They measure for the first time the rotation speed of a supermassive black hole

All black holes have an inherent rotation that has been shaped by their cosmic encounters over time. Under certain circumstances, the black hole can increase its rotation speed by swallowing matter, but in other cases, such as if its mass grows by merging with that of another black hole, the interaction between the rotation of one and that of the other can make the black hole resulting from the merger have a slower rotation speed than the two previous black holes.

When a black hole rotates on itself, it drags the space-time around it with it.

Normally, this effect would not be obvious around black holes, since objects with this much gravity do not emit light.

But in recent years, some physicists have proposed that, in cases such as when a star is torn apart by the gravitational field of a black hole when it passes too close to it, tracking the light of the stellar remains as they are dragged could give an opportunity to measure the rotation speed of the black hole.

That kind of shattering occurs when a black hole causes colossal tides on a passing star, exceeding its structural integrity. As the star is torn apart by the black hole’s immense tidal forces, part of it may become an intensely hot accretion disk of stellar material rotating around the black hole.

By analyzing certain details of the behavior of this rotating disk of stellar material, it is possible to find out what the rotation speed of the black hole is.

And that is what some scientists have managed to do, demonstrating the viability of the method.

The achievement is the work of an international team led by Dheeraj R. Pasham, from the Kavli Institute for Astrophysics and Space Research, attached to the Massachusetts Institute of Technology (MIT) in the United States.

Artistic recreation of a black hole. (Photo: NASA Goddard Space Flight Center)

Pasham and his colleagues measured the rotation of a supermassive black hole in a galaxy about a billion light-years away. They deduced the rotation speed by following the pattern of X-ray flashes that the black hole caused after tearing apart a star that passed too close. The team followed the flashes for several months and finally managed to determine that the black hole rotates at less than 25% of the speed of light, that is, relatively slowly for what black holes are.

The study is titled “Lense-Thirring Precession after a Supermassive Black Hole Disrupts a Star.” And it has been published in the academic journal Nature. (Fountain: NCYT by Amazings)