**Title**:

Global structure of three distinct accretion flows and outflows around black holes through two-dimensional radiation-magnetohydrodynamic simulations

**Authors**:

Ken Ohsuga and Shin Mineshige**First Author’s Institution**:

National Astronomical Observatory of Japan and Graduate University of Advanced Study

*Today’s Astrobite was contributed by Warrick Ball, a graduate student at the Institute of Astronomy at Cambridge. If you would like to contribute to Astrobites, please contact us!*

### Simulation Setup

The authors have written a code that numerically solves the equations of radiation magnetohydrodynamics (RMHD). In other words, they solve a set of partial differential equations that describe radiation, magnetic fields, and fluid dynamics.

As always, it isn’t possible to simulate reality perfectly, so a number of approximations are made. The most important is that the flow is presumed to be symmetric around the disk’s axis of rotation. This is a common and reasonable assumption and it’s important for making the problem solvable. Solving for two dimensions is a lot easier than solving for three! They further assume that the flow is symmetric across the equatorial plane. Gas that would cross the plane just bounces back. Finally, in order to solve efficiently for radiation,they make the flux-limited diffusion approximation. Basically, the energy density of the radiation diffuses out at a rate that is limited by how quickly radiation can scatter through the gas.To capture general relativistic effects, the authors approximate the black hole’s gravity through the Paczynski-Wiita potential. This is a pretty good fit for the potential around a black hole, but it only describes a non-rotating black hole. The models can’t capture any effects due to the black hole’s spin, which are thought to be important for the formation of black hole jets.The simulation kicks off with a doughnut of material at a distance equal to 40 times the radius of the event horizon. The authors allow the flow to evolve without radiation effects for a few orbital periods before switching on radiation. Because the magnetic field describes the viscous nature of the fluid, the only free parameter that’s expected to make a qualitative difference, presuming that the gas is ideal, is the gas density.

### Results

Despite all the approximations, the simulation produces three distinct accretion flows for different choices of the density. Their models A, B, and C, shown in figure 2 of the paper (reproduced above), correspond to density parameters 1, 10^{-4} and 10^{-8} g.cm^{-3}.

### Discussion

The authors discuss their models in great detail and go on to consider them in light of a range of observational properties. They finally note some effects that their simulations cannot capture, but the fact remains that by varying just one parameter, they can produce three major modes of black hole accretion. Even though the detailed, quantitative structure doesn’t perfectly match theory, this looks like a big step in unifying our understanding of black hole accretion.

Good summary, but with one (important) inaccuracy: Actually, ADAFs were NOT found first by Narayan and Yi, but much earlier by Ichimaru in 1977 following up on even earlier work by people like Chandrasekhar. This early work has been nicely summarized by Roland Svensson, see http://arxiv.org/abs/astro-ph/9902205