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 observed farther out in the universe. However, new findings from NASA’s James Webb Space Telescope highlight 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*, emits approximately five or six significant flares daily, in addition to several smaller bursts occurring in between.
The observations were detailed in The Astrophysical Journal Letters.
“In our data, we saw a constantly changing, bubbling brightness. And then boom! A big burst of brightness suddenly popped up. Then it calmed down again,” reported Dr. Farhad Yusef-Zadeh, the study's lead author, from Northwestern University in Illinois. “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.”
Dr. Yusef-Zadeh and his colleagues observed Sagittarius A* using JWST’s Near-Infrared Camera, or NIRCam, accumulating a total of 48 hours of observations, segmented into eight to ten-hour increments over the course of a year. While they anticipated seeing flares, the level of activity they observed around the black hole was unexpected.
The researchers propose that two distinct processes could be responsible for the observable light show. Smaller flares may arise due to turbulence in the accretion disk, which compresses the disk's hot, magnetized gas. Such disturbances could eject brief bursts of radiation, akin to solar flares.
“It’s similar to how the sun’s magnetic field gathers together, compresses, and then erupts in a solar flare,” Yusef-Zadeh elaborated. “Of course, the processes are more dramatic because the environment around a black hole is much more energetic and extreme.”
In contrast, the more substantial bursts may result from magnetic reconnection events, occurring when two magnetic fields collide, propelling bright blasts of particles traveling at velocities nearing the speed of light. Yusef-Zadeh explained, “A magnetic reconnection event is like a spark of static electricity, which, in a sense, also is an ‘electric reconnection.’”
Moreover, one of the unexpected findings from this research pertains to the brightness variation of the flares when viewed at two different wavelengths. Events observed at shorter wavelengths were found to brighten slightly before their longer-wavelength counterparts.
“This is the first time we have seen a time delay in measurements at these wavelengths,” Yusef-Zadeh stated. “We simultaneously observed these wavelengths with NIRCam and noticed the longer wavelength lags behind the shorter one by a very small amount — maybe a few seconds to 40 seconds.”
These observations may offer essential clues regarding the physical processes occurring within the disk swirling around the black hole. It could indicate that particles expelled by the flares lose energy more rapidly at shorter wavelengths than they do at longer wavelengths. This pattern is consistent with particles spiraling along magnetic field lines in a cosmic synchrotron context.
Researchers are now seeking an extended observation period with the JWST, anticipating it will help them minimize observational noise and produce a more detailed understanding of events at the heart of our galaxy.
“When you observe such weak flaring events, you must contend with noise,” cautioned Yusef-Zadeh. “If we can observe for 24 hours, we can reduce the noise and uncover features that were previously invisible. This would be astounding. Additionally, we can determine whether these flares repeat or if they are genuinely random.”
In addition to Yusef-Zadeh, the authors of the study in The Astrophysical Journal Letters, titled “Nonstop Variability of Sgr A* Using JWST at 2.1 and 4.8 μm 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.
