We actually don’t know how fast a supermassive black hole in the Milky Way is rotating but there might be a way to find out

Unless Einstein was wrong, a black hole is defined by three properties: mass, rotation, and electric charge. The charge of the black hole should be nearly zero because the material captured by the black hole is electrically neutral. The mass of a black hole determines its size Event horizon, It can be measured in a number of ways, from the brightness of the material around it to the orbital motion of nearby stars. The rotation of a black hole is difficult to study.

How do x-rays from a black hole tell us about its rotation. Credit: NASA / JPL-Caltech

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 do not have a physical surface like stars and planets do. The rotation of a black hole, like mass, is a property of space-time. Spin defines how space is twisted around a black hole. To measure the rotation of a black hole, you need to study the behavior of matter near it.

The rotations of some supermassive black holes have been measured. Through a small number of active black holes, we can study the X-rays emitted by the disks of their accumulation. The X-ray light from the disk is given a boost of energy from the rotation, and by measuring this strengthening we can determine the rotation. Another method is to take a live image of the black hole, as we did with the one at the center of the M87. The ring of light that we see is brighter on the side that turns towards us.

One side is brighter due to the rotation of the black hole. Credit: EHT Collaboration

But we don’t know the rotation of the nearest supermassive black hole in our galaxy. Our black hole is not very active, and it is much smaller than the one on the M87. We cannot measure its rotation by observing the light near it. But a new sheet is in The Astrophysical Journal Letters He argues that there is another way to measure rotation.

Their method uses a property known as frame drag. As it rotates a block, it slightly twists the space around it. We know it’s real because we did Measurement of the effect of tire pulling on Earth’s rotation. The rotation of the black hole creates the same type of tire drag, and by measuring it, we can determine the rotation of the black hole. We can’t put a probe in orbit around a black hole like we did with Earth, but we can use the next best thing.

A star cluster S orbiting a black hole in our galaxy. Credit: NCSA, UCLA / Keck

Hundreds of stars orbiting a black hole at the center of our galaxy. About forty of them, known as S stars, have orbits close to a black hole. Over time, their orbits change through the effect of tire dragging. If we can measure these shifts, we can measure rotation – the higher the rotation, the greater the orbital displacement.

In this new work, the team studied the orbits of the S-stars and found no change in tire drag. 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 could be no more than 0.1 on a scale from 0 to 1, which means it rotates less than 10% of the black hole’s maximum possible rotation. In contrast, the rotation of the black hole in M87 is at least 0.4.

reference: Fragione, Giacomo and Abraham Loeb. “An upper limit of SgrA’s rotation * based on stellar orbits in proximity. ” The Astrophysical Journal Letters 901.2 (2020): L32.

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reference: Niemen, Rodrigo. “M87 rotation *. ” The Astrophysical Journal Letters 880.2 (2019): L26.

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