The study was published today (Sept. 20) in The Astrophysical Journal.

According to new high-resolution 3D simulations, spinning black holes twist up the surrounding space-time, ultimately ripping apart the violent whirlpool of gas (or accretion disk) that encircles and feeds them. This results in the disk tearing into inner and outer subdisks. Black holes first devour the inner ring. Then, debris from the outer subdisk spills inward to refill the gap left behind by the wholly consumed inner ring, and the eating process repeats.

One cycle of the endlessly repeating eat-refill-eat process takes mere months — a shockingly fast timescale compared to the hundreds of years that researchers previously proposed.

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

“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. 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.”

Kaaz is a graduate student in astronomy at Northwestern’s Weinberg College of Arts and Sciences and member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Kaaz is advised by paper co-author Alexander Tchekhovskoy, an associate professor of physics and astronomy at Weinberg and a CIERA member.

Mistaken assumptions
Accretion disks surrounding black holes are physically complicated objects, making them incredibly difficult to model. Conventional theory has struggled to explain why these disks shine so brightly and then abruptly dim — sometimes to the point of disappearing completely.

Previous researchers have mistakenly assumed that accretion disks are relatively orderly. In these models, gas and particles swirl around the black hole — in the same plane as the black hole and in the same direction of the black hole’s spin. Then, over a time scale of hundreds to hundreds of thousands of years, gas particles gradually spiral into the black hole to feed it.

“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.”
— Nick Kaaz, lead author

“For decades, people made a very big assumption that accretion disks were 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 hole is rotating, so why would they automatically be aligned? Changing the alignment drastically changes the picture.”

The researchers’ simulation, which is one of the highest-resolution simulations of accretion disks to date, indicates that the regions surrounding the black hole are much messier and more turbulent places than previously thought.