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The ultraluminous state revisited : fractional variability and spectral shape as diagnostics of super-Eddington accretion

Sutton, A.D.; Roberts, T.P.; Middleton, M.J.

The ultraluminous state revisited : fractional variability and spectral shape as diagnostics of super-Eddington accretion Thumbnail


A.D. Sutton

M.J. Middleton


Although we are nearing a consensus that most ultraluminous X-ray sources (ULXs) below 1041 erg s−1 represent stellar mass black holes accreting in a super-Eddington ‘ultraluminous’ accretion state, little is yet established of the physics of this extreme accretion mode. Here, we use a combined X-ray spectral and timing analysis of an XMM–Newton sample of ULXs to investigate this new accretion regime. We start by suggesting an empirical classification scheme that separates ULXs into three classes based on the spectral morphologies observed by Gladstone et al.: a singly peaked broadened disc class, and two-component hard ultraluminous and soft ultraluminous regimes, with the spectra of the latter two classes dominated by the harder and softer component, respectively. We find that at the lowest luminosities (LX < 3 × 1039 erg s−1) the ULX population is dominated by sources with broadened disc spectra, whilst ULXs with two-component spectra are seen almost exclusively at higher luminosities, suggestive of a distinction between ∼Eddington and super-Eddington accretion modes. We find high levels of fractional variability are limited to ULXs with soft ultraluminous spectra, and a couple of the broadened disc sources. Furthermore, the variability in these sources is strongest at high energies, suggesting it originates in the harder of the two spectral components. We argue that these properties are consistent with current models of super-Eddington emission, where a massive radiatively driven wind forms a funnel-like geometry around the central regions of the accretion flow. As the wind provides the soft spectral component this suggests that inclination is the key determinant in the observed two-component X-ray spectra, which is very strongly supported by the variability results if this originates due to clumpy material at the edge of the wind intermittently obscuring our line-of-sight to the spectrally hard central regions of the ULX. The pattern of spectral variability with luminosity in two ULXs that straddle the hard/soft ultraluminous regime boundary is consistent with the wind increasing at higher accretion rates, and thus narrowing the opening angle of the funnel. Hence, this work suggests that most ULXs can be explained as stellar mass black holes accreting at and above the Eddington limit, with their observed characteristics dominated by two variables: accretion rate and inclination.


Sutton, A., Roberts, T., & Middleton, M. (2013). The ultraluminous state revisited : fractional variability and spectral shape as diagnostics of super-Eddington accretion. Monthly Notices of the Royal Astronomical Society, 435(2), 1758-1775.

Journal Article Type Article
Publication Date Oct 21, 2013
Deposit Date Dec 13, 2013
Publicly Available Date Jan 27, 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 435
Issue 2
Pages 1758-1775
Keywords Accretion discs, Black hole physics, X-rays, Binaries, Galaxies.


Published Journal Article (554 Kb)

Copyright Statement
This article has been published in the Monthly Notices of the Royal Astronomical Society. © 2013 The Authors Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved.

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