Astronomers utilized the James Webb Space Telescope (JWST) to achieve a new perspective on flares emanating from Sagittarius A* (Sgr A*), the supermassive black hole at the core of the Milky Way galaxy. The analysis of these observations is expected to reveal the mechanisms by which black hole flares are launched and the influence of magnetic fields on the surrounding cosmic matter.
Bridging the Spectral Gap
The research team, which includes Sebastiano von Fellenberg of the Max Planck Institute for Radio Astronomy, successfully observed flaring from Sgr A* in the Mid-infrared observations regime for the first time. Prior observations were routinely conducted in the near-infrared and other wavelengths.
According to von Fellenberg, the new JWST data, first revealed in January 2025, is crucial as it “closes the gap between the radio and near-infrared regimes” in the spectrum of Sgr A* flares. This mid-infrared window was previously a “gaping hole.”
The ability to observe the source simultaneously at four different wavelengths with a single instrument allowed the team to measure the mid-infrared spectral index, an important parameter for understanding the flare’s evolution.
Unraveling the Mechanism of Flaring
While black holes themselves do not emit light, Sgr A* frequently emits flares, or “burps,” of light. Simulations suggest these flares may result from interactions between surrounding magnetic fields, where the reconnection of field lines releases vast amounts of energy and, as a byproduct, emits synchrotron radiation.
The finding that the mid-infrared spectral index of the Sgr A* flare changes over its lifetime revealed that a phenomenon called Synchrotron cooling is occurring around Sgr A*. Synchrotron cooling describes high-speed electrons losing energy by emitting synchrotron radiation, which powers the observed mid-infrared emissions.
Von Fellenberg explained that this behavior, though expected, had not been confirmed until these high-sensitivity multi-frequency observations by JWST.
“What is cool about this is that since the speed of this cooling, the cooling time scale, depends on the Magnetic field strength, we can now measure it for the given flare,” von Fellenberg stated.
This new method provides a “clean” way of determining the magnetic field strength, independent of other variables, offering valuable constraints for theoretical models which are currently poorly defined for Sgr A*. These observations were made possible specifically by the Medium-Resolution Spectrometer (MRS) mode of JWST’s Mid-Infrared Instrument (MIRI).
