According to a study published in The Astrophysical Journal Letters, a Northwestern University-led team of astrophysicists used NASA’s James Webb Space Telescope (JWST) to get the longest, most detailed look yet into the void in the center of the Milky Way galaxy.
This artist’s concept portrays the supermassive black hole at the center of the Milky Way galaxy. Several flaring hot spots that resemble solar flares, but on a more energetic scale, are seen in the disk. Image Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
The spinning disk of gas and dust (or accretion disk) around the Milky Way's supermassive black hole, known as Sagittarius A*, emits a steady stream of flares with no periods of rest, the researchers discovered.
While some flares are weak flickers that last only a few seconds, others are blindingly intense eruptions that occur every day. There are even fainter flickers that appear for months at a time. The level of activity varies with time, ranging from brief interludes to extended periods.
The new findings may help physicists better comprehend the underlying nature of black holes, how they interact with their surroundings, and the dynamics and evolution of our own galaxy.
Flares are expected to happen in essentially all supermassive black holes, but our black hole is unique. 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.
Farhad Yusef-Zadeh, Study Lead and Professor, Department of Physics and Astronomy, Northwestern University
Yusef-Zadeh is a physics and astronomy professor at Northwestern's Weinberg College of Arts and Sciences who specializes on the core of the Milky Way. The worldwide 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 and Smithsonian, and Claire Chandler of the National Radio Astronomy Observatory.
Random Fireworks
Yusef-Zadeh and his team conducted the study using the JWST’s near infrared camera (NIRCam), which can see two infrared colors at the same time for extended periods of time. The researchers used the imaging technology to examine Sagittarius A* for a total of 48 hours, divided into 8-to-10-hour intervals across a year. This allowed scientists to observe how the black hole evolved over time.
While Yusef-Zadeh expected to witness flares, Sagittarius A* was more active than he had anticipated. Simply put, the observations indicated ongoing fireworks with varying brightness and duration. The accretion disk surrounding the black hole produced five to six large flares every day, with several minor sub-flares in between.
Yusef-Zadeh added, “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.”
Two Separate Processes at Play
Although astrophysicists do not yet fully comprehend the processes at work, Yusef-Zadeh believes two distinct processes are responsible for the short bursts and prolonged flares. If the accretion disk were a river, the short, faint flickers would be like little ripples on the river's surface, fluctuating randomly. Longer, brighter flares, on the other hand, resemble tidal waves created by larger catastrophes.
According to Yusef-Zadeh, small disturbances within the accretion disk are most likely responsible for the weak flickers. Turbulent fluctuations within the disk can compress plasma (a hot, electrically charged gas), resulting in a brief burst of radiation. Yusef-Zadeh compares the event to a solar flare.
He explained, “It is 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.”
Magnetic reconnection events, in which two magnetic fields meet and release energy in the form of accelerated particles, are the cause of the large, bright flares, according to Yusef-Zadeh. These particles travel at speeds close to the speed of light and release intense radiation bursts.
“A magnetic reconnection event is like a spark of static electricity, which, in a sense, also is an ‘electric reconnection,’” Yusef-Zadeh added.
Double Vision
Since 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.
The researchers were taken by surprise once more. Surprisingly, they found that shorter-wavelength events changed brightness a little before longer-wavelength occurrences.
“This is the first time we have seen a time delay in measurements at these wavelengths. 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,” Yusef-Zadeh noted.
This time delay revealed more information about the physical processes occurring around the black hole. One possibility is that the particles lose energy throughout the flare, losing energy faster at shorter wavelengths than at longer wavelengths. Such changes are expected when particles spiral around magnetic field lines.
Aiming for an Uninterrupted Look
Yusef-Zadeh plans to use the JWST to spend more time observing Sagittarius A*. He recently proposed observing the black hole for an uninterrupted 24 hours. The longer observation period will assist in minimizing noise, allowing the researchers to notice even finer features.
“When you are looking at such weak flaring events, you have to compete with noise. 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,” Yusef-Zadeh stated.
NASA and the National Science Foundation provided funding for this study.
Journal Reference:
Yusef-Zadeh, F. et. al. (2025) Nonstop Variability of Sgr A* Using JWST at 2.1 and 4.8 μm Wavelengths: Evidence for Distinct Populations of Faint and Bright Variable Emission. The Astrophysical Journal Letters. doi.org/10.3847/2041-8213/ada88b