Abstract
The standard thin accretion disc model predicts that discs around stellar mass black holes become radiation pressure dominated and thermally unstable once their luminosity exceeds L>0.02 L_Edd. Observationally, discs in the high/soft state of X-ray binaries show little variability in the range 0.01 L_Edd < L < 0.5 L_Edd, implying that these discs in nature are in fact quite stable. In an attempt to reconcile this conflict, we investigate one-zone disc models including turbulent and convective modes of vertical energy transport. We find both mixing mechanisms to have a stabilizing effect, leading to an increase in the L threshold up to which the disc is thermally stable. In the case of stellar mass black hole systems, convection alone leads to only a minor increase in this threshold, up to ~5 per cent of Eddington. However turbulent mixing has a much greater effect -- the threshold rises up to ~20 per cent Eddington under reasonable assumptions. In optimistic models with superefficient turbulent mixing, we even find solutions that are completely thermally stable for all accretion rates. Similar results are obtained for supermassive black holes, except that all critical accretion rates are a factor ~10 lower in Eddington ratio.
Highlights
Accretion discs are ubiquitous in our universe; their physics governs the production of powerful jets from Active Galactic Nuclei, the formation of planets, and the growth of black holes
We find that turbulence acts in a similar way to convection, i.e. turbulent discs are substantially more resilient to the onset of thermal instability
We find that disc stability is strongly modified by the inclusion of turbulent mixing, which pushes the stability threshold to much higher accretion rates
Summary
Accretion discs are ubiquitous in our universe; their physics governs the production of powerful jets from Active Galactic Nuclei, the formation of planets, and the growth of black holes. The model predicts that the transition should occur at an accretion rate above a few tenths of a per cent of Eddington, depending on the mass of the central object This rate, we expect limit-cycle behaviour due to the onset of both thermal instability (Shakura & Sunyaev 1976; Honma, Matsumoto & Kato 1992; Szuszkiewicz & Miller 1998; Janiuk, Czerny & Siemiginowska 2002) and viscous instability (Lightman & Eardley 1974). Gierlinski & Done (2004) show that discs around stellar mass black holes remain stable up to 50 per cent Eddington, which conflicts with the prediction of standard thin-disc theory by more than an order of magnitude
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