Sagittarius A*, the 4.3-million-solar-mass black hole that resides at the center of the Milky Way Galaxy, is spinning so quickly it is warping the spacetime surrounding it into a shape that can look like a football, according to an analysis of data gathered by NASA’s Chandra X-ray Observatory and NSF’s Karl G. Jansky Very Large Array.
Black holes have two fundamental properties: their mass (how much they weigh) and their spin (how quickly they rotate).
Determining either of these two values tells astrophysicists a great deal about any black hole and how it behaves.
Penn State University’s Dr. Ruth Daly and colleagues applied a new method that uses X-ray and radio data to determine how quickly Sagittarius A* is spinning based on how material is flowing towards and away from the black hole.
They found that Sagittarius A* is spinning with an angular velocity (number of revolutions per second) that is about 60% of the maximum possible value, a limit set by material not being able to travel faster than the speed of light.
In the past, different astronomers made several other estimates of Sagittarius A*’s rotation speed using different techniques, with results ranging from Sagittarius A* not spinning at all to it spinning at almost the maximum rate.
“Our work may help settle the question of how fast our Galaxy’s supermassive black hole is spinning,” Dr. Daly said.
“Our results indicate that Sagittarius A* is spinning very rapidly, which is interesting and has far reaching implications.”
A rotating black hole pulls spacetime and nearby matter around as it spins. Spacetime around the spinning black hole is also squashed down.
Looking down on a black hole from the top, along the barrel of any jet it produces, spacetime is a circular shape.
Looking at the spinning black hole from the side, however, the spacetime is shaped like a football. The faster the spin the flatter the football.
A black hole’s spin can act as an important source of energy. Spinning supermassive black holes can produce collimated outflows, that is narrow beams of material such as jets, when their spin energy is extracted, which requires that there is at least some matter in the vicinity of the black hole.
Because of limited fuel around Sagittarius A*, this black hole has been relatively quiet in recent millennia with relatively weak jets.
The new study, however, shows that this could change if the amount of material in the vicinity of Sagittarius A* increases.
“Jets powered and collimated by a galaxy’s spinning central black hole can profoundly affect the gas supply for an entire galaxy, which affects how quickly and even whether stars can form,” said Dr. Megan Donahue, an astronomer at Michigan State University.
“The Fermi bubbles seen in X-rays and gamma rays around our Milky Way’s black hole show the black hole was probably active in the past. Measuring the spin of our black hole is an important test of this scenario.”
To determine the spin of Sagittarius A*, the astronomers used an empirically based theoretical method referred to as the outflow method that details the relationship between the spin of the black hole and its mass, the properties of the matter near the black hole, and the outflow properties.
The collimated outflow produces the radio waves, while the disk of gas surrounding the black hole is responsible for the X-ray emission.
Using this method, the researchers combined data from NASA’s Chandra X-ray Observatory and NSF’s Karl G. Jansky Very Large Array with an independent estimate of the black hole’s mass from other telescopes to constrain the black hole’s spin.
“We have a special view of Sagittarius A* because it is the nearest supermassive black hole to us,” said Dr. Anan Lu, an astronomer at McGill University.
“Although it’s quiet right now, our work shows that in the future it will give an incredibly powerful kick to surrounding matter.”
“That might happen in a thousand or a million years, or it could happen in our lifetimes.”
The study was published in the Monthly Notices of the Royal Astronomical Society.
Ruth A. Daly et al. 2024. New black hole spin values for Sagittarius A* obtained with the outflow method. MNRAS 527 (1): 428-436; doi: 10.1093/mnras/stad3228