Unless Einstein is wrong, black holes are defined by three properties: mass, spin, and electric charge. The charge of a black hole should be almost zero because the matter 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 surrounding material to the orbit of nearby stars. The spin of a black hole is more difficult to study.
The spin of a black hole is basically its rotation. Make black holes in the same way that stars and planets rotate on their axis. The difference is that black holes have no physical surface like stars and planets. Black hole spin, like mass, is a space-time property. The spin determines how space is wrapped around the black hole. To measure the spin of a black hole, you need to study how the object behaves close to it.
The spin of some supermassive black holes has been measured. With a few active black holes, we can study the X-rays emitted by their action discs. The X-ray light from the disc is given an increase in energy radiation from the rotation, and by measuring that boost, we can determine the spin. Another way is to take a direct image of the black hole, as we did with the middle of the M87. The ring of light we see is bright on the side turning towards us.
But we don’t know the spin of the nearest supermassive black hole, which is in our own galaxy. Our black hole is not very active, and it is much smaller than the M87. We cannot measure its spin by observing light near it. But a new paper Astrophysical Journal Letters Argues that there is another way to measure spin.
Their method uses a property known as frame pulling. When a mass rotates, it rotates slightly around the space around it. We know that it is real because we have measured the frame-dragging effect of the Earth’s rotation. The spin of a black hole creates a similar frame-dragging, and by measuring it, we can determine the spin of a black hole. We can’t investigate the way we did with the Earth around the black hole, but we can use the next best thing.
Hundreds of stars orbit a black hole in the center of the galaxy. About forty of them, known as S-Stars, have orbits with close approaches to black holes. Over time, their orbits are displaced by the frame-pulling effect. If we can measure this shift, we can measure the spin – the larger the spin, the greater the orbital shift.
In this new task, the team studied the orbits of the S-stars and found no frame pull shift. Given how well we know the orbits of these stars, we know that black holes in the center of our galaxy must be moving slowly. The team has determined that its spin cannot be more than 0.1 on a scale of 0 to 1, meaning that it rotates less than 10% of the maximum possible spin for a black hole. In contrast, the spin of the M87’s black hole is at least 0.4.
Reference: Fragione, Giacomo and Abraham Loeb. “A higher limit on SGRA’s spin * based on the orbit of the stars in its vitality.” Astrophysical Journal Letters 901.2 (2020): L32.
Reference: Nemman, Rodrigo. “Spin of M87″. ” Astrophysical Journal Letters 880.2 (2019): L26.