Accretion disk theory has been undergoing a slow but steady revolution ever since the discovery that turbulence driven by the magnetorotational instability (MRI) is likely to be responsible for outward angular momentum transport. Thanks in large part to simulations of MRI turbulence, scientists are for the first time getting a handle on the true vertical structure of accretion disks, free of ad hoc assumptions. This project will analyze new simulations with radiation pressure much larger than gas pressure. This radiation pressure-dominated regime is the most uncertain and at the same time the most important for understanding the innermost regions of luminous accretion disks around black holes. The study will also rebuild spectral models of accretion disks, applicable to both stellar mass black hole X-ray binaries and supermassive black holes in quasars and active galactic nuclei. The techniques include Monte Carlo methods to handle radiative transfer.
In further activity, the group will analyze local and global MRI simulations, perform analytic calculations of linear modes and nonlinear driving and damping, and simulate linear instabilities in magnetically dominated configurations. The end result will be a complete understanding of the true vertical structure of accretion disks around black holes in all regimes, from the gas pressure dominated outer regions to the radiation pressure dominated inner regions. The spectral models will be fully predictive for the first time, as they are based on physics and not ad hoc assumptions. This research will lead to a deep understanding of the expected variability of MRI flow structures based on combining analytic and quasi-analytic linear mode theory and local and global simulations.
The project will be the main vehicle for the theses of two graduate students, one concentrating on numerical simulations, and the other focused mainly on analytic calculations and numerical spectral modeling. The researchers will also continue to engage the public in exciting developments in black hole astrophysics, using movies and images from numerical simulations as well as music-based ideas to explain black hole X-ray variability.
Energy released as matter works its way deeper into the gravity well of a central mass is a process of fundamental importance to many phenomena in the universe, particularly when that matter adopts the form of a rotating disk. Such "accretion disks" are observed to form around young stars, where they are the nurseries of planets, and are also known to form around white dwarfs, neutron stars, and black holes - both stellar mass black holes in orbit around other stars in binary systems, and supermassive black holes in the centers of galaxies. The work in this project was intended to further our theoretical understanding of the fundamental physical processes in these flows, and to investigate how these processes might affect their observed properties, building toward the ultimate goal of testing our theories with observations. I highlight the major results below. (1) Just as the earth's spin axis is not perpendicular to its orbital plane, these accretion disks can be misaligned with respect to the spin axis of the central mass around which they orbit. This turns out to produce interesting phenomena, particularly around black holes, where the spacetime itself rotates with the black hole and drags the accretion disk around with it. This project has shown that strong shocks form in such flows, and these shocks dissipate a significant amount of power into light. Moreover, transient blobs that form and reform in the accretion disk produce enhanced variability in the light output when they traverse these shocks. This might be important in our understanding of the accretion flow around our local supermassive black hole at the center of our Milky Way galaxy, as well as some of the variability phenomena that we observed in stellar mass black holes in binary systems. (2) Hot accretion disks around black holes and other compact objects often have high electrical conductivities, and can be highly magnetized. These magnetic fields profoundly affect the physics of these flows. We have shown that buoyant concentrations of such fields continually form and carry heat outward in the most luminous accretion flows, and therefore play an important role in determining how much of the power liberated in the flow can actually reach observers who are far away. A number of fluid instabilities attend these magnetic fields, and we have increased our understanding of how such instabilities work. Moreover, we have also increased our understanding of how these magnetic processes affect the spectral energy distributions (the "colors", if you like) that we observe from these flows. (3) Both theoretically and observationally, the inflowing matter in an accretion disk can experience instabilities which cause the entire disk to undergo a transient outburst in luminosity. Using numerical simulations of the magnetic turbulence that exists in these disks, we have made progress in understanding the physical basis for one of these instabilities, which is believed to drive the outbursts in the systems which have by far and away the best observations: so-called dwarf novae, which are outbursting disks around white dwarfs. We have shown that thermal convection (fluid motions that are similar to those occurring in a boiling kettle of water) significantly alters the character of magnetic turbulence in a manner that might explain the puzzling rapid variability observed when these systems are in outburst. This will hopefully enable us to use these well-observed systems to constrain our theories of magnetic turbulence, which is widely believed to be the fundamental physical process in all accretion disks in the universe. At the same time, though, we have failed to explain why luminous black holes, which should theoretically undergo outbursts from different instability in their innermost regions, apparently do not. So there are still things that we need to understand better. These projects have formed the basis for two PhD theses of two graduate students, who now teach physics at the college level, as well as research projects for two undergraduates, who have themselves gone on to pursue PhD's of their own. I have also given many public talks in the community at various venues on these research areas, and welcome the opportunity to give more.
