Thermal stability in turbulent accretion discs

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 0.02L Edd. Observationally, discs in the high/soft state of X-ray binaries show little variability in the range 0.01L Edd < L < 0.5L 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 of ∼10 lower in Eddington ratio.