Feeding frenzy: Study reveals the eating habits of black holes

A groundbreaking study led by Northwestern University has recently reshaped astrophysicists’ understanding of the “feeding habits” of supermassive black holes. 

Contrary to earlier beliefs that these black holes “eat” slowly, new high-definition 3D simulations suggest a much quicker consumption rate.

Consumption cycle

According to new high-resolution 3D simulations, spinning black holes contort the surrounding space-time, ultimately ripping apart the gas whirlpool – known as the accretion disk – that orbits and nourishes them. This tumult results in the formation of inner and outer subdisks. 

The black holes voraciously consume the inner subdisk first. Subsequently, debris from the outer subdisk cascades inward, filling the void left by the ingested inner ring, marking the beginning of another consumption cycle. 

Erratic behavior 

Although scientists have long thought that this process spans hundreds of years, this new study shows that eating and refilling happens surprisingly quickly, taking just a few months. 

Such rapid cycles might elucidate the erratic behavior of some of the brightest objects in the night sky, such as quasars, which abruptly flare up and then disappear without explanation.

Drastic variation 

“Classical accretion disk theory predicts that the disk evolves slowly,” said lead author Nick Katz, a graduate student in astronomy at Northwestern. “But some quasars – which result from black holes eating gas from their accretion disks – appear to drastically change over time scales of months to years.”

“This variation is so drastic. It looks like the inner part of the disk – where most of the light comes from – gets destroyed and then replenished.”

“Classical accretion disk theory cannot explain this drastic variation. But the phenomena we see in our simulations potentially could explain this. The quick brightening and dimming are consistent with the inner regions of the disk being destroyed.”

Past theories

Historically, understanding the physics of accretion disks – the orbiting gas structures that feed black holes – has been extremely challenging. Past theories struggled to delineate the intermittent brightness and sudden dimming of these disks. 

As a consequence, previous research has often oversimplified their structure and behavior, treating them as much more orderly structures as they actually are. The longstanding assumption was that gas and particles orbited black holes in the same plane as the black hole and in the same direction as the black hole’s spin. 

“For decades, people made a very big assumption that accretion disks were aligned with the black hole’s rotation,” Katz said. “But the gas that feeds these black holes doesn’t necessarily know which way the black hole is rotating, so why would they automatically be aligned? Changing the alignment drastically changes the picture.”

Focus of the study 

In one of the most detailed simulations of its kind, the researchers found that the region around black holes is in fact highly chaotic and tumultuous. 

Using Summit, a leading supercomputer at Oak Ridge National Laboratory, they crafted a realistic model that factored in gas dynamics, magnetism, and general relativity. 

What the researchers discovered 

The results highlighted the effects of frame-dragging, a phenomenon where spinning black holes drag surrounding space, creating an environment where gas from different parts of the disk collide, driving material closer to the black hole.

“Black holes are extreme general relativistic objects that affect space-time around them. So, when they rotate, they drag the space around them like a giant carousel and force it to rotate as well – a phenomenon called ‘frame-dragging.’ This creates a really strong effect close to the black hole that becomes increasingly weaker farther away,” Katz explained.

Violent shocks 

Frame-digging makes the entire disk wobble in circles, in a similar way in which a gyroscope precesses. However, since the inner disk attempts to wobble much faster than the outer parts, this mismatch of forces causes the entire disk to warp, making gas from different parts of the disk to collide. These collisions create bright shocks which violently drive material increasingly closer to the black hole.

As the warping becomes more pronounced, the accretion disk’s innermost region continues to wobble increasingly faster until it separates from the rest of the disk. Afterwards, the subdisks start evolving independently from one another at different speeds and angles like the wheels in a gyroscope.

“When the inner disk tears off, it will precess independently. It precesses faster because it’s closer to the black hole and because it’s small, so it’s easier to move,” Katz said.

Feeding frenzy

In the simulation, the “feeding frenzy” truly begins in the tearing region, where the inner and outer subdisks split. 

“There is competition between the rotation of the black hole and the friction and pressure inside the disk. The tearing region is where the black hole wins. The inner and outer disks collide into each other. The outer disk shaves off layers of the inner disk, pushing it inwards,” Katz explained.

The subdisks now meet at varying angles. Material from the outer disk cascades onto the inner disk. This additional weight propels the inner disk closer to the black hole, leading to its consumption. Subsequently, the gravitational force of the black hole attracts gas from the outer section to replenish the previously emptied inner area.

Changing-look quasars 

According to the experts, the rapid sequences of eat-refill-eat might provide insights into the mysterious “changing-look” quasars. 

Defined by their intense luminosity, quasars release energy that is about 1,000 times greater than the cumulative energy of the 200 billion to 400 billion stars in the Milky Way. Changing-look quasars take this extremity a notch higher. 

Remarkably, these celestial objects seem to switch between their bright and dark states in just a few months – quite a short time span for a regular quasar.

While classical theories present certain assumptions about the rate at which accretion disks brighten or dim, the observed behaviors of changing-look quasars hint at an even brisker transformation pace.

“The inner region of an accretion disk, where most of the brightness comes from, can totally disappear – really quickly over months,” Kaaz said. 

“We basically see it go away entirely. The system stops being bright. Then, it brightens again and the process repeats. Conventional theory doesn’t have any way to explain why it disappears in the first place, and it doesn’t explain how it refills so quickly.”

Enduring mysteries 

However, these novel simulations offer more than just a better understanding of quasars. They might also unravel some of the enduring mysteries surrounding black holes.

“How gas gets to a 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,” Katz concluded.

The study is published in The Astrophysical Journal.

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