February 26, 2024

Telescopes show the Milky Way’s black hole is ready to spring into action

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This artist’s illustration depicts the findings of a new study on the supermassive black hole at the center of our galaxy called Sagittarius A* (abbreviated as Sgr A*). As reported in our last press release, this result revealed that Sgr A* is spinning so quickly that it is distorting spacetime – that is, time and the three dimensions of space – so that it can look more like a soccer ball. Credit: Chandra X-ray Center

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This artist’s illustration depicts the findings of a new study on the supermassive black hole at the center of our galaxy called Sagittarius A* (abbreviated as Sgr A*). As reported in our last press release, this result revealed that Sgr A* is spinning so quickly that it is distorting spacetime – that is, time and the three dimensions of space – so that it can look more like a soccer ball. Credit: Chandra X-ray Center

The supermassive black hole at the center of the Milky Way is spinning so quickly that it is warping the spacetime around it into a shape that may look like a football, according to a new study using data from NASA’s Chandra X-ray Observatory. NASA and National Science. The Foundation’s Karl G. Jansky Very Large Array (VLA).

Astronomers call this giant black hole Sagittarius A* (abbreviated Sgr A*), which is located about 26,000 light-years away from Earth, at the center of our galaxy.

Black holes have two fundamental properties: their mass (how much they weigh) and their spin (how fast they rotate). Determining either of these two values ​​tells scientists a lot about any given black hole and how it behaves.

A team of researchers has applied a new method that uses X-ray and radio data to determine how fast Sgr A* rotates, based on how material flows toward and away from the black hole. They found that Sgr A* is spinning with an angular velocity – the number of rotations per second – that is about 60% of the maximum possible value, a limit set by the material not being able to travel faster than the speed of light.

In the past, different astronomers have made various other estimates of the rotational speed of Sgr A* using different techniques, with results ranging from Sgr A* not rotating at all to rotating at almost maximum speed.

“Our work could help resolve the question of how fast our galaxy’s supermassive black hole spins,” said Ruth Daly of Penn State University, lead author of the new study. “Our results indicate that Sgr A* is rotating very quickly, which is interesting and has far-reaching implications.”


Credit: Chandra X-ray Center

A rotating black hole pulls on “spacetime” (the combination of time and the three dimensions of space) and nearby matter as it spins. The spacetime around the spinning black hole is also compressed. Looking down at a black hole, down the barrel of any jet it produces, spacetime has a circular shape. Looking sideways at the spinning black hole, spacetime is shaped like a football. The faster the spin, the flatter the ball.

The rotation of a black hole can act as an important source of energy. Rotating supermassive black holes can produce collimated streams, that is, narrow beams of material, like jets, when their rotational energy is extracted, which requires there to be at least some matter in the vicinity of the black hole.

Due to limited fuel around Sgr A*, this black hole has been relatively quiet for the past few millennia, with relatively weak jets. This work, however, shows that this could change if the amount of material in the vicinity of Sgr A* increases.

“A spinning black hole is like a rocket on the launch pad,” said Biny Sebastian, co-author at the University of Manitoba in Winnipeg, Canada. “When the material gets close enough, it’s as if someone has fueled the rocket and pressed the ‘launch’ button.”


Chandra X-ray image of Sagittarius A* and surrounding region. Credit: NASA/CXC/Univ. of Wisconsin/Y.Bai, et al.

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Chandra X-ray image of Sagittarius A* and surrounding region. Credit: NASA/CXC/Univ. of Wisconsin/Y.Bai, et al.

This means that in the future, if the properties of matter and the strength of the magnetic field near the black hole change, some of the enormous energy from the black hole’s rotation could generate more powerful outflows. This source material could come from gas or the remains of a star torn apart by the black hole’s gravity if that star gets too close to Sgr A*.

“The jets powered and collimated by a galaxy’s spinning central black hole can profoundly affect the gas supply to an entire galaxy, which affects how quickly and even whether stars can form,” said co-author Megan Donahue of Michigan StateUniversity. “The ‘Fermi bubbles’ seen in X-rays and gamma rays around our Milky Way’s black hole show that the black hole was likely active in the past. Measuring our black hole’s rotation is an important test of this scenario.”

To determine the spin of Sgr A*, the authors used an empirically based theoretical method called the “exit method”, which details the relationship between the black hole’s spin and its mass, the properties of matter near the black hole, and the output properties.

The collimated flow produces radio waves, while the disk of gas surrounding the black hole is responsible for emission of X-rays. Using this method, researchers combined data from Chandra and the VLA with an independent estimate of the black hole’s mass obtained from other telescopes to constrain the black hole’s rotation.

“We have a special view of Sgr A* because it is the closest supermassive black hole to us,” said co-author Anan Lu of McGill University in Montreal, Canada. “Although it is calm at the moment, our work shows that in the future it will give an incredibly powerful boost to the surrounding matter. This could happen within a thousand or a million years, or it could happen within our lifetimes.”

The study is published in the journal Monthly Notices of the Royal Astronomical Society.

More information:
Ruth A Daly et al, New black hole spin values ​​for Sagittarius A* obtained with the exit method, Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad3228

Diary information:
Monthly Notices of the Royal Astronomical Society

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