  Studies of X-ray continuum emission and flux variability have not
conclusively revealed the nature of ultra-luminous X-ray sources (ULXs) at the
high-luminosity end of the distribution (those with Lx > 1e40 erg/s). These are
of particular interest because the luminosity requires either super-Eddington
accretion onto a black hole of mass ~10 Msun, or more standard accretion onto
an intermediate-mass black hole. Super-Eddington accretion models predict
strong outflowing winds, making atomic absorption lines a key diagnostic of the
nature of extreme ULXs. To search for such features, we have undertaken a long,
500 ks observing campaign on Holmberg IX X-1 with Suzaku. This is the most
sensitive dataset in the iron K bandpass for a bright, isolated ULX to date,
yet we find no statistically significant atomic features in either emission or
absorption; any undetected narrow features must have equivalent widths less
than 15-20 eV at 99% confidence. These limits are far below the >150 eV lines
expected if observed trends between mass inflow and outflow rates extend into
the super-Eddington regime, and in fact rule out the line strengths observed
from disk winds in a variety of sub-Eddington black holes. We therefore cannot
be viewing the central regions of Holmberg IX X-1 through any substantial
column of material, ruling out models of spherical super-Eddington accretion.
If Holmberg IX X-1 is a super-Eddington source, any associated outflow must
have an anisotropic geometry. Finally, the lack of iron emission suggests that
the stellar companion cannot be launching a strong wind, and that Holmberg IX
X-1 must primarily accrete via roche-lobe overflow.
