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Durham University

Department of Physics

Staff profile

Publication details for Prof Tim Roberts

Walton, D.J., Miller, J.M., Harrison, F.A., Fabian, A.C., Roberts, T.P., Middleton, M.J. & Reis, R.C. (2013). X-Ray Outflows and Super-Eddington Accretion in the Ultraluminous X-Ray Source Holmberg IX X-1. Astrophysical Journal Letters 773(1): L9.

Author(s) from Durham


Studies of X-ray continuum emission and flux variability have not conclusively revealed the nature of ultraluminous
X-ray sources (ULXs) at the high-luminosity end of the distribution (those with LX  1040 erg s−1). These are
of particular interest because the luminosity requires either super-Eddington accretion onto a black hole of mass ∼10 M 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 data set 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.