Physics
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011an....332..388m&link_type=abstract
Astronomische Nachrichten, Vol.332, Issue 4, p.388-391
Physics
2
Accretion: Accretion Disks, Black Hole Physics, Xrays: Binaries, X-Rays: Individual (Ngc 5408 X-1)
Scientific paper
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&sun;. Here we compare in greater detail our analysis to that of previous work on this source's variability characteristics.
Done Christine
Jackson Floyd E.
Middleton Matthew J.
Roberts Timothy P.
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