Publication: The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1
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Issued Date
2016
Resource Type
File Type
application/pdf
ISSN
358711
Other identifier(s)
2-s2.0-84980385968
Rights Holder(s)
Scopus
Bibliographic Citation
Monthly Notices of the Royal Astronomical Society. Vol 460, No.4 (2016), p.4417-4432
Suggested Citation
Luangtip W., Roberts T.P., Done C. The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1. Monthly Notices of the Royal Astronomical Society. Vol 460, No.4 (2016), p.4417-4432. doi:10.1093/mnras/stw1282 Retrieved from: https://hdl.handle.net/20.500.14740/5263
Author(s)
Abstract
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 ~1040 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. © 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.