The supermassive black hole at the center of the Milky Way appears to be having a party — and it is weird, wild and wonderful.
Using NASA's James Webb Space Telescope (JWST), a Northwestern University-led team of astrophysicists has gained the longest, most detailed glimpse yet of the void that lurks in middle of our galaxy.
The swirling disk of gas and dust (or accretion disk) orbiting the central supermassive black hole, called Sagittarius A*, is emitting a constant stream of flares with no periods of rest, the researchers found. 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. The level of activity occurs over a wide range of time — from short interludes to long stretches.
The new findings could help physicists better understand the fundamental nature of black holes, how they interact with their surrounding environments and the dynamics and evolution of our own galactic home.
The study will be published on Tuesday (Feb. 18) in The Astrophysical Journal Letters.
"Flares are expected to happen in essentially all supermassive black holes, but our black hole is unique," said Northwestern's Farhad Yusef-Zadeh , who led the study. "It is always bubbling with activity and never seems to reach a steady state. 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."
An expert on the Milky Way's center, Yusef-Zadeh is a professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences . The international team of coauthors includes Howard Bushouse of the Space Telescope Science Institute, Richard G. Arendt of NASA, Mark Wardle of Macquarie University in Australia, Joseph Michail of Harvard & Smithsonian and Claire Chandler of the National Radio Astronomy Observatory.
Random fireworks
To conduct the study, Yusef-Zadeh and his team used the JWST's near infrared camera (NIRCam), which can simultaneously observe two infrared colors for long stretches of time. With the imaging tool, the researchers observed Sagittarius A* for a total of 48 hours — in 8-to-10-hour increments across one year. This enabled scientists to track how the black hole changed over time.
While Yusef-Zadeh expected to see flares, Sagittarius A* was more active than he anticipated. Simply put: the observations revealed ongoing fireworks of various brightness and durations. 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," 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."
Two separate processes at play
Although astrophysicists do not yet fully understand the processes at play, Yusef-Zadeh suspects 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.
Yusef-Zadeh posits that 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. Yusef-Zadeh likens the event to a solar flare.
"It's similar to how the sun's magnetic field gathers together, compresses and then erupts a solar flare," he explained. "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."
Yusef-Zadeh attributes 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,'" Yusef-Zadeh said.
Double vision
Because the JWST's NIRCam can observe two separate wavelengths (2.1 and 4.8 microns) at the same time, Yusef-Zadeh and his collaborators were able to compare how the flares' brightness changed with each wavelength. Yusef-Zadeh said capturing light at two wavelengths is like "seeing in color instead of black and white." By observing Sagittarius A* at multiple wavelengths, he captured a more complete and nuanced picture of its behavior.
Yet again, the researchers were met with a surprise. Unexpectedly, they discovered events observed at the shorter wavelength changed brightness slightly before the longer-wavelength events.
"This is the first time we have seen a time delay in measurements at these wavelengths," Yusef-Zadeh said. "We observed these wavelengths simultaneously with NIRCam and noticed the longer wavelength lags behind the shorter one by a very small amount — maybe a few seconds to 40 seconds."
This time delay provided more clues about the physical processes occurring around the black hole. One explanation is that the particles lose energy over the course of the flare — losing energy quicker at shorter wavelengths than at longer wavelengths. Such changes are expected for particles spiraling around magnetic field lines.
Aiming for an uninterrupted look
To further explore these questions, Yusef-Zadeh hopes to use the JWST to observe Sagittarius A* for a longer period of time. He recently submitted a proposal to observe the black hole for an uninterrupted 24 hours. The longer observation run will help reduce noise, enabling the researchers to see even finer details.
"When you are looking at such weak flaring events, you have to compete with noise," 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 study, "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," was supported by NASA and the National Science Foundation.
