Suction in the jaws of gravity: Some black holes are particularly voracious

Suction in the throat gravity
Some black holes are particularly voracious

September 23, 2023 at 2:10 pm

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Some black holes in space appear to swallow more matter than others. Researchers now offer an explanation for this. It also explains why some so-called quasars lose their brightness.

The relativistic effect helps some black holes gobble up matter more quickly than previously thought: as they spin they pull in the space around them, tearing the disk of hot gas surrounding them in two and thus causing the black hole to fall more quickly. In their gravity-fed jaws. the mentioned A research team from the United States of America and the Netherlands relied on computer simulations in the Astrophysical Journal. This could explain why some quasars change their brightness dramatically over a period of months.

Quasars are black holes millions or even billions of times the mass of our Sun located in the centers of distant galaxies. The gas flowing in from the outside forms a rotating disk — astronomers call an accretion disk — around the black hole. Friction causes the gas in this disk to heat up and glow brightly. That’s why quasars emit up to 1,000 times more light than the surrounding galaxy. Gas flows into the black hole from the inner edge of the disk, causing it to grow slowly.

A special role of different brightness

“In the conventional description, accretion disks are thin and axially symmetrical,” Nicholas Kaz of Northwestern University in Evanston (Ill.) and colleagues wrote. In particular, the disks of gas rotate in the equatorial plane of the black hole, because this simplifies the calculation of gas flows.

“But the gas flowing from the outside into the black hole doesn’t know how to rotate,” Kaz explains. “So why are they accumulating at the equatorial level?”

Although the simple model can successfully explain a large proportion of quasars, it fails when it comes to those that change their brightness rapidly. These organisms often change dramatically over the scale of months. “It’s as if the interior of the accretion disk has been completely destroyed and then refilled,” says Kaz, describing the phenomenon. “This kind of thing can’t be explained using the classical model of accretion disks.”

Computer simulation creates a new phenomenon

Kaz and his colleagues have now taken a new approach: Using a supercomputer, they simulated an accretion disk tilted strongly — by 65 degrees — toward the black hole’s equatorial plane. A new, completely unexpected phenomenon has appeared. As the black hole pulls on space around it as it spins, as predicted by Albert Einstein’s theory of general relativity, the accretion disc begins to oscillate — more strongly near the black hole than outside. This difference eventually becomes so large that the disc breaks into two pieces. The inner and outer discs roll around the black hole independently of each other.

They repeatedly collide with each other at their edges, resulting in a powerful flow of mass from the outer disk to the inner disk. As a result, gas from the inner disk falls into the black hole much faster – so quickly that it is completely destroyed. “In fact, we see that the inner disk disappears completely,” says Kaz. “The brightness of the quasar is thus extinguished.” Just as observed with rapidly changing quasars.

The whole process starts again

After the quasar apparently extinguishes, gas continues to flow from the outer disk into the now empty inner region – where a new accretion disk is formed. “The quasar slowly gets brighter again, and the whole process starts again,” Kaz says. Computer simulations fully describe the behavior observed in some quasars. “On the other hand, the conventional theory offers no explanation for why the inner disk disappeared in the first place, nor how it reappeared so quickly.”

However, Kaz and his colleagues have so far only been able to demonstrate this phenomenon in a single simulation with a pre-determined configuration of the black hole and accretion disk. According to the researchers, it is now necessary to perform several simulations with different black hole masses and different inclination angles of the accretion disks. This is the only way to know the situations in which the brightness of quasars can change so quickly.