Unless Einstein is wrong, a black hole is defined by three characteristics: mass, rotate, and electric charge. The charge of a black hole should be almost zero because the object obtained 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 many ways, from the brightness of the material around it to the orbital motion of nearby stars. Rotating a black hole is more difficult to study.
The rotation of a black hole is usually its rotation. Just as stars and planets rotate on their axis, so do black holes. The difference is that black holes have no physical surface like stars and planets. Black hole spin, like mass, is a property of spacetime. Rotate determines how the space is trapped around a black hole. To measure the rotation of a black hole, you need to analyze how close it is.
Measured the rotation of some supermassive black holes. In some active black holes, we can analyze the x-rays emitted by their accretion disks. X-ray light from the disk is given the help of energy from rotation, and by measuring that boost, we can determine the rotation. Another way is to take a direct image of the black hole, as we did in the middle of the M87. The ring of light we see is brighter on the side that rotates towards us.
But we do not know the rotation of the nearest supermassive black hole, one of our own galaxy. Our black hole is not very active, and it is smaller than the one on the M87. We cannot measure its rotation by observing the light near it. But a new role in Letters in the Astrophysical Journal Argued that there was another way to measure rotation.
Their technique uses an asset known as frame dragging. When a mass rotates, it rotates the space around it slightly. We know this to be true because we measure the effect of the rotation frame of the Earth’s rotation frame. Rotating a black hole creates the same type of frame-dragging, and by measuring it, we can determine the rotation of the black hole. We can’t put a probe in orbit around the black hole the way we did on Earth, but we can use the next best thing.
Hundreds of stars revolve around the black hole in the center of our galaxy. About forty of these, known as S-stars, have orbits with close proximity to the black hole. Over time their orbits are changed by the drag effect. If we can measure these shifts, we can measure rotation – the larger the rotation, the greater the orbit shift.
In this new task, the team studied the orbits of S-stars and found no frame dragging shift. Because of how well we know the orbits of these stars, we know that the black hole in the center of our galaxy must be rotating slowly. The team determined that its rotation could be no more than 0.1 on a scale from 0 to 1, meaning it rotates less than 10% of the maximum possible rotation for a black hole. In contrast, the rotation of the black hole of the M87 is at least 0.4.
References: Fragione, Giacomo, and Abraham Loeb. “A High Limit on SgrA Rotation * Based on Stellar Orbits for Its Benefit.” The Letters in the Astrophysical Journal 901.2 (2020): L32.
References: Nemmen, Rodrigo. “The Spin of the M87 *.” The Letters in the Astrophysical Journal 880.2 (2019): L26.