New research finds that supermassive black holes eat up surrounding matter faster than previously thought. The insight comes from high-resolution simulations and could explain why quasars flare and fade so quickly. A new study led by Northwestern University is changing the way astrophysicists understand the feeding habits of supermassive black holes. Previous researchers assumed that black holes eat slowly, but new simulations show that black holes gobble up food much faster than conventional wisdom thought.
The research was published in the Astrophysical Journal on September 20.
Simulation Insights
According to new high-resolution three-dimensional simulations, a spinning black hole warps the space-time around it, eventually tearing apart the violent vortex of gas, or accretion disk, that surrounds and feeds the black hole. This causes the accretion disk to tear into two sub-disks, an inner one and an outer one. The black hole devours the inner ring first. Then, fragments of the outer sub-disk spill inward, refilling the void left by the fully devoured inner ring, and the devouring process repeats itself.
A cycle of endlessly repeating the process of "eat" - "eat" - "eat again" takes only a few months - a shockingly fast timescale compared to the hundreds of years previously proposed by researchers.
The new discovery helps explain the dramatic behavior of some of the brightest objects in the night sky, including quasars, which suddenly burst into flames and then disappear for no apparent reason.
Nick Kaaz of Northwestern University, who led the study, said: "Classical accretion disk theory predicts that the accretion disk will evolve slowly. But some quasars - black holes devouring gas in the accretion disk - appear to undergo dramatic changes on timescales of months to years. The changes are so dramatic. It looks like the inner part of the accretion disk - where most of the light comes from - is destroyed and then replenished. Classical accretion disk theory cannot explain this dramatic change, but it is possible that the rapid brightening and dimming seen in our simulations is consistent with the destruction of the inner regions of the disk."
Kaaz is a graduate student in astronomy at Northwestern University's Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Kaaz was supervised by co-author Alexander Tchekhovskoy, associate professor of physics and astronomy at Weinberg College and a CIERA member.
wrong assumption
The accretion disks surrounding black holes are physically very complex objects and therefore difficult to model. Conventional theories have struggled to explain why these disks shine so brightly and then suddenly dim - and sometimes disappear entirely.
Previous researchers mistakenly believed that accretion disks were relatively ordered. In these models, gas and particles orbit the black hole -- in the same plane as the black hole and in the same direction as the black hole's spin. Then, over timescales of hundreds to hundreds of thousands of years, gas particles gradually spiral into the black hole, feeding it.
"For decades, people made a big assumption that the accretion disk was aligned with the black hole's rotation," Kaaz said. "But the gas that feeds these black holes doesn't necessarily know which way the black holes are spinning, so why do they align themselves? Changing the alignment changes the picture dramatically."
The researchers' simulation, one of the highest-resolution simulations of an accretion disk to date, shows that the region around a black hole is much more chaotic and turbulent than previously thought.
More like a gyroscope than a plate
Researchers used Summit, one of the world's largest supercomputers at Oak Ridge National Laboratory, to perform three-dimensional general relativistic magnetohydrodynamics (GRMHD) simulations of a tilted thin accretion disk. While previous simulations were not powerful enough to include all the necessary physics needed to construct a real black hole, the Northwestern-led model incorporates gas dynamics, magnetic fields and general relativity to build a more complete picture.
"Black holes are extreme general relativistic objects that affect the surrounding space-time," Kaaz said. "So when black holes spin, they drag the space around them like a giant merry-go-round, forcing space to spin too - a phenomenon known as 'frame drag'. This creates a very strong effect close to the black hole, and becomes weaker and weaker further away."
Frame drag causes the entire disk to wobble in a circle, similar to the preprocessing of a gyroscope. But the inside of the disk swings much faster than the outside. This force mismatch causes the entire disk to warp, causing gases from different parts of the disk to collide. The strong shock waves created by the collision violently push material closer and closer to the black hole.
As the warping becomes more severe, the innermost region of the accretion disk continues to wobble faster and faster until it breaks away from the rest of the disk. Then, based on new simulation results, the subdisks begin to evolve independently of each other. Instead of moving smoothly together like flat plates around the black hole, the subdisks wobble independently at different speeds and angles like the wheels in a gyroscope.
"As the inner disk tears apart, it preprocesses independently. Its forward motion is faster because it is closer to the black hole, and because it is smaller, it can move more easily," Kaaz said.
Where black holes win
According to the new simulations, the tear zone—where the inner and outer subdisks disconnect—is where the feeding frenzy really begins. While friction tries to hold the disk together, the spinning black hole's distortion of spacetime tries to tear it apart.
"There is a competition between the black hole's rotation and the friction and pressure inside the disk," Katz said. "The tear zone is where the black hole wins. The inner and outer disks collide with each other. The outer disk shaves away layers of the inner disk, pushing it inward."
Now, the sub-discs intersect at different angles. The outer disk dumps the material onto the inner disk. This extra mass also pushes the inner disk toward the black hole, swallowing it up. The black hole's own gravity then pulls gas from the outer regions toward the now-empty inner regions, refilling it.
Connections between quasars
This rapid cycle of "eat-and-eat-eat" could explain the so-called "changing appearance" of quasars, Katz said. Quasars are extremely bright objects that emit 1,000 times the energy of the 200 billion to 400 billion stars in the entire Milky Way. Changing quasars are even more extreme. They appear to flicker on and off over the course of several months - an extremely short time for a typical quasar.
Although classical theory makes assumptions about the speed of accretion disk evolution and brightness changes, observations of distorted quasars suggest that they actually evolve much faster.
"The inner region of the accretion disk, which is where most of the brightness comes from, can disappear completely -- rapidly within a matter of months. We can basically see it disappear completely. The system no longer lights up. Then, it gets brighter again, and the process repeats. Conventional theory can't explain why it disappears in the first place, or how it refills so quickly."
The new simulation has the potential not only to explain quasars, but also to answer long-standing questions about the mysterious nature of black holes.
"How gas enters a black hole to power it is a central question in accretion disk physics," Katz said. "If you know how this happens, you can tell how long the disk lasts, how bright it is, and what the light should look like when we look at it with a telescope."