  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.
