Using the James Webb Space Telescope (JWST), an international team of scientists has detected for the first time a mid-infrared (IR) flare from the supermassive black hole (SMBH) at the heart of the Milky Way Galaxy.
The team includes Assistant Professor Bart Ripperda and graduate student Braden Gail from the Faculty of Arts & Science’s Canadian Institute for Theoretical Astrophysics (CITA).
The supermassive black hole (SMBH) is known as Sgr A* — pronounced “Sagittarius A star.” It is roughly 4 million times the mass of the Sun and has been the subject of scientific scrutiny 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, understand how it emits flares and on what timescale 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.
The reported mid-IR detection happened during a flare that lasted about 40 minutes, a duration similar to near-IR and X-ray flares. Infrared light is a type of electromagnetic radiation with longer wavelengths than visible light, but shorter wavelengths than radio waves. 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 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.
“The flare observed at the centre of the Milky Way Galaxy with JWST was so well-monitored that we are not just able to infer the properties of the radiation but we can learn something about the electrons that orbit the black hole and emit the photons. The data is so rich that we could really test our theories of how these flares work via simulations,” shared CITA faculty Bart Ripperda.
Scientists aren’t 100 per cent sure what causes flares, so they rely on models and simulations, which they compare with observations to try to understand what causes flares. 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 radiation emitted by moving electrons. 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.
“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 National Science Foundation Astronomy and Astrophysics Postdoctoral Fellow at the Center for Astrophysics, Harvard & Smithsonian. “For over 20 years, we’ve known what happens in the radio and near-infrared ranges, but the connection between them was never 100 per cent clear. This new observation in mid-IR fills in that gap.”
Braden Gail, who ran simulations on a Canadian supercomputer, adds: “This kind of research is interesting because we're able to probe the fundamental physics of how supermassive black holes accrete material, a process that is known to reshape and evolve galaxies like our own. The recent mid-infrared observation, in addition to existing near-infrared, x-ray and radio, are all critical pieces in solving this puzzle. Discovering the change in spectral index is particularly important in understanding how emitted energy from these flares evolve over time, helping us better understand and model the processes related to their creation and evolution.”
The observations were presented today in a press conference at the 245th proceedings of the American Astronomical Society (AAS) in National Harbor, Maryland, and have been accepted for publication in the Astrophysical Journal Letters (ApJL).
With files from the Center for Astrophysics, Harvard & Smithsonian.