April 25
Abstract:
The inner regions of black hole accretion disks are expected to be
radiation dominated, that is the radiation field contains a significant
fraction of both the energy and momentum in the flow. In order to
understand how gas is accreted towards the black hole, it is very
important to study how the saturation level of the magneto-rotational
instability (MRI) will be affected by radiation. Previous numerical
simulations of black hole accretion disks adopt the flux-limited diffusion
(FLD) approximation, which may not be appropriate for the optically thin
regions of the disk atmosphere, and cannot be used when the momentum of
the photons is significant. I will describe a new Godunov radiation MHD
algorithm based on a variable Eddington tensor. This new algorithm does
not adopt any diffusion-like approximation. It works in both optically
thin and thick regimes, and works for both radiation or gas pressure
dominated flows. I will show a set of tests to demonstrate that the code
is working accurately as expected for different regimes. Finally I will
show the results when we apply the code to study the saturation levels of
MRI turbulence in unstratified shearing-box simulations of the MRI to
explore the midplane regions of disks with different radiation pressures.
We find that the Reynold stress is damped by Compton drag when the
momentum of photons is significant, which has important implications for
the structure of radiation dominated accretion disks.