The new discovery may explain why quasars burn out and fade so quickly

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A new study shows that by pulling in spacetime, supermassive black holes tear apart the violent spirals of debris around them (or accretion disks) leading to an inner and outer sub-disc. Credit: Nick Goss/Northwestern University

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A new study shows that by pulling in spacetime, supermassive black holes tear apart the violent spirals of debris around them (or accretion disks) leading to an inner and outer sub-disc. Credit: Nick Goss/Northwestern University

A new Northwestern University-led study is changing the way astrophysicists understand the feeding habits of supermassive black holes.

While previous researchers have assumed that black holes eat slowly, the new simulations show that black holes eat food much faster than conventional understanding.

The study is published in the title „Nozzle shocks, disc tearing and streamers drive rapid accretion in 3D GRMHD simulations of warped thin discs.” The Astrophysical Journal.

According to new high-resolution 3D simulations, rotating black holes twist spacetime around them, eventually tearing apart the violent vortex (or accretion disk) of gas that feeds around them. This results in the disc tearing into inner and outer subcircles. Black holes first swallow the inner ring. Then, debris from the outer companion disk scatters inward to refill the gap left by the completely consumed inner ring, and the eating process repeats.

One cycle of the endlessly repeating feeding-replenishing-eating process takes mere months—a staggeringly fast time span compared to the hundreds of years researchers had previously proposed.

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

„Classical accretion disk theory predicts that the disk evolves slowly,” said Northwestern’s Nick Gass, who led the study. „But some quasars—the result of black holes eating gas from their accretion disks—vary sharply on timescales of months to years. This variability is very drastic. It looks like the inner part of the disk—where most of the light comes from—is destroyed and then replenished. Classical accretion disk theory explains this drastic change. The contrast cannot be explained. But the phenomena we see in our simulations may explain it. The rapid brightening and dimming is consistent with the destruction of the inner parts of the disk.”

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Kass is a graduate student in astronomy at Northwestern’s Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Studies and Research (CIERA) in the Department of Astrophysics, and is advised by paper co-author Alexander Tsekovsky, associate professor in the Department of Physics and Astronomy. Weinberg and member of CIERA.

Incorrect assumptions

Accretion disks around black holes are physically complex objects that are incredibly difficult to model. Conventional theories struggle with why these discs shine so brightly and then suddenly dim — sometimes to the point of disappearing entirely.

Previous researchers mistakenly assumed that accretion discs were relatively regular. In these models, the gas and particles rotate around the black hole—in the same plane as the black hole and in the same direction as the black hole’s rotation. Then, over hundreds to hundreds of thousands of years, gas particles gradually spiral into and feed on the black hole.

„For decades, people made the huge assumption that accretion disks are aligned with the rotation of the black hole,” Gauss said. „But the gas feeding these black holes doesn’t know which way the black hole is spinning, so why would they automatically align? Changing the alignment changes the picture drastically.”

The researchers’ simulation, one of the highest-resolution simulations of accretion discs to date, indicates that the region around a black hole is a much more chaotic and turbulent place than previously thought.

Like a gyroscope, like a plate

Using Summit, one of the world’s largest supercomputers located at Oak Ridge National Laboratory, the researchers conducted a 3D general relativistic magnetohydrodynamics (GRMHD) simulation of a thin, tilted accretion disk. Although previous simulations were not powerful enough to cover all the physics needed to create a realistic black hole, the Northwestern-led model included gas dynamics, magnetic fields, and general relativity.

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This still from a simulation shows how the accretion disk of a supermassive black hole can tear into two sub-discs, which are misshapen in this image. Credit: Nick Goss/Northwestern University

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This still from a simulation shows how the accretion disk of a supermassive black hole can tear into two sub-discs, which are misshapen in this image. Credit: Nick Goss/Northwestern University

„Black holes are very general relativistic objects that affect the spacetime around them,” Gauss said. „So, as they spin, they pull the space around them like a giant carousel, forcing it to spin too – a phenomenon known as 'frame-tracking’. This creates an effect that is very strong near the black hole and becomes weaker far away.”

Frame-dragging causes the entire disk to oscillate in circles, similar to how a gyroscope moves forward. But the inner disk tends to wobble much faster than the outer ones. This mismatch of forces distorts the entire disc, causing gas to collide from different parts of the disc. Collisions create bright shocks that violently propel objects closer and closer to the black hole.

As the warping intensifies, the inner part of the accretion disk wobbles faster and faster until it separates from the rest of the disk. Then, according to the new simulations, the sub-discs begin to form independently of each other. Instead of moving steadily like a flat plate around a black hole, the subdiscs oscillate independently at different speeds and angles, like the wheels on a gyroscope.

„When an internal disc tears, it progresses independently,” said Kass. „It’s moving faster because it’s closer to the black hole and it’s smaller and easier to move around.”

’Where the black hole wins’

According to the new simulation, the tearing region—where the inner and outer subdiscs are torn apart—is where the feeding frenzy truly begins. As friction tries to hold the disc together, the twisting of spacetime by the spinning black hole wants to tear it apart.

„There is a competition between the rotation of the black hole and the friction and pressure inside the disk,” Gauss said. „The tear-off is where the black hole wins. The inner and outer discs collide with each other. The outer disc shaves off layers of the inner disc, pushing them inward.”

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Now the subcircles intersect at different angles. The outer disc pours material over the inner disc. This extra mass pushes the inner disk toward the black hole, where it is swallowed up. Then, the black hole’s own gravity pulls gas from the outer region to refill the now-empty inner region.

The quasar connection

These fast feeding-refeeding cycles may explain quasars with so-called „variable appearances,” Kass said. Quasars are extremely luminous objects, emitting 1,000 times more energy than the 200 billion to 400 billion stars in the entire Milky Way. Looking quasars are even more extreme. They appear to turn on and off for months at a time – a short time for a typical quasar.

Although classical theory has made assumptions about how quickly accretion disks form and change brightness, observations of quasars with changing origins indicate that they actually form much faster.

„The inner part of the accretion disk, where most of the luminosity comes from, disappears completely—very quickly in months,” Kass said. „Essentially we see it disappear completely. The system stops being bright. Then, it gets bright again and the process repeats. Conventional theory has no way to explain why it disappears in the first place, and it doesn’t explain how. It fills up very quickly.”

The new simulations could not only explain quasars, but also answer current questions about the mysterious nature of black holes.

„How gas feeds a black hole is a central question in accretion-disk physics,” Gauss said. „If you know how that happens, it tells you how long the disk lasts, how bright it is, and what the light should look like when we observe it through telescopes.”

More information:
Tip shocks, disc tearing, and streamers drive rapid accretion of warped thin discs in 3D GRMHD simulations, The Astrophysical Journal (2023) on arXiv: DOI: 10.48550/arxiv.2210.10053

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