Do black holes eat differently than expected?

Supermassive black holes can apparently split up the swirl of debris surrounding them and then quickly devour the inner disk. © Nick Kaaz/Northwestern University

On the trail of sinister “table manners”: A new model simulation shows that black holes absorb their food faster and in more complex ways than previously assumed. Accordingly, the gravitational giants divide the disk of matter orbiting them into two partitions. The inner disc is then eaten and then reformed by inflow from the outer disc. The individual cycles of “eating-refilling-eating” only last a few months, according to the simulations. This process could now also explain unusually fast activity patterns in quasars, say the scientists.

Due to their gigantic mass and gravitational force, they bend space so much that not even light can escape: Black holes are among the most fascinating objects in the universe. In addition to the physical phenomena, their voracity is also a particularly exciting aspect: matter that falls under their spell is attracted, torn apart and ultimately forms a so-called accretion disk, which the black holes feed on. But what exactly is going on?

For a long time it was assumed that accretion disks around supermassive black holes were structured in a relatively orderly manner. In these models, gas and particles rotate in a disk around the black hole, and the rotation occurs in the same direction as the spin of the black hole. Over a period of hundreds to hundreds of thousands of years, the material from the accretion disk is then drawn spirally into the black hole - this is the traditional assumption about feeding behavior. But this straightforward model does not seem to fit some of the phenomena observed with black holes in recent years.

Simulation of astronomical meals

For this reason, researchers led by Nick Kaaz from Northwestern University in the USA have now dedicated a new model analysis to the processes in the accretion disk and feeding behavior. One of the world's largest supercomputers was used at the Oak Ridge National Laboratory in Tennessee. With the help of “Summit,” the scientists developed a 3D simulation of the dynamic processes in an accretion disk around a supermassive black hole. As they explain, earlier computers were not powerful enough to take into account the many physical aspects in more complex modeling. But according to the team, the new simulation based on general relativity has now been able to combine the gas dynamics, magnetic fields and other factors into a clearer picture of the processes.

“Black holes are extreme objects that affect the space-time around them in complex ways,” says Kaaz. This is exactly what was now reflected in the results of the new simulation, as he and his colleagues report: The regions around a black hole are much more chaotic and turbulent than previously assumed. Black holes were found to tear apart the massive whirlpool of matter and ultimately partition it. The complex gravitational and dynamic processes therefore lead to the accretion disk dividing into an inner and an outer sub-disk. The two units begin to move independently of each other: they develop different speeds and change their orientation.

Quickly consumed portions

According to the simulation, the black hole then quickly eats up the inner ring. A new formation then occurs: debris from the outer disk spills inward to fill the gap left by the completely consumed inner ring, the researchers report. This is followed by another meal. As can be seen from the simulations, the cycle of the repetitive process of eating-refilling-eating lasts only a few months. This means that supermassive black holes can “feed” much faster than previously thought.

As the researchers point out, the results can now also help to explain the abrupt lighting up and subsequent dimming of certain celestial objects, which was previously difficult to explain. “Some quasars – formed by black holes eating matter from their accretion disks – appear to change dramatically over timescales ranging from months to years. The classical accretion disk theory cannot explain this drastic variation. “But the phenomena we see in our simulations could now potentially explain this: the rapid brightening and dimming are consistent with the inner areas of the disk being destroyed,” says the astronomer.

Source: Northwestern University

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