If Einstein is not mistaken, a black hole is defined by three properties: mass, spin, and electric charge. The charge of a black hole should be almost zero since the matter of being trapped 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 a variety of ways, from the brightness of the surrounding material to the orbital motion of nearby stars. The spin of a black hole is much more difficult to study.
The spin of a black hole is basically its rotation. Just as stars and planets revolve around an axis, so do black holes. The difference is that black holes do not have physical surfaces like stars and planets. Blackhole spin, as mass, is a spatial property. Spin determines how the space around the black hole is wrapped. To measure the spin of a black hole you need to study how the matter behaves close to it.
The spin of some supermassive black holes has been measured. With the help of a few active black holes we can study the X-rays emitted by their accretion disks. The X-ray light from the disk increases the energy from rotation and by measuring that boost we can determine the spin. Another way is to take a direct picture of the black hole, as we put it in the center of the M877. The light ring that we see is brighter on the side that turns towards us.
However, we do not know the spin of the nearest supermassive black hole in our own galaxy. Our black hole is not very active and is much smaller than the M887. We cannot measure its spin by reviewing the light around 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 twists some space around it. We know this is real because we have measured the frame-pulling effect of the earth’s rotation. The spin of a black hole causes the same kind of frame-pulling and by measuring it we can determine the spin of a black hole. We can’t put any probes into orbit around the way we’ve blackholed the Earth, but we can use the next best thing.
Hundreds of stars orbit a black hole in the center of our galaxy. About forty of these, known as S-stars, orbit near the location of the black hole. Over time their orbits are shifted by the effect of frame-pulling. If we can measure these shifts, we can measure the spin – the greater the spin, the greater the space in the orbit.
In this new task the team studied the orbits of the S-stars and found the shift to pull the frame. No matter how well we know the orbits of these stars, the black hole at the center of our galaxy must be spinning slowly. Turning. In contrast, the spin of the M87’s black hole is at least 0.4.
References: Frasion, Giacomo and Abraham Loeb. “SGRA has a higher limit on spin * based on Starler orbits at its VCT” Astrophysical Journal Letters 901.2 (2020): L32.
References: Nemon, Rodrigo “Spin of M87” * Astrophysical Journal Letters 880.2 (2019): L26.