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The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1

Luangtip, Wasutep and Roberts, Timothy P. and Done, Chris (2016) The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1. Monthly Notices of the Royal Astronomical Society, 460 (4). pp. 4417-4432. ISSN 0035-8711. doi:10.1093/mnras/stw1282.

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We present a new analysis of X-ray spectra of the archetypal ultraluminous X-ray source (ULX) Holmberg IX X-1 obtained by the Swift, XMM–Newton and NuSTAR observatories. This ULX is a persistent source, with a typical luminosity of ∼10^(40) erg s^(−1), that varied by a factor of 4–5 over eight years. We find that its spectra tend to evolve from relatively flat or two-component spectra in the medium energy band (1–6 keV), at lower luminosities, to a spectrum that is distinctly curved and disc-like at the highest luminosities, with the peak energy in the curved spectrum tending to decrease with increased luminosity. We argue that the spectral evolution of the ULX can be explained by super-Eddington accretion models, where in this case we view the ULX down the evacuated funnel along its rotation axis, bounded by its massive radiatively driven wind. The spectral changes then originate in enhanced geometric beaming as the accretion rate increases and wind funnel narrows, causing the scattered flux from the central regions of the supercritical flow to brighten faster than the isotropic thermal emission from the wind, and so the curved hard spectral component to dominate at the highest luminosities. The wind also Compton down-scatters photons at the edge of the funnel, resulting in the peak energy of the spectrum decreasing. We also confirm that Holmberg IX X-1 displays spectral degeneracy with luminosity, and suggest that the observed differences are naturally explained by precession of the black hole rotation axis for the suggested wind geometry.

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URLURL TypeDescription Paper
Roberts, Timothy P.0000-0001-8252-6337
Additional Information:© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2016 May 26. Received 2016 May 26; in original form 2015 September 24. Published: 30 May 2016. We thank the anonymous referee for their comments, that have helped improve this paper. WL acknowledges financial support in the form of funding for a PhD studentship from the Royal Thai Government. TPR and CD thank STFC for support as part of the consolidated grant ST/L00075X/1. We also would like to thank Matthew Middleton and Dominic Walton for helpful discussions. This work is in part based on observations obtained with XMM–Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This research has also made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL, and has utilized the NUSTARDAS software package, jointly developed by the ASDC (Italy) and Caltech (USA).
Funding AgencyGrant Number
Royal Thai GovernmentUNSPECIFIED
Science and Technology Facilities Council (STFC)ST/L00075X/1
European Space Agency (ESA)UNSPECIFIED
Subject Keywords:accretion, accretion discs – black hole physics –X-rays: binaries – X-rays: individual: Holmberg IX X-1
Issue or Number:4
Record Number:CaltechAUTHORS:20160527-090731589
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Official Citation:Wasutep Luangtip, Timothy P. Roberts, Chris Done; The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1, Monthly Notices of the Royal Astronomical Society, Volume 460, Issue 4, 21 August 2016, Pages 4417–4432,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:67428
Deposited By: Joy Painter
Deposited On:27 May 2016 16:40
Last Modified:11 Nov 2021 00:32

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