South Pole Telescope Spots Energetic Stellar Flares at Galaxy's Core

Researchers from the South Pole Telescope project team looked deep into the center of the Milky Way, discovering powerful, surprising bursts of light from two accreting white dwarf systems. It marks the first time such events have been captured in a millimeter-wavelength survey and opens a new window of research opportunity into the dynamic environments at the center of the galaxy. The events were published in The Astrophysical Journal. Texas Tech University's Tom Maccarone, professor in the Department of Physics and Astronomy within the College of Arts & Sciences, was a collaborator on the project. The South Pole Telescope was originally developed to measure the cosmic microwave background, and as the team started making surveys of the Galactic Plane, they recruited Maccarone for his expertise in interacting binary stars to help understand new phenomena as they were discovered. The discovery is important because, rather than targeting a pre-selected list of candidate objects, the survey repeatedly scanned a large portion of the Galactic Plane, uncovering short-lived bursts of light from two known accreting white dwarf systems within one of the most complex regions of the sky. "We've only searched for transients in two years and already found two remarkable events," Maccarone said. "This is a great example of the adage among astronomers that opening new windows on the universe produces new, unexpected, exciting results. We've only scratched the surface of what can be done with millimeter transient surveys of the Galactic Plane and are looking forward to discoveries of many more new events in years to come." The team suspects the flares were triggered by sudden magnetic explosions in the accretion flow—an analog to solar flares, but operating in a far more extreme setting. In the sun, magnetic reconnection can rapidly convert stored magnetic energy into heat and energetic particles. In an accretion disk around a compact object, similar processes can occur at much higher densities and energies, potentially producing bright, short-lived bursts that propagate outward and radiate across multiple bands. If this interpretation holds, millimeter observations may offer new insight into the magnetic physics of accretion disks, which is central to understanding how compact binaries evolve, how they transport angular momentum and how they generate outflows. The international South Pole Telescope team, led by University of Illinois Urbana-Champaign graduate student Yujie Wan, collaborated to develop the analysis necessary to identify transients: sudden, short-lived bursts of emission that appear and fade on timescales of hours to days. In this case, each flare lasted roughly one day. Though longer compared to milliseconds-long radio bursts, it's brief in the context of most astronomical variability and places strong constraints on the size and physical mechanism of the emitting region. Importantly, the events were detected in millimeter bands where transient discovery has historically been far less common than in optical or X-ray surveys. The sources of the outbursts are accreting white dwarfs locked in close binary orbits with companion stars. As a white dwarf's gravity pulls gas off its companion, the material forms a swirling accretion disk that heats up and can drive powerful variability across the electromagnetic spectrum.