408 X-1 Thumbnail


M.J. Middleton

C. Done

F.E. Jackson


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 100M⊙. 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.


Middleton, M., Roberts, T., Done, C., & Jackson, F. (2011). Challenging times: a re-analysis of NGC 5408 X-1. Monthly Notices of the Royal Astronomical Society, 411(1), 644-652.

Journal Article Type Article
Publication Date Feb 11, 2011
Deposit Date Jan 30, 2012
Publicly Available Date Jan 24, 2014
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 411
Issue 1
Pages 644-652
Keywords Accretion discs, Black hole physics, X-rays, Binaries.


Published Journal Article (702 Kb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2011 The Authors Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved.

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