X-ray Outflows and Super-Eddington Accretion in the Ultraluminous X-ray Source Holmberg IX X-1

TL;DR

This study used deep X-ray observations of Holmberg IX X-1 to search for atomic wind features, finding none, which challenges models of spherical super-Eddington accretion and suggests anisotropic outflows or different accretion mechanisms.

Contribution

The paper provides the most sensitive iron K bandpass dataset for a bright ULX, setting stringent limits on atomic features and constraining accretion and outflow models.

Findings

No significant atomic features detected in the iron K band.

Limits on line equivalent widths are below expectations for super-Eddington winds.

Results suggest the absence of spherical outflows and minimal stellar wind contribution.

Abstract

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.