Webb Space Telescope Tracks Fireworks Around Our Galaxy’s Black Hole

The supermassive black hole at the center of our Milky Way galaxy may not be as voracious as the gas-gobbling monsters that astronomers have seen farther out in the universe, but new findings from NASA’s James Webb Space Telescope reveal that its surroundings are flaring with fireworks.

JWST’s readings in two near-infrared wavelengths have documented cosmic flares that vary in brightness and duration. Researchers say the accretion disk of hot gas surrounding the black hole, known as Sagittarius A*, throws off about five or six big flares a day, and several smaller bursts in between.

The observations are detailed today in The Astrophysical Journal Letters.

“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,” study lead author Farhad Yusef-Zadeh of Northwestern University in Illinois said in a news release. “We couldn’t find a pattern in this activity. It appears to be random. The activity profile of this black hole was new and exciting every time that we looked at it.”

Yusef-Zadeh and his colleagues observed Sagittarius A* using JWST’s Near-Infrared Camera, or NIRCam, for a total of 48 hours, broken up into eight- to 10-hour increments over the course of a year. They expected to see flares, but they didn’t expect the black hole’s surroundings to be as active as they are.

The researchers suggest that two separate processes are sparking the light show. The smaller flares may be due to turbulence in the accretion disk, compressing the disk’s hot, magnetized gas. Such disturbances could throw off brief bursts of radiation that Yusef-Zadeh likens to solar flares.

“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.”

The bigger bursts could be due to magnetic reconnection events. That would occur when two magnetic fields collide, throwing off bright blasts of particles that travel at velocities near the speed of light. “A magnetic reconnection event is like a spark of static electricity, which, in a sense, also is an ‘electric reconnection,’” Yusef-Zadeh said.

Another unexpected finding has to do with how the flares brighten and dim when seen in two different wavelengths. 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.”

Those observations could serve as clues to the physical processes at work in the disk swirling around the black hole. It could be that the particles thrown off by the flares lose energy more quickly at shorter wavelengths than at longer wavelengths. That’s the pattern you’d expect for particles spiraling around magnetic field lines in a cosmic synchrotron.

Now researchers are hoping to get a longer stretch of time on JWST, which should help them reduce the noise in their observations and produce a more detailed picture of what’s going on at the center of our home galaxy.

“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 repeat themselves, or if they are truly random.”

In addition to Yusef-Zadeh, the authors of the study in The Astrophysical Journal Letters, “Nonstop Variability of Sgr A* Using JWST at 2.1 and 4.8 ?m Wavelengths: Evidence for Distinct Populations of Faint and Bright Variable Emission,” include H. Bushouse, R.G. Arendt, M. Wardle, J.M. Michail and C.J. Chandler.

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