Armed by an understanding that the fundamental mechanism of accretion is angular momentum transport due to magnetohydrodynamic (MHD) turbulence driven by the magneto-rotational instability, it has become possible to investigate at a detailed level the astrophysical properties of accretion onto black holes. With previous NSF support, this team has developed a suite of Newtonian and general relativistic MHD numerical simulation codes suitable for exploring the global properties of accretion flows onto black holes and the jets they sometimes generate. This collaborative project, led by Dr. Krolik, will now extend prior work to consider accretion thermodynamics and energetics, the impact of large-scale magnetic fields, and how an accretion flow oriented obliquely to the black hole rotation axis at large distances interacts with Lense-Thirring torques. Emphasis will be placed throughout on creating quantitative links between dynamical simulation data and observable properties.
This work should provide genuine guidance for the controversy over how much large-scale magnetic flux may be brought deep into the accretion flow. It should also reveal how accretion flows stressed by magnetic fields with intrinsic large-scale structure may differ from those without. In addition, computing MHD stresses within the accretion flow is essential to the question of the extent of disk bending by gravitomagnetic forces, and it should soon be possible to calculate disk structure with sufficient resolution to address this issue.
Both simulation codes and data from specific models will be made available to the community, strengthening the software infrastructure of the discipline. The project will provide graduate student training in numerical simulation techniques. Successful public outreach efforts based on simulation results will continue, including planetarium shows and television documentaries.
Black holes when described in popular media are generally portrayed as the darkest objects possible, and that is true of black holes considered in complete isolation. But real black holes in Nature can often be exactly the opposite: the sites of extremely powerful light generation. This happens because their extremely strong gravity enables them to capture surrounding gas. When that gas orbits close outside the black hole, its orbital speed is a significant fraction of the speed of light. However, just as in the Solar System, orbital speeds decline outward, so there can be substantial friction between fluid orbiting on adjacent paths. With such large amounts of kinetic energy available, the friction can make the gas extremely hot, and therefore extremely bright. The work supported by this grant is part of a major effort by many astrophysicists to understand how "accreting black holes" actually work. In order for gas to move close to a black hole, it is necessary for it to lose angular momentum; otherwise, it would simply orbit forever at whatever large distance its initial angular momentum and energy determine. We now understand that in the conditions of gas orbiting black holes, magnetic fields become attached to the fluid, and the interaction of those magnetic fields with the gas's orbital motion leads to a particularly violent kind of turbulence. One consequence of that turbulence is the amplification of the magnetic field itself. Unfortunately, there are no good pencil-and-paper tools for studying this sort of turbulence; only numerical simulations demanding the largest supercomputers can handle the problem. Over the past decade, it has become possible to run such simulations, computing the detailed behavior of this magnetohydrodynamic (MHD) turbulence with all the effects of general relativity. The goal of our particular work was two-fold: to explore several previously murky areas of the dynamics of accretion flows around black holes, and to develop techniques to use the results of these dynamical simulations to predict specific properties of the light emitted as black holes accrete gas so that observational astronomers might be able to test these calculations. A particularly interesting result of our dynamical work involved an exotic property of black holes: when they rotate, they drag nearby spacetime into rotation along with them, but their ability to do so weakens rapidly with increasing distance. One consequence of this "frame-dragging" is that gas orbiting in a plane different from the equatorial plane of the black hole rotation is forced to precess around the black hole spin axis, but with a precession frequency that decreases with distance away from the black hole. For forty years it had been speculated that in such a situation the inner rings of such a system would be forced to align with the black hole rotation, but there was little real understanding of how that would happen. We were the first to construct a direct calculation showing how that alignment proceeds. Perhaps the most striking result of our effort to create observational predictions has to do with a surprising property of accretion flows onto black holes: the radiate light in a way strikingly different from the way stars do. The surfaces of stars are thermal radiators; that is, the light they generate is very close to what any object of their temperature would radiate, and its spectrum is strongly concentrated at the photon energies characteristic of that temperature. Initially, people thought accretion flows onto black holes would be similar---until it was found that typically tens of percent of the energy produced by black hole accretion flows comes in high-energy X-rays, photons of energy much greater than the thermal emission should produce. It has been speculated since the late 1970s that somehow magnetic processes in the accretion flow would energize "coronae" above the surfaces of accretion disks where the X-ray could be made, but no one could calculate how much or how specifically it would happen. We used numerical simulations of MHD turbulence to show that it naturally leads to just such coronae, and then calculated the specific spectrum of high-energy X-rays that would be produced. The spectral shapes we predicted match very closely to those observed, vindicating this picture.
