Thermal Stability of Radiation-Dominated Accretion Disks

Julian Krolik, Johns Hopkins University

In their classic 1973 paper on the structure of accretion disks, Shakura and Sunyaev showed that, very nearly independent of the details of the stress, the inner regions of all relativistic disks accreting at more than a small fraction of the Eddington rate should be radiation-dominated. Three years later, they also showed that a straightforward extension of their alpha model for estimating the stress in disks predicts that any radiation-dominated region should be thermally unstable. Using a 3-d MHD simulation code that carefully accounts for the energy budget, radiation forces, and radiation diffusion in stratified shearing boxes, we have now shown that this instability does not actually take place. Its absence can be explained simply: fluctuations in the rate of magnetic dissipation in the turbulence drive fluctuations in the pressure, with little evidence for feedback in the other direction.