A team of astronomers has managed to remotely calculate the speed of rotation of a supermassive black hole thanks to the object’s chance encounter with a star, which instantly destroyed it.
All black holes have spin, which they develop through their interactions with other matter in space. When black holes grow by accumulating matter, they can spin at greater speeds; when they grow through mergers with other massive objects, they tend to slow down. In their recent work, the team managed to infer the spin of a supermassive black hole by measuring the wobble of its accretion disk after a star is disrupted (a polite word for torn apart) by the giant object. They found that the black hole’s spin was less than 25% 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 has grown by steady accretion of gas over billions of years will tend to spin high, while a black hole that has grown by mergers with other black holes should spin slowly,” said Dheeraj Pasham, an astronomer at the MIT and the lead author of the new paper, in an email to Gizmodo.
Black holes are regions of spacetime with gravitational fields so intense that not even light can escape it beyond a certain point, called an event horizon. But black holes also pull a lot of material into their environment, which is brilliant, allowing researchers to study the physics of these shadowy behemoths. The material – a collection of rocky debris, dust and gas – is the black hole’s accretion disk, and its bright glow allows the Event Horizon Telescope to see directly image the shadows of black holes.
“There are other modes in which supermassive black holes – and therefore their host galaxies – can grow over time, and each mode has a specific prediction for the spin distribution,” Pasham added. “So if we can measure the spin perturbation of supermassive black holes, we can determine how they (and their host galaxies) grew over cosmic time.”
Occasionally, unlucky stars that pass too close to a black hole become trapped by the tidal force and are torn to pieces; 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 believed 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 the source’s X-ray emissions peaked every 15 days. This led the team to conclude that these spikes occurred when the accretion disk was directly aligned with the telescope. Looking back at this apparent wobble of the accretion disk, the team took into account the estimated mass of the black hole and that of the star it had taken material from. They arrived at an estimate for 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 that also included Pasham found a signal that they associated with a black hole spinning at about half the speed of light. But as Pasham told Gizmodo, the nature of that signal is “still a mystery,” while the new measurement matches the black hole’s spin in line with relevant theories. Although 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 some of the most extreme objects in the universe.
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), 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 swallowed by the black hole. MIT has also produced 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 tricks. In 2021, a team including Pasham determined that there may have been a strange object in space known as ‘the Cow’. a black hole being born; in 2022 there appeared to be another flash from an object 8.5 billion light years away the farthest tidal disruption observed to dateand who saw his black hole fire a beam of superheated material directly at the Earth.
The team will continue to catalog tidal disruption events, with the aim of discovering the spin distribution of supermassive black holes. The arc to understanding the black hole universe is long, but deciphering their physics could help us unravel some of the universe’s greatest mysteries.
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