Global Radiation MHD Simulations of Black Hole Accretion Disks

The goal of this project is to study the structures of black hole accretion disks with various accretion rates (from sub-Eddington to super-Eddington accretion flows) based on first principle 3D radiation MHD simulations. We solve the time dependent radiative transfer equation for the specific intensities directly. Therefore we can calculate the angular distribution of the radiation field, the radiative efficiency and the radiation driven outflow rates. Particularly, we can predict the spectrum of the accretion disks, which can be compared with observations directly.

The first simulation we have done is a super-Eddington accretion disk with an accretion rate 200 L_Edd/c^2. There is a strong radiation-driven outflow along the rotation axis. The mechanical energy flux carried by the outflow is ~20% of the radiative energy flux. The total mass flux lost in the outflow is about 29% of the net accretion rate. The radiative luminosity of this flow is ~10 L_Edd. This yields a radiative efficiency ~4.5%, which is comparable to the value in a standard thin disk model. In our simulation, vertical advection of radiation caused by magnetic buoyancy transports energy faster than photon diffusion, allowing a significant fraction of the photons to escape from the surface of the disk before being advected into the black hole. The simulation results have broad implications for the growth of supermassive black holes in the early universe and provide a basis for explaining the spectrum and population statistics of ultraluminous X-ray sources.