Astrophysicists surprised by light show around the Milky Way's supermassive black hole
Sagittarius A* is the supermassive black hole at the center of our home Milky Way galaxy. It has a mass equal to billions of suns and has an accretion disk made up of gas and dust surrounding it. Accretion disks are also the main light source from a black hole. At only 26,000 light years away from Earth, Sagittarius A* is one of the few black holes that scientists can observe to watch the flow of gas and dust in its accretion disk.
'Flares are expected to happen in essentially all supermassive black holes, but our black hole is unique,' Farhad Yusef-Zadeh, a study co-author and astrophysicist at Northwestern University, said in a statement. '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.'
Studying Sagittarius A* can help physicists better understand the fundamental nature of black holes, how they interact with their surroundings, and even the evolution of our own galactic home.
[ Related: Can the black hole in the center of our galaxy expand to our solar system? ]
In the new study, the team used JWST's near infrared camera (NIRCam). This instrument can simultaneously observe two infrared colors for long periods of time. They observed Sagittarius A* with their NIRCam for a total of 48 hours, using 8-to-10-hour increments across one Earth year. This allowed them to track how the black hole changed over time, similar to a time-lapse video.
Using NASA's James Webb Space Telescope, Northwestern astrophysicists gained the longest, most detailed glimpse yet of the supermassive black hole at the center of the Milky Way. They found the black hole's accretion disk emits a constant stream of flares with no periods of rest. This video shows the 2.1 micron data taken on April 7, 2024. CREDIT: Farhad Yusef-Zadeh/Northwestern University.
While flares were expected, Sagittarius A* was more active than would be anticipated. The team saw 'ongoing fireworks' of various brightness and durations. About five to six big flares with several smaller sub-flares in between spewed out of the accretion disk.
'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.'
The team suspects that there are two separate processes behind the short bursts and longer flares. These short and faint flickers are like the small ripples that fluctuate randomly at the surface of a body of water. However, the longer and brighter flares are more similar to tidal waves and are caused by more significant events.
The minor disturbances within the accretion disk likely generate the faint flickers. Turbulent fluctuations within the disk can compress a hot, electrically charged gas called plasma and create a temporary burst of radiation. According to Yusef-Zadeh, these events are similar to solar flares.
'It's similar to how the sun's magnetic field gathers together, compresses and then erupts a solar flare,' said Yusef-Zadeh. '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 big, bright, and dramatic flares are likely more similar to magnetic reconnection events. This is when two magnetic fields collide and release energy in the form of accelerated particles. These particles travel at velocities near the speed of light and shoot out 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.
JWST's NIRCam can observe two separate wavelengths (2.1 and 4.8 microns) at the same time. With this, the team could compare how the flares' brightness changed with each wavelength. Capturing light at two wavelengths is similar to 'seeing in color instead of black and white,' according to Yusef-Zadeh. By observing Sagittarius A* at multiple wavelengths, the team captured a more complete and nuanced picture of the black hole's behavior.
Even with the powerful NIRCam, the team was still surprised. They unexpectedly discovered that events observed at the shorter wavelength actually changed brightness just before the longer-wavelength events.
[ Related: Gaze upon the supermassive black hole at the center of our galaxy. ]
'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.'
According to the team, this time delay provided more clues about what physical processes are occurring around the black hole. One explanation is that the particles lose energy over the course of the flare and could be losing energy quicker at shorter wavelengths than they do at longer wavelengths. These changes are expected for particles that are spiraling around magnetic field lines.
In future studies, Yusef-Zadeh hopes to use the JWST to observe Sagittarius A* for a longer period of time, potentially for an uninterrupted 24 hour period. A longer observation could help reduce noise and allow scientists to observe 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.'
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