Astronomers have made a record-breaking measurement of a black hole’s spin, one of the two fundamental properties of black holes. NASA’s Chandra X-ray Observatory shows that this black hole spins more slowly than most of its smaller cousins.
Supermassive black holes contain millions or even billions of times more mass than the Sun. Astronomers think that almost every large galaxy has a supermassive black hole in the middle. While the existence of supermassive black holes is not up for debate, scientists are still trying to understand how they grow and evolve. A crucial piece of information is how fast the black holes are spinning.
“Any black hole can be defined by just two numbers: its spin and its mass,” said Julia Sisk-Reynes of the Institute of Astronomy (IoA) at the University of Cambridge in the UK, who led the new study. “While that sounds pretty simple, it has proven incredibly difficult to calculate those values for most black holes.”
For this result, researchers observed X-rays bouncing off a disk of material swirling around the black hole in a quasar known as H1821+643. Quasars contain fast-growing supermassive black holes that generate large amounts of radiation in a small area around the black hole. Located in a cluster of galaxies about 3.4 billion light-years from Earth, the black hole of H1821+643 is located between about three and 30 billion solar masses, making it one of the most famous. In contrast, the supermassive black hole at the center of our galaxy weighs about four million suns.
The strong gravitational forces near the black hole change the intensity of X-rays at different energies. The greater the change, the closer the disk’s inner edge must be to the point where the black hole cannot return, known as the event horizon. Because a spinning black hole drags into space and spins matter closer than is possible for a non-spinning black hole, the X-ray data can show how fast the black hole is spinning.
“We found that the black hole in H1821+643 spins about half as fast as most black holes weighing between about one million and ten million suns,” said study co-author Christopher Reynolds, also of the IoA. “The million dollar question is, why?”
The answer may lie in how these supermassive black holes grow and evolve. This relatively slow spin supports the idea that the most massive black holes such as H1821+643 undergo most of their growth by merging with other black holes, or by drawing gas in random directions when their large disks are disrupted.
Supermassive black holes that grow in this way are likely to often undergo major changes of spin, including being slowed down or pulled in the opposite direction. The prediction is therefore that the most massive black holes should have a wider range of spin velocities than their less massive relatives.
On the other hand, scientists expect less massive black holes to gather most of their mass from a disk of gas orbiting them. Because such disks are expected to be stable, the incoming matter always approaches from a direction that will make the black holes spin faster until they reach the maximum speed possible, namely the speed of light.
“The moderate spin for this ultramassive object may be evidence of the violent, chaotic history of the universe’s largest black holes,” said study co-author James Matthews, also of the IoA. “It may also provide insight into what will happen to our galaxy’s supermassive black hole billions of years in the future, when the Milky Way collides with Andromeda and other galaxies.”
This black hole provides information that complements what astronomers have learned about the supermassive black holes we’ve seen in our galaxy and in M87, captured by the Event Horizon Telescope. In those cases, the masses of the black hole are well known, but the spider is not.
A paper describing these results from Sisk-Reynes and her collaborators appears in the Monthly Notices from the Royal Astronomical Society†
NASA’s Marshall Space Flight Center manages the Chandra program. The Chandra X-ray Center at the Smithsonian Astrophysical Observatory controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
Júlia Sisk-Reynés et al, Evidence for a moderate spin from X-ray reflection from the high-mass supermassive black hole in the cluster-hosted quasar H1821+643, Monthly Notices from the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac1389
Chandra X-ray Center
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