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Sagittarius A* (image courtesy NASA)
Physicists at PPPL and Princeton University have developed a rigorous new method for modeling the accretion disk that fuels the supermassive black hole at the center of our Milky Way galaxy. The method provides a much-needed foundation for simulation of the extraordinary processes involved.
Accretion disks are clouds of plasma that orbit and gradually swirl into massive bodies such as black holes — intense gravitational fields produced by stars that collapse to a tiny fraction of their original size. These collapsed stars are bounded by an “event horizon,” from which not even light can escape. As accretion disks flow toward event horizons, they power some of the brightest and most energetic sources of electromagnetic radiation in the universe.
The colossal black hole at the center of the Milky Way — called “Sagittarius A*” because it is found in the constellation Sagittarius — has a gravitational mass that is four million times greater than our own sun. Yet the accretion disk plasma that spirals into this mass is “radiatively inefficient,” meaning that it emits much less radiation than one would expect.
“So the question is, why is this disk so quiescent?” asks Matthew Kunz, an assistant professor of astrophysical sciences at Princeton University, a physicist at PPPL and lead author of the research published in the journal Physical Review Letters. Co-authors include James Stone, Princeton professor of astrophysical sciences, and Eliot Quataert, director of theoretical astrophysics at the University of California, Berkeley.
According to the researchers, the reason lies in the fact that the plasma in the Sagittarius A* accretion disk is so hot and dilute that it is collisionless, meaning that the trajectories of protons and electrons inside the plasma rarely intersect. This distinguishes the Sagittarius A* disk from brighter and more radiative collisional disks that orbit other black holes.
However, traditional formulas for modeling collisional disks don’t work for the collisionless Sagittarius A*. So researchers replaced the traditional formulas that treat the motion of collisional plasmas as a macroscopic fluid with a method called “kinetic” that traces the paths of individual collisionless particles. This complex approach produced a set of equations better able to model behavior of the disk that orbits the supermassive black hole.
The goal of the new method “will be to produce more predictive models of the emission from black-hole accretion at the galactic center for comparison with astrophysical observations,” said Kunz. Such observations come from instruments such as the Chandra X-ray observatory, an Earth-orbiting satellite that NASA launched in 1999, and the upcoming Event Horizon Telescope, an array of nine Earth-based radio telescopes located in countries around the world.
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