Lauren Fuge
It’s party time all the time at the heart of the galaxy, according to new observations from the James Webb Space Telescope (JWST). The space observatory has taken the longest and most detailed look yet at Sagittarius A*, the supermassive black hole at the centre of the Milky Way, and found that it’s constantly emitting a stream of flares.
Some are blindingly bright and occur daily; some are faint, lasting only seconds; and others are fainter again but last for months on end.
“Flares are expected to happen in essentially all supermassive black holes, but our black hole is unique,” says Farhad Yusef-Zadeh, an astrophysicist at Northwestern University in the US and leader of the new study.
“In our data, we saw constantly changing, bubbling brightness. And then boom! A big burst of brightness suddenly popped up. Then, it calmed down again.
“We couldn’t find a pattern in this activity. It appears to be random. The activity profile of the black hole was new and exciting every time that we looked at it.”
Studying this ongoing, spectacular light show could tell us more about the nature of these monstrous black holes and how they interact with their environment.
Wait, what’s a supermassive black hole?
There are several types of black hole, including ‘stellar-mass’ and ‘supermassive.’
Stellar-mass black holes form when a dying star – more than eight times the mass of our sun – explodes as a supernova, then collapses in on itself to form a black hole. These can continue to grow and gain mass by eating stars or colliding with other black holes.
Supermassive black holes are much, much bigger – from hundreds of thousands to billions of times the sun’s mass. Their origins are mysterious, but they are found at the centre of almost every galaxy. It’s known that they can grow by feeding on stars, or by merging with other supermassive black holes in galactic collisions.
But black holes aren’t just dark, hungry voids. They can emit significant amount of radiation from their accretion disks, the ring of matter caught in the black hole’s orbit and heated to extraordinary temperatures.
Peering into the heart of the galaxy
Yusef-Zadeh and team used the JWST’s near infrared camera to look at Sagittarius A* at two different infrared wavelengths (2.1 and 4.8 microns) at the same time. Their observations totalled 48 hours, but split into 8-10 hours chunks spread across an entire year to see how the black hole changed over time.
Throughout the year, the team saw how the black hole’s accretion disk emitted 5 to 6 large flares per day, of varying lengths and brightnesses, plus smaller flares in between.
“[Sagittarius A*] is always bubbling with activity and never seems to reach a steady state,” Yusef-Zadeh says. “We observed the black hole multiple times throughout 2023 and 2024, and we noticed changes in every observation. We saw something different each time, which is really remarkable. Nothing ever stayed the same.”
In their paper published in The Astrophysical Journal Letters, the team outlines two possible ideas for the processes driving these flares. The faint flickers may be caused by turbulent fluctuations in the accretion disk, which could compress plasma and trigger a burst of radiation.
“It’s similar to how the sun’s magnetic field gathers together, compresses and then erupts a solar flare,” Yusef-Zadeh says. “Of course, the processes are more dramatic because the environment around a black hole is much more energetic and much more extreme.”
The larger and brighter flares, on the other hand, may be caused by two fast-moving magnetic fields colliding and releasing accelerated particles. These magnetic reconnection events also have a solar parallel.
The next step is to observe Sagittarius A*’s fireworks show for a longer, uninterrupted time, to parse out even finer details. “If we can observe for 24 hours, then we can reduce the noise to see features that we were unable to see before,” Yusef-Zadeh says. “We also can see if these flares show periodicity (or repeat themselves) or if they are truly random.”