Flickers and flares: Milky Way’s central black hole constantly bubbles with light

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.