The ultraluminous state revisited: fractional variability and spectral shape as diagnostics of super-Eddington accretion (1307.8044v1)

Abstract: 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. We find high levels of variability are limited to ULXs with soft ultraluminous spectra, and a few broadened disc sources. Furthermore, the variability is strongest at high energies, suggesting it originates in the harder spectral component. These properties are consistent with current models of super-Eddington emission, where a wind forms a funnel around the central regions of the accretion flow. As the wind provides the soft component this suggests that inclination is the key determinant in the observed X-ray spectra, which is very strongly supported by the variability results if this originates due to clumpy material at the edge of the wind intermittently obscuring our line-of-sight to the central regions of the ULX. The pattern of spectral variability with luminosity in two ULXs that straddle the hard/soft ultraluminous regime boundary is consistent with the wind increasing at higher accretion rates, and thus narrowing the opening angle of the funnel. Hence, this work suggests that most ULXs can be explained as stellar-mass black holes accreting at and above the Eddington limit, with their observed characteristics dominated by two variables: accretion rate and inclination. (abridged)

Assessing Super-Eddington Accretion in Ultraluminous X-ray Sources

The paper "The ultraluminous state revisited: fractional variability and spectral shape as diagnostics of super-Eddington accretion" by Sutton et al. conducts an in-depth analysis of ultraluminous X-ray sources (ULXs), which are thought to be stellar-mass black holes accreting matter at super-Eddington rates. The research draws on observations from the {\it XMM-Newton} archive to elucidate the spectral states and variability patterns of these fascinating objects.

Empirical Classification of Spectral States

The authors propose an empirical classification system that distinguishes ULXs into three spectral types: broadened disc, hard ultraluminous, and soft ultraluminous states. This classification leverages the multi-component nature of ULX spectra, which demarcates the observations into regimes based on the contributions of a multi-color disc (MCD) model and a power-law component. This system builds upon prior work by Gladstone et al., refining the distinction between Eddington and super-Eddington accretion states.

Spectral and Timing Characteristics

A key finding is that broadened disc spectra dominate at lower luminosities, particularly below 3 × 10^{39} erg s^{-1}, whereas hard and soft ultraluminous spectra predominantly appear at higher luminosities. This suggests a transition from ~Eddington to super-Eddington accretion. The paper also finds notable differences in fractional variability with soft ultraluminous sources showing significantly higher variability, especially in harder spectral components, indicating complex dynamics linked to accretion geometries.

Implications and Future Directions

The analysis supports the notion that inclination plays a crucial role in the observed spectral states of ULXs. High energy variability in soft ultraluminous sources, hypothesized to arise from clumpy wind material obscuring central regions, offers support for a model involving massive radiatively driven winds. These findings suggest the potential for a unifying model where inclination angle and accretion rate dictate observed characteristics.

Looking ahead, more detailed observations may further clarify the role of wind-driven absorption features and multi-wavelength properties such as associated radio nebulae. Understanding these complexities will enhance theoretical models of super-Eddington accretion and aid in defining the intrinsic properties of stellar-mass black holes across various galactic environments.

Conclusion

This paper makes a significant contribution to our understanding of ULXs, presenting data-backed models that aim to unify their diverse observational properties. By focusing on detailed spectral and timing analyses, it paves the way for advancing theories of black hole accretion dynamics, reinforcing the importance of future observational campaigns to further investigate the spectral transitions and variability mechanisms within these potent sources.