As far as supermassive black holes go, the one at the center of the Milky Way is relatively sedate.
But, even in its supposed quiescent state, Sagittarius A* is prone to the occasional belch or rupture – and now, using JWST, astronomers have recorded it doing something we’ve never seen before.
On 6 April 2024, the black hole let out a flare observed in mid-infrared wavelengths, followed by a radio flare counterpart.
Although Sgr A* belches out the occasional flare, this is the first time we’ve captured it in mid-infrared – one of the missing pieces of the puzzle of the black hole’s behavior, according to a team led by astronomer Sebastiano von Fellenberg from the Max Planck Institute for Radio Astronomy in Germany.
“Sgr A*’s flare evolves and changes quickly, in a matter of hours, and not all of these changes can be seen at every wavelength,” says astrophysicist Joseph Michail of the Smithsonian Astrophysical Observatory.
“For over 20 years, we’ve known what happens in the radio and Near-infrared (NIR) ranges, but the connection between them was never 100 percent clear. This new observation in mid-infrared fills in that gap.”
Supermassive black holes are a crucial component to the ordering of the Universe as we know it, the nuclei around which galaxies cluster and revolve. They range from millions to billions of times the mass of the Sun, and exhibit a range of activity levels, from ravenously rampageous as they scarf down matter at a tremendous rate, to calm and quiescent.
Sgr A*, at the heart of the Milky Way and clocking in at 4.3 million solar masses, is the closest supermassive black hole we have access to. It’s also on the quiescent end of the activity scale, which means we have a front row seat to small-scale black hole behavior that would be too faint to see were it taking place in another galaxy.
Astronomers have been closely watching the galactic center for decades in a range of wavelengths to record its strange blips and burps to learn more about the activity and dynamics of the most gravitationally extreme environment in the Milky Way galaxy.
Sgr A*’s presence creates a wild, turbulent region of space, with a huge torus of dust roiling around the supermassive black hole. Astronomers don’t know what causes the flares in the region, but simulations suggest that it’s an interaction between magnetic field lines in the disk of material that most closely orbits the black hole.
When two field lines get close enough together, the simulations suggest, they can join together in a way that releases a huge amount of energy that we can see as synchrotron emission – the radiation emitted by electrons accelerating along the magnetic field lines.
But we couldn’t be sure, because we didn’t have mid-infrared observations of one of these flares.
“Because mid-infrared sits between the submillimeter [far-infrared to microwave] and the near-infrared, it was keeping secrets locked away about the role of electrons, which have to cool to release energy to power the flares,” Michail explains.
“Our new observations are consistent with the existing models and simulations, giving us one more strong piece of evidence to support the theory of what’s behind the flares.”
The observations were collected using JWST’s mid-infrared instrument (MIRI); the Submillimeter Array jointly operated by the Smithsonian Astrophysical Observatory and Academia Sinica; NASA’s Chandra X-ray Observatory; and NASA’s Nuclear Spectroscopic Telescope Array, a gamma-ray observatory riding the International Space Station.
When JWST caught a flare that lasted around 40 minutes, they turned to the other instruments to see what they may have collected. There were no detections in the X- and gamma-ray regimes – likely because the electron acceleration wasn’t high enough – but the Submillimeter Array caught a flare of radio waves lagging around 10 minutes behind the mid-infrared.
These results, the researchers say, are consistent with synchrotron radiation from a single population of cooling electrons accelerating through magnetic reconnection, magnetic turbulence, or a combination of both. However, there is a lot we still don’t know – which means there’s more work to be done.
“While our observations suggest that Sgr A*’s mid-infrared emission does indeed result from synchrotron emission from cooling electrons, there’s more to understand about magnetic reconnection and the turbulence in Sgr A*’s accretion disk,” von Fellenberg says.
“This first-ever mid-infrared detection, and the variability seen with the Submillimeter Array, has not only filled a gap in our understanding of what has caused the flare in Sgr A* but has also opened a new line of important inquiry.”
The research was presented at the 245th meeting of the American Astronomical Society. It has also been accepted into The Astrophysical Journal Letters, and is available on preprint server arXiv.
