The most mysterious objects in the universe can shed light on important phenomena here on Earth. With this in mind, one Princeton professor has set out to quantify the behavior of black holes at a level of detail never before achieved. Using the nation’s most powerful supercomputers, Professor James Stone, Chair of the Department of Astrophysics with a joint appointment in the Program in Applied and Computational Mathematics, is seeking to achieve new insight into the accretion of plasma onto black holes in x-ray binary systems and active galactic nuclei. Professor Stone is Director of Princeton Institute for Computational Science and Engineering (PICSciE), , which provides computational hardware and supports high-performance computing on campus in partnership with OIT’s Research Computing department.
In an x-ray binary system, a normal star is in a very close orbit with a black hole. “They’re orbiting together, closer than the radius of the sun,” Stone explains. Over time, plasma from the star is stripped off and falls into the black hole, forming what is known as an accretion disk, or a rotationally supported plasma flow. In an active galactic nucleus, a supermassive black hole which may be millions or billions of times more massive than the sun is accreting matter from the inner regions of the galaxy which hosts it. “As matter falls into the gravitational potential of the black hole, it releases a tremendous amount of energy, which makes the plasma very, very hot,” Stone says. “It produces radiation that we can observe on Earth.”
In addition to radiation, the accretion also produces collimated outflows of matter, or jets, which can “affect the surroundings, and potentially can impact the growth of the black hole over cosmic time and in the case of active galactic nuclei, also the environment of the black hole, including the far reaches of the galaxy which host it.” Stone and his team hope, in developing numerical models to describe the behavior, to quantify the rate of accretion, the fraction of mass that is subsumed into the black hole vs. blown out as a jet, and--perhaps most relevant to projects here on Earth--how much energy is produced by the accretion flows.
“There are aspects of the plasma physics of the problem that are relevant to fusion power,” Stone explains. As such, his team is involved in a collaboration involving the Department of Astrophysics, the Princeton Plasma Physics Laboratory, and the Max Planck Institutes in Germany which seeks to coordinate the insights of plasma scientists, fusion scientists and astrophysicists. “Some of the basic processes of the plasma--for example, turbulence; magnetic reconnection, which is a process by which a magnetic field is destroyed in a plasma; and also particle acceleration and propagation--these occur in black hole accretion flows, but also in a tokamak, in a fusion device.”
“ Physicists are trying to create a star on Earth, and produce nuclear fusion in a laboratory, while that’s what we study: stars, accretion, and high-energy phenomena. So actually the physics is very similar, “ Stone says.
In attempting to quantify radiation transfer in general relativity using direct numerical simulation, Professor Stone relies on the massive computing power of machines including Mira at Argonne National Laboratory, which has 800,000 cores. In 2015, Stone and his team were awarded 47 million processing hours through a Department of Energy INCITE grant. Stone commends the DOE’s commitment to making these machines available for “pure science,” and describes PICSciE’s high-performance computing resources on campus as critical to the development of software and code primed to leverage the scale of the national supercomputer centers.
“Without PICSciE, we probably would not have been able to develop the software.” Stone explains. “We have to study three-dimensional magnetohydrodynamical flows in general relativity, including three-dimensional, time-dependent radiation transport. There’s a lot going on.”
The project pushes the limits of computation in astrophysics, as well as our understanding of high-energy plasma physics. Given the partnership with PPPL and the Max Planck Institute, Stone is optimistic that work for the project will prove valuable in other contexts. “In terms of algorithms… obviously there’s not much application for general relativity in commercial software,” Stone says. “But the lessons learned in using big machines, and the lessons learned from plasma dynamics, are applicable for fusion science, potentially, and for computational fluid dynamics more generally.”
At heart, however, Stone-- an advocate for greater focus on computational science across the disciplines--views the project as fascinating in its own right.
“I think there’s just a basic, fundamental, human interest in understanding what’s out there in the universe, and figuring out how black holes form and grow. That’s just cool,” Stone says. “Interpreting observations of some unusual galaxy being affected by an accreting black hole at its core--that’s cool. So that’s what we’re trying to do.”
Learn more about PICSciE’s HPC resources, and visit Professor Stone’s website to learn more about this and other research projects.