Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, produces bursts of varying intensity. A team of astrophysicists used NASA’s James Webb Space Telescope to investigate the cause, with findings that could shed light on similar phenomena in other supermassive black holes.
On May 12, 2022, the first image of Sgr A*, located 26,000 light-years from Earth, was published. Now, a team led by Northwestern University has obtained the most detailed and prolonged observation of it using the James Webb telescope.
The black hole, estimated at 4 million solar masses, continuously emits flares from its accretion disk. Some are weak and last seconds, while others are intensely bright. The findings could improve understanding of black hole behavior, their interactions with their surroundings and the Milky Way’s evolution.
"Flares are expected to happen in essentially all supermassive black holes, but our black hole is unique," said Prof. Farhad Yusef-Zadeh of Northwestern University’s physics and astronomy department, 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,” he added.
First image of Sagittarius A*
The team used the telescope’s Near-Infrared Camera (NIRCam), which observes in two infrared wavelengths simultaneously over extended periods. They found Sgr A* was far more active than expected, producing five to six major flares daily along with smaller ones.
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Though the exact mechanisms behind these flares remain unclear, Prof. Yusef-Zadeh believes two separate processes drive the flares: short bursts result from turbulence compressing plasma in the accretion disk, while longer flares likely stem from magnetic field line collisions — similar to solar flares but on a much larger scale.
"The black hole’s environment is far more energetic and extreme than the Sun’s. This process involves colliding magnetic fields that release energy in the form of accelerated particles moving near the speed of light," he explained.
Since the James Webb Space Telescope’s NIRCam can capture two wavelengths (2.1 and 4.8 microns) at once, researchers compared how flare brightness changed at each wavelength. "Capturing light in two wavelengths is like seeing in color instead of black and white." Prof. Yusef-Zadeh explained.
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Black hole simulation
(Illusration: NASA, ESA, CSA, Ralf Crawford (STScI)/Handout, Reuters)
Unexpectedly, researchers found that shorter-wavelength flares faded faster than longer-wavelength ones, suggesting particles lose energy more quickly at shorter wavelengths — a common trait of particles spiraling around magnetic field lines.
The study, published in Astrophysical Journal Letters, has prompted researchers to continue using the James Webb telescope for further observations. They now aim to observe Sgr A* for a full 24 hours to determine whether the flares follow a pattern or occur randomly.
"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," Prof. Yusef-Zadeh concluded.