While what exactly goes on within the event horizon of a black hole is still well within the realm of theoretical physics (and it’s said that at the very heart of a black hole physics as we know it gets a serious kick in the pants) researchers are learning more and more about what happens in the immediate vicinity around a black hole, within the flattened disk of superheated material falling inexorably in toward the center. Using supercomputers, scientists can model the behavior of black holes’ accretion disks and see how gas behaves as it gets accelerated and drawn inward, heated to millions and even billions of degrees.
Here, an animation shows the activity around an active, non-rotating stellar-mass black hole. Taking 27 days to complete on a supercomputer at UT Austin, it shows “a turbulent froth orbiting the black hole” at relativistic speeds — that is, very close to the speed of light. Using this data, scientists are able to see how a black hole heats gas and emits different kids of x-rays… it’s the next best thing to being there! (Actually, it’s probably a much better thing than being there.)
“Our work traces the complex motions, particle interactions and turbulent magnetic fields in billion-degree gas on the threshold of a black hole, one of the most extreme physical environments in the universe,” said lead researcher Jeremy Schnittman, an astrophysicist at NASA’s Goddard Space Flight Center.
Gas falling toward a black hole initially orbits around it and then accumulates into a flattened disk. The gas stored in this disk gradually spirals inward and becomes greatly compressed and heated as it nears the center. Ultimately reaching temperatures up to 20 million degrees Fahrenheit (12 million C) — some 2,000 times hotter than the sun’s surface — the gas shines brightly in low-energy, or soft, X-rays.
For more than 40 years, however, observations have shown that black holes also produce considerable amounts of “hard” X-rays, light with energy tens to hundreds of times greater than soft X-rays. This higher-energy light implies the presence of correspondingly hotter gas, with temperatures reaching billions of degrees.
The new study bridges the gap between theory and observation, demonstrating that both hard and soft X-rays inevitably arise from gas spiraling toward a black hole.
Working with Julian Krolik, a professor at Johns Hopkins University in Baltimore, and Scott Noble, a research scientist at the Rochester Institute of Technology in Rochester, N.Y., Schnittman developed a process for modeling the inner region of a black hole’s accretion disk, tracking the emission and movement of X-rays, and comparing the results to observations of real black holes.
Noble developed a computer simulation solving all of the equations governing the complex motion of inflowing gas and its associated magnetic fields near an accreting black hole. The rising temperature, density and speed of the infalling gas dramatically amplify magnetic fields threading through the disk, which then exert additional influence on the gas.
“Black holes are truly exotic, with extraordinarily high temperatures, incredibly rapid motions and gravity exhibiting the full weirdness of general relativity. But our calculations show we can understand a lot about them using only standard physics principles.”
– Julian Krolik, professor at Johns Hopkins University
The result is a turbulent froth orbiting the black hole at speeds approaching the speed of light. The calculations simultaneously tracked the fluid, electrical and magnetic properties of the gas while also taking into account Einstein’s theory of relativity.
Using the data generated by Noble’s simulation, Schnittman and Krolik developed tools to track how X-rays were emitted, absorbed, and scattered throughout both the accretion disk and the corona region. Combined, they demonstrate for the first time a direct connection between magnetic turbulence in the disk, the formation of a billion-degree corona, and the production of hard X-rays around an actively “feeding” black hole.
Lights in the Dark 