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Tour: Chandra Shows Giant Black Hole Spins Slower Than Its Peers

Astronomers have made a record-breaking measurement of a black hole's spin, one of two fundamental properties of black holes. NASA's Chandra X-ray Observatory shows this black hole is spinning slower than most of its smaller cousins.

This is the most massive black hole with an accurate spin measurement and gives hints about how some of the Universe's biggest black holes grow.

Supermassive black holes contain millions or even billions of times more mass than the Sun. Astronomers think that nearly every large galaxy has a supermassive black hole at their center. While the existence of supermassive black holes is not in dispute, scientists are still working to understand how they grow and evolve. One critical piece of information is how fast the black holes are spinning.

Every black hole can be defined by just two numbers: its spin and its mass. While that sounds fairly simple, figuring those values out for most black holes has proved to be incredibly difficult.

For this result, researchers observed X-rays that bounced off a disk of material swirling around the black hole in a quasar known as H1821+643. Located in a cluster of galaxies about 3.4 billion light-years from Earth, H1821+643 contains an actively growing black hole containing between about three and 30 billion solar masses, making it one of the most massive known. By contrast the supermassive black hole in the center of our galaxy weighs about four million suns.

The strong gravitational forces near the black hole alter the intensity of X-rays at different energies. The larger the alteration the closer the inner edge of the disk must be to the point of no return of the black hole, known as the event horizon. Because a spinning black hole drags space around with it and allows matter to orbit closer to it than is possible for a non-spinning one, the X-ray data can show how fast the black hole is spinning.

The scientists found that the black hole in H1821+643 is spinning about half as quickly as most black holes weighing between about a million and ten million suns. 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 like H1821+643 undergo most of their growth by merging with other black holes, or by gas being pulled inwards in random directions when their large disks are disrupted.

While there is much more work to be done, this result signifies an exciting step forward in scientists' attempt to uncover how the most massive black holes in the universe grow.

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