A team of astronomers was 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 interactions with other matter in the universe. 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 rotation of a supermassive black hole by measuring the wobbles of its accretion disk after the star was disrupted—the polite word for rupture—by a gigantic object. They found that the black hole’s rotation was 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 linked to its evolution. For example, a black hole that grew by steadily accreting gas over billions of years tends to spin rapidly, while a black hole that grew by merging with other black holes should spin slowly,” said Dheeraj Pasham, an astronomer at MIT. lead author of a new email article at Gizmodo.
Black holes are regions of spacetime with such intense gravitational fields that not even light can escape them beyond a certain point called the event horizon. But black holes also pull a lot of material into their vicinity, which is great, allowing researchers to study the physics of these shadowy monsters. The material—a cluster of rocky debris, dust, and gas—is the black hole’s accretion disk, and its bright glow allows the Event Horizon Telescope direct image of black hole shadows.
“There are other regimes in which supermassive black holes – and thus their host galaxies – can grow over time, and each regime has a specific prediction for the spin distribution,” Pasham added. “So if we can measure the perturbations in the rotation of supermassive black holes, we can constrain how they (and their host galaxies) have grown over cosmic time.”
Occasionally, unlucky stars that pass too close to a black hole get caught in its tidal force and torn apart; part of the star may be ejected into space, while part is stretched into a mass of superheated stellar material that becomes part of the black hole’s accretion disk.
The rotating giant was detected 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 believed to be a tidal disruption event, for more than 200 days using NASA’s NICER telescope, which observes the universe at X-ray wavelengths.
The team found that the X-ray emission from the source peaks every 15 days. This led the team to conclude that these peaks occurred when the accretion disk was directly aligned with the telescope. Working back from this apparent fluctuation of the accretion disk, the team considered the approximate mass of the black hole as well as the mass of the star from which it picked up the material. They arrived at an estimate of the rotation 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 that included Pasham found a signal which connected to a black hole rotating at about half the speed of light. However, as Pasham told Gizmodo, the nature of this signal “is still a mystery,” while the new measurement matches the rotation of the black hole in line with relevant theories. While a black hole rotating 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 some of the most extreme objects. in space.
Pasham 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). Kip Thorne counted in 1974. This maximum is due to the magnitude of the torque on the black hole generated by radiation emitted from the accretion disk and absorbed by the black hole. MIT also created a helpful video to walk people through the new insights, 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 deceptions. In 2021, a team including Pasham discovered that a strange object in space known as a “cow” may have been a black hole is born; in 2022, another flare from an object 8.5 billion light years away showed up the most distant tidal disruption event to dateand which saw its black hole shoot a stream of superheated material directly at Earth.
The team will continue to catalog tidal disruption events with the aim of discovering the spin distribution of supermassive black holes. The curve to understanding the universe of black holes is long, but deciphering their physics could help us unravel some of the universe’s greatest mysteries.
More: Astronomers have discovered the heaviest stellar black hole in the Milky Way