The black hole of our galaxy, like a verbena

NASA’s James Webb Space Telescope (JWST) has obtained the longest and detailed vision of Sagittarius A*, the supermassive hole in the center of our galaxy, the Milky Way. The astrophysicists, led by the Northwestern University (USA), discovered that the rotating and powder disc surrounds the giant emits a constant flow of calling, without rest.

While some flares are weak flashes that last just a few seconds, others are blind and bright eruptions that occur daily. There are also even weaker flashes that are maintained for months. The level of activity occurs in a wide range of time, from brief interludes to prolonged periods, according to the authors in ‘The Astrophysical Journal Letters’.

“It is expected that flares in virtually all supermassive black holes, but our black hole is unique,” says Farhad Yusef-Zadeh, professor of Physics and Astronomy in Northwestern, who directed the study. «It is always full of activity and never seems to achieve a stable state. We observe the black hole several times over 2023 and 2024, and notice changes in each observation. We saw something different every time, which is really remarkable. Nothing remained the same.

Random fireworks

To carry out the study, Yusef-Zadeh and his team used the near infrared chamber (NIRCAM) of the JWST, which can simultaneously observe two infrared colors for long periods of time. With the image obtaining tool, the researchers observed Sagittarius A* for a total of 48 hours, in increases of 8 to 10 hours over a year. This allowed scientists to track how the black hole changed over time.

Although Yusef-Zadeh expected to see flares, Sagittarius A* was more active than he had planned. In a nutshell: the observations revealed continuous fireworks of different brightness and durations. The accretion album surrounding the black hole generated five to six large flares per day and several small subllamated in between.

“In our data, we saw a brightness in constant change and effervescence,” says Yusef-Zadeh. «And then, Boom! Suddenly, a great brightness explosion appeared. Then he calmed down again. We could not find a pattern in this activity. It seems to be random. The black hole activity profile was new and exciting every time we looked at it ».

Two separate processes

Although astrophysicists still do not completely understand the processes that intervene, Yusef-Zadeh suspects that there are two different processes that are responsible for short explosions and longer flares. If the accretion disc is a river, then short and weak flashes are like small waves that fluctuate randomly on the surface of the river. However, the longest and brighter flares are more similar to tidal, caused by more significant events.

The researcher postulates that small disturbances within the accretion disc probably generate weak flashes. Specifically, turbulent fluctuations inside the disc can compress plasma (a hot gas electrically loaded) and cause a temporary radiation explosion. Yusef-Zadeh compares the event with a solar flare.

“It is similar to how the magnetic field of the sun is concentrated, compressed and then causes a solar eruption,” he explains. «Of course, the processes are more intense because the environment around a black hole is much more energetic and much more extreme. But the surface of the sun also bubbles activity ».

On the other hand, the big and brilliant flares are attributed to magnetic reconnection events, a process in which two magnetic fields collide and release energy in the form of accelerated particles. These particles, which travel at speeds near light, emit bright radiation bursts.

“A magnetic reconnection event is like a static electricity spark, which, in a sense, is also an ‘electrical reconnection’,” said Yusef-Zadeh.

Double vision

As the JWST Nircam Chamber can observe two different wavelengths (2.1 and 4.8 microns) at the same time, Yusef-Zadeh and its collaborators could compare how the brightness of the flares with each wavelength changed. Yusef-Zadeh said that capturing the light in two wavelengths is like “seeing in color instead of black and white.” When Sagittarius A* in multiple wavelengths, he captured a more complete and nuanced image of his behavior.

Once again, the researchers took a surprise. Unexpectedly, they discovered that the events observed in the shortest wavelength changed the brightness slightly before the longest wavelength events.

“It is the first time we see a delay in measurements in these wavelengths,” says Yusef-Zadeh. “We observe these wavelengths simultaneously with NIRCAM and notice that the longest wavelength is delayed very little with respect to the shortest, perhaps between a few seconds and 40 seconds.”

This temporal delay provided more clues about the physical processes that occur around the black hole. An explanation is that particles lose energy along the flare, losing faster energy in shorter wavelengths than in longer wavelengths. Such changes are expected for spiral particles around magnetic field lines.

To explore these issues more thoroughly, Yusef-Zadeh hopes to use the JWST to observe Sagittarius A* for a longer period of time. Recently he presented a proposal to observe the black hole for uninterrupted 24 hours. The longest observation period will help reduce noise, which will allow researchers to see even faster details.

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.