The ultraluminous state revisited: fractional variability and spectral shape as diagnostics of super-Eddington accretion

Understand

Although we are nearing a consensus that most ULXs are stellar-mass black holes in a super-Eddington state, little is yet established of the physics of this accretion mode.

• Here, we use a combined X-ray spectral and timing analysis of a sample of ULXs to investigate this new accretion regime.
• We suggest a spectral classification scheme that separates ULXs into three classes: a broadened disc class, and two-component hard and soft ultraluminous regimes.
• At the lowest luminosities the ULX population is dominated by sources with broadened disc spectra, whilst two component spectra are seen at higher luminosities, suggestive of a distinction between ~ Eddington and super-Eddington accretion modes.