The rapid feast of black holes shocks scientists

New research reveals that supermassive black holes are consuming the material around them faster than previously thought. This insight, derived from high-resolution simulations, could explain why quasars burn up and fade away so quickly.

new Northwestern UniversityThe study he leads is changing the way astrophysicists understand the eating habits of supermassive black holes.

While previous researchers assumed that black holes eat slowly, new simulations suggest that black holes cover up food much faster than conventional understanding suggests.

The study was published on September 20 the Astrophysical Journal.

Simulation insights

According to new high-resolution 3D simulations, rotating black holes warp the spacetime surrounding them, eventually tearing apart the violent vortex of gas (or accretion disk) that surrounds and feeds them. This tears the disk into internal and external sub-disks. Black holes first devour the inner ring. Debris from the outer sub-disc then spills inward to fill the gap left by the completely consumed inner ring, and the eating process is repeated.

One cycle of this non-stop eating-refilling-eating process takes only months – a shockingly fast timescale compared to the hundreds of years previously suggested by researchers.

This new discovery could help explain the dramatic behavior of some of the brightest objects in the night sky, including quasars, which suddenly flare up and then disappear without explanation.

This simulation shows how a supermassive black hole’s accretion disk could split into two sub-disks, which are not aligned in this image. Credit: Nick Kaz/Northwestern University

“Classical accretion disk theory predicts that the disk evolves slowly,” said Nick Kaz of Northwestern University, who led the study. “But some quasars — which result from black holes eating gas from their accretion disks — appear to change radically over time over months to years. This difference is quite dramatic. The interior of the disk, where most of the light comes from, appears to be destroyed.” Then it regenerates. Classical accretion disk theory cannot explain this drastic difference. But the phenomena we see in our simulations can. The rapid brightening and dimming is consistent with the destruction of the inner regions of the disk.

Kaz 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). Kaz advised paper co-author Alexander Chekhovskoy, a Weinberg associate professor of physics and astronomy and a member of CIERA.

False assumptions

The accretion disks surrounding black holes are physically complex objects, which makes them very difficult to model. Traditional theory has struggled to explain why these discs shine brightly and then suddenly dim, sometimes to the point of disappearing altogether.

Previous researchers have incorrectly assumed that accretion disks are relatively organized. In these models, gases and particles rotate around Black hole – In the same plane as the black hole and in the same direction of rotation of the black hole. Then, over a time period of hundreds to hundreds of thousands of years, gas molecules gradually spiral into the black hole to feed it.

“How gas gets to the black hole to feed it is the central question in accretion disk physics. If you know how that happens, it will tell you how long the disk lasts, how bright it is, and what the light should look like when we observe it with telescopes.”
-Nick Kaz, lead author

“For decades, people have made a pretty big assumption that accretion disks correspond to black hole rotation,” Kaz said. “But the gas feeding these black holes doesn’t necessarily know which direction the black hole is rotating, so why would it automatically align? Changing the alignment radically changes the picture.”

The researchers’ simulation, which is one of the highest resolution simulations of accretion disks to date, suggests that the regions around a black hole are much more chaotic and turbulent places than previously thought.

More like a gyroscope, less like a painting

Using Summit, one of the world’s largest supercomputers located at Oak Ridge National Laboratory, the researchers performed a 3-D general magnetohydrodynamic (GRMHD) simulation of a thin, tilted accretion disk. While previous simulations were not robust enough to include all the physics needed to build a realistic black hole, the model led by Northwestern incorporates gas dynamics, magnetic fields, and general relativity to piece together a more complete picture.

“Black holes are extreme general relativistic objects that affect the spacetime surrounding them,” Kaz said. “So, when it spins, it pulls on the space around it like a giant carousel and forces it to spin as well — a phenomenon called ‘frame drag.’ This creates a really strong effect close to the black hole, which becomes increasingly weaker farther away.”

Pulling the frame makes the entire disc sway in circles, similar to how a gyroscope moves. But the inner disk wants to wobble much faster than the outer parts. This mismatch of forces causes the entire disk to warp, causing gas from different parts of the disk to collide. The collisions create bright shocks that violently push material closer and closer to the black hole.

As the twisting becomes more severe, the deeper region of the accretion disk continues to oscillate faster and faster until it separates from the rest of the disk. Then, according to the new simulations, the subdisks begin to evolve independently of each other. Instead of moving smoothly together like a flat plate surrounding a black hole, the subdisks sway independently at different speeds and angles like wheels on a gyroscope.

“When the inner disc ruptures, it will move on its own,” Kaz said. “It’s moving faster because it’s closer to the black hole and because it’s smaller, so it’s easier to move.”

“Where the black hole wins”

According to the new simulation, the rupture zone — where the inner and outer subdiscs separate — is where the feeding frenzy really begins. While friction tries to keep the disk together, the warping of spacetime by the rotating black hole wants to tear it apart.

“There is a competition between the black hole’s rotation and the friction and pressure inside the disk,” Kaz said. “The rupture zone is where the black hole wins. The inner and outer disks collide with each other. The outer disk shaves off the layers of the inner disk, pushing them inward.

Now the sub-disks intersect at different angles. The outer disc pours material over the inner disc. This extra mass also pushes the inner disk toward the black hole, where it is eaten. The black hole’s gravity then pulls gas from the outer region toward the now empty inner region to refill it.

Quasar contact

These rapid cycles of eating, filling and eating likely explain the quasar’s so-called “variable appearance,” Kaz said. Quasars are extremely luminous objects that emit 1,000 times more energy than the total energy milky way200 billion to 400 billion stars. Quasars with variable appearance are considered more extreme. They appear to turn on and off over a period of months, which is a small period of time for a typical quasar.

Although classical theory made assumptions about how quickly accretion disks evolve and change in brightness, observations of quasars with variable appearance suggest that they actually evolve much faster.

“The inner region of the accretion disk, where most of the brightness comes from, could disappear completely — very quickly, within months,” Kaz said. “We see it completely disappear. The system stops brightening. Then it lights up again and the process repeats. Conventional theory has no way of explaining why it disappeared in the first place, nor does it explain how it refills so quickly.”

Not only can the new simulations explain quasars, they could also answer persistent questions about the mysterious nature of black holes.

“How gas gets to the black hole to feed it is the central question in accretion disk physics,” Kaz said. “If you knew how that happens, it would tell you how long the disk lasts, how bright it is, and what the light should look like when we observe it with telescopes.”

Reference: “Nozzle shocks, disk tearing, and streamers lead to rapid accretion in 3D GRMHD simulations of thin twisted disks” by Nicholas Kaz, Matthew T. B. Liska, Jonathan Jacquemin Eade, Zachary L. Andalman, Jeboa Mosuki, Aleksandr Chekhovskoy, and Oliver Borth, September 20, 2023, Astrophysical Journal.
doi: 10.3847/1538-4357/ace051

The study was supported by the US Department of Energy and the National Science Foundation.