On the variation of black hole accretion disc radii as a function of state and accretion rate

Cabanac, C., Fender, R.P., Dunn, R.J.H. and Körding, E.G. (2009) On the variation of black hole accretion disc radii as a function of state and accretion rate. Monthly Notices of the Royal Astronomical Society, 396 (3), 1415-1440. (doi:10.1111/j.1365-2966.2009.14867.x).

Abstract

In response to major changes in the mass accretion rate within the inner accretion flow, black hole binary transients undergo dramatic evolution in their X-ray timing and spectral behaviour during outbursts. In recent years a paradigm has arisen in which 'soft' X-ray states are associated with an inner disc radius at, or very close to, the innermost stable circular orbit (ISCO) around the black hole, while in 'hard' X-ray states the inner edge of the disc is further from the black hole. Models of advective flows suggest that as the X-ray luminosity drops in hard states, the inner disc progressively recedes, from a few gravitational radii (R_g) at the ISCO, to hundreds of R_g. Recent observations which show broad iron line detections and estimates of the disc component strength suggest that a non-recessed disc could still be present in bright hard states. In this study we present a comprehensive analysis of the spectral components associated with the inner disc, utilizing bright states data from X-ray missions with sensitive low-energy responses (e.g. Swift, SAX), including re-analyses of previously published results. A key component of the study is to fully estimate systematic uncertainties associated with such spectral fits. In particular we investigate in detail the effect on the measured disc flux and radius of having a hydrogen column density that is fixed or free to vary. We conclude that at X-ray luminosities above ?0.01 of the Eddington limit, systematic uncertainties only allow us to constrain the disc to be ≤10R_g from spectral fits. There is, however, clear evidence that at X-ray luminosities between 10^-2 and 10^-3 of the Eddington rate, the disc does begin to recede. We include measurements of disc radii in two quiescent black hole binaries at bolometric luminosities of <10^-7 Eddington, and present the inferred evolution of disc luminosity, temperature, inner radius and accretion rate/efficiency across the entire range of bolometric luminosities 10^-8–1 Eddington. We compare our results with theoretical models, and note that the implied rate of disc recession with luminosity is consistent with recent empirical results on the X-ray timing behaviour of black holes of all masses.

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