Unless Einstein is wrong, a black hole is defined by three properties: mass, spin, and electric charge. The charge of a black hole should be nearly zero since the matter captured by a black hole is electrically neutral. The mass of a black hole determines the size of its event horizon and can be measured in several ways, from the brightness of the material surrounding it to the orbital motion of nearby stars. The rotation of a black hole is much more difficult to study.
The rotation of a black hole is basically its rotation. Just as stars and planets rotate on their axis, so do black holes. The difference is that black holes don’t have a physical surface like stars and planets. The spin of the black hole, like mass, is a property of spacetime. Spin determines how space is deformed around a black hole. To measure the spin of a black hole, it is necessary to study how the matter near it behaves.
The spin of some supermassive black holes was measured. With a few active black holes, we can study the X-rays emitted by their accretion discs. The X-ray light from the disk receives a boost of energy from the spin and by measuring that push, we can determine the spin. Another way is to acquire a direct image of the black hole, as we did with the one in the center of M87. The ring of light we see is brighter on the side that rotates towards us.
But we don’t know the rotation of the nearest supermassive black hole, the one in our galaxy. Our black hole is not very active and is much smaller than the one in M87. We cannot measure its rotation by looking at the light near it. But a new article in Letters from astrophysics journals argues that there is another way to measure spin.
Their method uses a property known as frame dragging. When a mass rotates, it slightly rotates the space around it. We know it’s real because we’ve measured the drag effect of the Earth’s rotation frames. The spin of a black hole creates the same kind of frame-drag and by measuring it, we can determine the spin of the black hole. We can’t put a probe into orbit around the black hole like we did with Earth, but we can use the next best thing.
Hundreds of stars orbit the black hole at the center of our galaxy. About forty of them, known as S stars, have orbits with a close approach to the black hole. Over time their orbits are shifted by the frame drag effect. If we can measure these changes, we can measure spin: the greater the spin, the greater the displacement of the orbit.
In this new work, the team studied the orbits of the S stars and found no frame drag displacement. Given how well we know the orbits of these stars, we know that the black hole at the center of our galaxy must rotate slowly. The team determined that its rotation cannot be greater than 0.1 on a scale of 0 to 1, which means it rotates less than 10% of the maximum possible rotation for a black hole. In contrast, the spin of the black hole of M87 is at least 0.4.
Reference: Fragione, Giacomo and Abraham Loeb. “An upper limit on SgrA’s rotation * Based on stellar orbits in its vicinity.” The Astrophysical Journal Letters 901.2 (2020): L32.
Reference: Nemmen, Rodrigo. “The Spin of M87 *.” The Astrophysical Journal Letters 880.2 (2019): L26.