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

• Published in: Astronomische Nachrichten 2011-05, Vol.332 (4), p.388-391
• Main Authors: Middleton, M.J., Roberts, T.P., Done, C., Jackson, F.E.
• Format: Article
• Language: English
• Subjects: accretion: accretion disks; black hole physics; Black holes; X-ray astronomy; X-rays; X-rays: binaries; X-rays: individual (NGC 5408 X-1)
• ISSN: 0004-6337; 1521-3994
• DOI: 10.1002/asna.201011506

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 NGC 5408 X‐1 is in a similar super‐Eddington state, placing a natural limit on the mass of ≤100 M⊙. Here we compare in greater detail our analysis to that of previous work on this source's variability characteristics (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)