A team of astronomers has been able to calculate the rotation rate of a distant supermassive black hole thanks to the object’s chance encounter with a star, which it promptly destroyed.
All black holes have spin, which they develop through their interaction with other matter in space. When black holes grow by accreting matter, they can spin at higher speeds; when they grow by merging with other massive objects, they tend to slow down. In their recent work, the team was able to infer the spin of a supermassive black hole by measuring the wobble of its accretion disk after a star has been destroyed—a polite word for torn apart—by the giant object. They found that the black hole spins at less than 25% of the speed of light — slow, at least for a black hole. The team’s research was published today in nature.
“The rotation of a black hole is related to its evolution. For example, a black hole that grew by steadily accreting gas over billions of years tends to spin high, while a black hole that grew by merging with other black holes should spin slowly,” said Dheeraj Pasham. , an MIT astronomer and lead author of the new paper, in an email to Gizmodo.
Black holes are regions of spacetime with gravitational fields so intense that even light cannot escape them beyond a certain point called the event horizon. But black holes also attract a lot of material near them, which is brilliant, allowing researchers to study the physics of these shadowy monsters. The material – a collection of rocky debris, dust and gas – is the black hole’s accretion disk, and its bright glow is what makes it possible for the Event Horizon telescope to direct image of black hole shadows.
“There are other regimes in which supermassive black holes – and therefore their host galaxies – can grow with time, and each regime has a specific spin distribution prediction,” Paschamp added. “Thus, if we can measure the spin disruption of supermassive black holes, we can constrain how they (and their host galaxies) have grown over cosmic time.”
Sometimes unlucky stars that pass too close to a black hole are caught up in its tidal force and torn apart; part of the star may be ejected into space, while part of it is stretched into an abundance of superheated stellar material that becomes part of the black hole’s accretion disk.
The spinning giant was discovered in February 2020 when the Zwicky Transient Facility detected a flash of light from an object 1 billion light-years from Earth. The team studied the light source, which they believe to be a tidal disturbance, for more than 200 days using NASA’s NICER telescope, which observes the cosmos at X-ray wavelengths.
The group found that X-ray emissions from the source peaked every 15 days. This led the team to conclude that these peaks appear when the accretion disk is directly aligned with the telescope. Working backwards from this apparent wobble of the accretion disk, the team took into account the approximate mass of the black hole as well as that of the star from which it had collected material. They arrived at an estimate of the spin of the black hole itself.
This is not the first time that the spin of a black hole has been calculated; in 2019, a team including Pasham found a signal that they associate with a black hole rotating at about half the speed of light. However, as Pascham told Gizmodo, the nature of this signal “is still a mystery,” while the new measurement matches the black hole’s rotation in accordance with relevant theories. While a black hole spinning at a quarter of the speed of light (167,654,156 miles per hour, or 74,948,114 meters per second) is still very fast in our ordinary human terms, we must remember that these are one of the most extreme objects in the universe.
Pascham added that a black hole cannot spin faster than 94% of the speed of light, or 630,379,631.62 mph (281,804,910.52 m/s), because Kip Thorne calculated in 1974. This maximum is due to the amount of torque on the black hole generated by radiation emitted by the accretion disk and absorbed by the black hole. MIT has also created a helpful video to walk people through the new findings, which you can watch below:
Flashes of X-rays from distant objects in space are often a sign of black holes up to their usual shenanigans. In 2021, a team including Pascham determined that a strange object in space known as “The Cow” may have been a black hole is born; in 2022 there was another flash from an object 8.5 billion light years away the most distant tidal disturbance event ever observedand who saw its black hole shoot a jet of superheated material directly toward Earth.
The team will continue to catalog tidal perturbation events in order to understand the spin distribution of supermassive black holes. The arc toward understanding the universe of black holes is long, but deciphering their physics can help us unravel some of the universe’s greatest mysteries.
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