Milky Way’s Supermassive Black Hole is More Active than Previously Thought

The accretion disk orbiting Sagittarius A* is emitting a constant stream of flares with no periods of rest, according to an analysis of new data from the NASA/ESA/CSA James Webb Space Telescope; while some flares are faint flickers, lasting mere seconds, other flares are blindingly bright eruptions, which spew daily; there also are even fainter flickers that surge for months at a time.

“Flares are expected to happen in essentially all supermassive black holes, but our black hole is unique,” said Dr. Farhad Yusef-Zadeh, an astronomer at Northwestern University.

“It is always bubbling with activity and never seems to reach a steady state.”

“We observed Sagittarius A* 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.”

Dr. Yusef-Zadeh and his colleagues used Webb’s NIRCam instrument to observe Sagittarius A* for a total of 48 hours — in 8-to-10-hour increments across one year.

While they expected to see flares, Sagittarius A* was more active than he anticipated.

The accretion disk surrounding the black hole generated five to six big flares per day and several small sub-flares in between.

“In our data, we saw constantly changing, bubbling brightness,” Dr. Yusef-Zadeh said.

“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.”

“We suspect two separate processes are responsible for the short bursts and longer flares. If the accretion disk is a river, then the short, faint flickers are like small ripples that fluctuate randomly on the river’s surface.”

“The longer, brighter flares, however, are more like tidal waves, caused by more significant events.”

“Minor disturbances within the accretion disk likely generate the faint flickers. Specifically, turbulent fluctuations within the disk can compress plasma (a hot, electrically charged gas) to cause a temporary burst of radiation.”

“It’s similar to how the Sun’s magnetic field gathers together, compresses and then erupts a solar flare.”

“Of course, the processes are more dramatic because the environment around a black hole is much more energetic and much more extreme. But the Sun’s surface also bubbles with activity.”

The astronomers attribute the big, bright flares to magnetic reconnection events — a process where two magnetic fields collide, releasing energy in the form of accelerated particles. Traveling at velocities near the speed of light, these particles emit bright bursts of radiation.

“A magnetic reconnection event is like a spark of static electricity, which, in a sense, also is an electric reconnection,” Dr. Yusef-Zadeh said.

The team hopes to use Webb to observe Sagittarius A* for a longer period of time.

“When you are looking at such weak flaring events, you have to compete with noise,” Dr. Yusef-Zadeh said.

“If we can observe for 24 hours, then we can reduce the noise to see features that we were unable to see before. That would be amazing. We also can see if these flares show periodicity (or repeat themselves) or if they are truly random.”

The findings will be published in the Astrophysical Journal Letters.

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F. Yusef-Zadeh et al. 2025. Non-stop Variability of Sgr A* using JWST at 2.1 and 4.8 micron Wavelengths: Evidence for Distinct Populations of Faint and Bright Variable Emission. ApJL, in press; arXiv: 2501.04096