Gluttonous Black Holes Eat Faster Than Thought. Does That Explain Quasars?

At the heart of large galaxies like our Milky Way, there resides a supermassive black hole (SMBH.) These behemoths draw stars, gas, and dust toward them with their irresistible gravitational pull. When they consume this material, there’s a bright flare of energy, the brightest of which are quasars.

While astrophysicists think that SMBHs eat too slowly to cause a particular type of quasar, new research suggests otherwise.

The research is published in The Astrophysical Journal, and the title is “Nozzle Shocks, Disk Tearing, and Streamers Drive Rapid Accretion in 3D GRMHD Simulations of Warped Thin Disks.” The lead author is Nick Kaaz, a graduate student in astronomy at Northwestern University.

The new research is based on computer simulations called 3D general relativistic magnetohydrodynamics (GRMHD) simulations. A powerful supercomputer called Summit, one of the world’s fastest computers and a 340-ton behemoth itself, carried out the simulations at the Oak Ridge National Laboratory.

“It looks like the inner part of the disk — where most of the light comes from — gets destroyed and then replenished.”

Nick Kaaz, lead author, Northwestern University

When SMBHs draw material toward themselves, the material doesn’t fall directly into the hole. Instead, it forms an accretion disk, a whirling disk of gas and dust. The rotating disk of material heats up and gives off electromagnetic energy that we can see with different telescopes when it falls into the black hole. When they’re extremely luminous, they’re called quasars.

But these disks are very difficult to study. They’re extremely complicated. Sometimes they flare brightly and then suddenly grow dimmer, on timescales of mere months, which is an extraordinarily short period of time for an astrophysical object. Current theory can’t explain it.

The new simulations show that SMBHs eat faster than thought. The encounter between the disk and the hole is violent and tears the whirlpool of gas into two pieces, an inner sub-disk and an outer sub-disk.

The SMBH consumes the inner sub-disk first. It takes only a matter of months for the black hole to consume this inner ring of swirling dust, and as it does so, it releases an enormous amount of energy as a quasar. Then material from the outer disk moves inward forming a new inner disk, and the entire process repeats.

If the simulations are correct, and this eat, refill, eat cycle takes mere months, instead of hundreds of years, then it can explain some observed quasars that last only a few months. There are different types of quasars, and astrophysicists have struggled to explain what they call “changing look” quasars. Up to 50% of observed quasars are changing look quasars, and these results seem to explain them.

“Classical accretion disk theory predicts that the disk evolves slowly,” said Northwestern’s Nick Kaaz, who led the study. “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.”

This is much different than how classical theory describes black hole accretion disks. In existing theory, matter near a black hole settles into a fairly predictable disk that’s hot, bright, and rotating the same way as the black hole. But it can’t explain changing look quasars.

“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.”

Black holes are powerful, complicated, and difficult to study objects. They literally warp space-time around them, according to Einstein’s general theory of relativity, and that means we can throw our intuitive ideas out the window.

“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,’ ” Kaaz said. “This creates a really strong effect close to the black hole that becomes increasingly weaker farther away.”

This is what allows the SMBH to tear an inner disk away from the outer disk and consume it. Relativistic frame-dragging makes the entire disk wobble as it spins, but since the inner regions are more strongly affected, they wobble more strongly. “When the inner disk tears off, it will precess independently,” Kaaz said. “It precesses faster because it’s closer to the black hole and because it’s small, so it’s easier to move.”

Forces in the disk are mismatched, and that warps the entire disk. Gas from different parts of the disk collide and create bright shocks, according to the Summit simulations. Those shocks drive more material toward the black hole.

When the disk tears into two, it also creates what researchers call streamers. These streamers rain down on the inner sub-disk on both sides. This drives further accretion into the inner disk, increasing the size of the black hole’s meal. “After they collide, some of the material from the streamers “spills” over the inner sub-disk,” the team writes in their paper.

This is a lot different from a more classical understanding of black holes. From that viewpoint, an SMBH’s unified disk moves predictably, and material sometimes falls into the black hole. But these simulations show that there’s nothing sedate about the process. The powerful forces of nature near the black hole rip the disk into two, and they begin to wobble independently, unleashing shocks and streamers that further complicate the process.

This is a battle between the black hole’s immense gravity and warping of space-time and the energy in the spinning disk. The black hole always wins, and that’s what tears the disk apart. The region where the tearing occurs is critical. “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.”

The inner and outer disks are misaligned, and this complicates things even more. They intersect at different angles, and the outer disk pours material onto the top of the inner disk. The increased mass helps drive the inner disk toward the black hole, hastening its meal. With the inner disk disappearing into the hole, material from the outer disk takes its place and fills up the inner disk again.

“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.”

But if this simulation is correct, we now know what’s happening. It also sheds more light on black holes in general, not just quasars.

Believe it or not, astrophysicists still don’t specifically know how gas gets from the accretion disk to the black hole. Kaaz calls it “the central question in accretion-disk physics.” But this research is getting closer to this very important answer in astrophysics.

“If you know how that happens,” Kaaz said, “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.”