Using the MIRI instrument onboard the James Webb Space Telescope, an international team of scientists made the first-ever detection of a mid-IR flare from Sagittarius A*, the supermassive blackhole at the heart of the Milky Way. In simultaneous radio observations, the team found a radio counterpart of the flare lagging behind in time. The paper is published on the arXiv preprint server.
Scientists have been actively observing Sagittarius A* (Sgr A)—a supermassive black hole roughly 4 million times the mass of the sun— since the early 1990s. Sgr A regularly exhibits flares that can be observed in multiple wavelengths, allowing scientists to see different views of the same flare and better understand how it emits light and how the emission is generated. Despite a long history of successful observations, and even imaging of the cosmic beast by the Event Horizon Telescope in 2022, one crucial piece of the puzzle— mid-infrared observations (Mid-IR)—was missing until now.
Infrared (IR) light is a type of electromagnetic radiation that has longer wavelengths than visible light, but shorter wavelengths than radio light. Mid-IR sits in the middle of the infrared spectrum, and allows astronomers to observe objects, like flares, that are often difficult to observe in other wavelengths due to impenetrable dust. Until the recent study, no team had yet successfully detected Sgr A*’s variability in the mid-IR, leaving a gap in scientists’ understanding of what causes flares, and questions about whether theoretical models are complete.
“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 Joseph Michail, one of the lead authors on the paper, a Postdoctoral Fellow at the Harvard CfA. “For over 20 years, we’ve known what happens in the radio and what happens in the near infrared, but the connection between them was never 100% clear or certain. This new observation in mid-IR fills in that gap and connects the two.”
Scientists aren’t 100% sure what causes flares, so they rely on models and simulations, which they compare with observations, to try to understand where they come from. Many simulations suggest that flares in Sgr A* are caused by the bunching of magnetic field lines in the supermassive black hole’s turbulent accretion disk. When two magnetic field lines approach they can connect to each other and release a large amount of their energy.
The byproduct of this magnetic reconnection—synchrotron emission—occurs when energized electrons travel at speeds close to the speed of light along the magnetic field lines of the supermassive black hole. They emit high-energy radiation photons that power the flare.
Because the mid-IR spectral range sits between the submillimeter and the near-infrared (NIR), it is keeping secrets locked away about the role of electrons, which have to cool to release energy to power the flares. The new observations are consistent with the existing models and simulations, giving one more strong piece of evidence to support the theory of what’s behind the flares.
“Our research indicates that there may be a connection between the observed variability at millimeter wavelengths and the observed mid-IR flare emission,” says Sebastiano von Fellenberg, a postdoctoral researcher at the Max Planck Institute for Radio Astronomy (MPIfR) and the lead author on the new paper.
He adds that the results underscore the importance of expanding multi-wavelength studies of not just Sgr A, but other supermassive black holes, like M87, to get a clear picture of what’s really happening within and beyond their accretion disks.
“While our observations suggest that Sgr A‘s mid-IR 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,” says von Fellenberg. “This first-ever mid-IR detection, and the variability seen with the SMA, 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.”
Simultaneous observations with the Submillimeter Array (SMA), the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Chandra X-ray Observatory filled in an additional part of the story. No flare was detected during the X-ray observations, likely because this particular flare didn’t accelerate electrons to energies as high as some other flares do. But the team was successful when they turned to the SMA, which detected a millimeter-wave flare lagging roughly 10 minutes behind the mid-IR flare.
“Working on reducing and calibrating the data from James Webb—which is presently one of the best telescopes we have—was a dream come true for me, and I’m really grateful for the amazing mentorship of Sebastiano von Fellenberg and Gunther Witzel. I look forward to working further in this area by pursuing a Ph.D. after graduating this year,” says Tamojeet Roychowdhury, currently a student of the Indian Institute of Technology in Bombay.
“We are building an increasingly detailed picture of the processes that take place in the immediate vicinity of a supermassive black hole. The quality of our mid-infrared data is yet another testament to the James Webb Space Telescope’s enormous technical capabilities,” concludes Witzel, staff scientist at the MPIfR. https://phys.org/news/2025-01-mid-infrared-flare-sagittarius-milky.html
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