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Newswise — National Harbor, MD— Using the James Webb Space Telescope, a team of scientists led by astronomers from the Center for Astrophysics | Harvard & Smithsonian (CfA) detected a Mid-Infrared (mid-IR) flare from the supermassive black hole (SMBH) at the heart of the Milky Way galaxy for the first time. In simultaneous observations, the team found a radio counterpart flare lagging behind.
Scientists have been actively observing Sgr A*— a SMBH 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. This characteristic also helps them understand how it emits flares and on what timescales they occur. Despite a long history of successful observations, including imaging of this cosmic beast by the Event Horizon Telescope in 2022, one crucial piece of the puzzle— mid-IR observations— was missing until now.
Infrared light is a type of electromagnetic radiation. It has longer wavelengths than visible light, but shorter wavelengths than radio light. Mid-IR sits in the middle of the IR spectrum, and allows astronomers to observe objects, such as 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 range, leaving a gap in scientists’ understanding of what causes flares, and questions about whether their 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,” said Joseph Michail, one of the lead authors on the paper and a NSF Astronomy and Astrophysics Postdoctoral Fellow at the Smithsonian Astrophysical Observatory, which is a part of the CfA. “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% clear. This new observation in mid-IR fills in that gap.”
Scientists aren’t 100% sure what causes flares, so they rely on models and simulations, which they compare with observations to try to understand the cause. Many simulations suggest that the flares in Sgr A* are caused by the interaction of magnetic field lines in the SMBH’s turbulent accretion disk. When two magnetic field lines approach each other they can connect to each other and release a large amount of their energy. A byproduct of this magnetic reconnection is synchrotron emission. The emission seen in the flare intensifies as energized electrons travel along the SMBH’s magnetic field lines at close to the speed of light.
Michail said, “Because mid-IR sits between the submillimeter and the NIR, it was keeping secrets locked away about the role of electrons, which have to cool to release energy to power the flares. 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.”
Simultaneous observations with the SAO’s Submillimeter Array— a facility of the CfA, NASA’s Chandra X-ray Observatory (which is operated by the SAO), and NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) 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 hit paydirt when they turned to the SMA, which detected a millimeter (mm) flare lagging roughly 10 minutes behind the mid-IR flare.
“Our research indicates that there may be a connection between the observed mm variability and the observed MIR flare emission,” said Sebastiano D. von Fellenberg, a postdoctoral researcher at the Max Planck Institute for Radio Astronomy (MPIfR) and the lead author on the new paper. He added that the results underscore the importance of expanding multi-wavelength studies of not just Sgr A*, but other SMBHs, 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,” said 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.”
Michail added, “We still want to know, and need to find out… what other secrets is Sgr A* holding that the mid-IR can unlock? What’s really behind the flare’s variable emission? There’s a wealth of knowledge stored up inside this black hole’s region just waiting for us to access it.”
The new observations were presented today in a press conference at the 245th proceedings of the American Astronomical Society (AAS) in National Harbor, Maryland, and are accepted for publication in the Astrophysical Journal Letters (ApJL).
Located near the summit of Maunakea on the Big Island of Hawaii, the Submillimeter Array (SMA) is one of the flagship observatories of the CfA. The observatory consists of eight radio dishes working together as one telescope, giving astronomers a window on a wide range of astronomical objects and phenomena: planets and comets in our own Solar System; the birth of stars and planets; and the supermassive black holes hidden at the centers of the Milky Way and other galaxies. The SMA is operated jointly by the CfA and the Academia Sinica in Taiwan.
Another version of this press release was issued by the Max Planck Institute for Radio Astronomy (MPIfR).
Resource
von Fellenberg, S., Roychowdhury, T., Michail, J. et al. “First mid-infrared detection and modeling of a flare from Sgr A*,” Astrophysical Journal Letters, arxiv: http://arxiv.org/abs/2501.07415
About the Center for Astrophysics | Harvard & Smithsonian
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