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

TL;DR

This study analyzes the spectral evolution of the ultraluminous X-ray source Holmberg IX X-1 over eight years, revealing a transition from flat to curved spectra with increasing luminosity, explained by super-Eddington accretion and geometric beaming effects.

Contribution

It provides new insights into the spectral evolution of Holmberg IX X-1, linking it to super-Eddington accretion models and wind geometry, with evidence of spectral degeneracy explained by precession.

Findings

Spectra evolve from flat/two-component to curved/disc-like with luminosity

Peak energy decreases as luminosity increases due to wind Compton down-scattering

Spectral degeneracy explained by precession of the black hole axis

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 ~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.