South Pole Telescope Detects Rare Flares From Galactic White Dwarfs

The center of the Milky Way is one of the busiest places in the sky. Stars are packed tightly together, clouds of gas drift between them, and the remains of dead stars are scattered throughout the region. Many of these objects change over time, sometimes slowly and sometimes very suddenly.

Most of this activity has been studied using visible light, X-rays, or radio waves. Millimeter waves, which sit between infrared and radio, have mostly stayed in the background. Until recently, astronomers simply did not have the tools to watch the millimeter sky change from day to day.

That situation is now changing. A new study reports the detection of two brief millimeter-wave flares coming from near the Galactic center. Each flare lasted only about a day, but both carried clear signals of energetic activity in compact stellar systems.

Why Millimeter Waves Are Different

Millimeter waves have wavelengths of just a few millimeters. For a long time, astronomers mainly used them to study cold material, such as dust clouds and molecular gas. They were also essential for mapping the faint glow left over from the Big Bang.

Fast events were harder to catch. The instruments were sensitive, but they usually looked at small patches of sky. That made it easy to miss short-lived flashes.

Newer telescopes have changed the picture. They can scan wide areas of sky again and again, building a timeline instead of a single snapshot. This makes it possible to see when something suddenly brightens and then fades away.

Millimeter waves are especially useful for studying magnetic processes. When charged particles move through magnetic fields, they can produce a type of light called synchrotron radiation. This kind of emission often shows up strongly at millimeter wavelengths.

A Telescope at the End of the World

The detections came from the South Pole Telescope, a 10-meter observatory located in Antarctica. It sits at the Amundsen-Scott South Pole Station, one of the coldest and driest places on Earth. Those conditions are actually good for millimeter astronomy because there is very little water vapor in the air.

The telescope was built mainly to study the cosmic microwave background. Over time, its instruments have become powerful enough to do much more.

The camera used in this study, called SPT-3G, observes the sky at three millimeter-wave frequencies. These bands allow astronomers to measure how a source behaves across different wavelengths at the same time.

Just as important, the telescope repeatedly scans the same regions of sky. That repetition makes it possible to track changes, even subtle ones.

Watching the Galactic Plane Over and Over

For this project, the researchers focused on a section of the Galactic Plane near the center of the Milky Way. This region is crowded and complex, filled with bright emission from gas, dust, and many overlapping sources.

The survey covered about 100 square degrees of sky. Observations were made during two observing seasons, in early 2023 and early 2024. On many days, the telescope revisited the same area, sometimes spending only about 20 minutes per scan.

Over the two years, the team collected roughly 500 hours of usable data. That adds up to around 1,500 individual observations.

Working with this data was not simple. The bright background of the Galactic Plane can easily hide or mimic faint changes. To deal with this, the researchers carefully removed steady background signals and masked areas dominated by very bright sources.

After that, they compared each single-day observation with an average map built from many days. Any strong difference stood out as a possible transient event.

Short Flashes That Refused to Be Ignored

To make sure they were seeing something real, the researchers set strict rules. A flare had to appear clearly above the background noise, and it had to show up at more than one observing frequency at the same location.

Using these criteria, two events stood out.

Both appeared suddenly, reached their peak brightness within about a day, and then faded away. At their brightest, they were strong enough to be detected even from near the Galactic center, tens of thousands of light-years away.

Each flare reached a brightness of at least 50 millijanskys at one of the observed frequencies. While that number may sound small, at such distances it represents a significant release of energy.

The duration was also striking. Many known stellar flares last minutes or hours. These lasted about a full day, suggesting a larger and slower process at work.

Tracing the Light Back to Its Source

Finding out what caused the flares required knowing exactly where they came from. The South Pole Telescope does not have the sharpest vision compared to optical telescopes, but repeated observations allowed the team to narrow down the positions to within a few arcseconds.

With those positions, the researchers searched existing catalogs. In both cases, the millimeter flares lined up closely with known X-ray sources observed by NASA’s Chandra X-ray Observatory.

The chance of this happening randomly was very small. That strongly suggested a real physical connection.

The same locations also matched sources seen in optical and infrared surveys. Together, these clues pointed to two compact binary systems that include white dwarfs.

Understanding Accreting White Dwarfs

A white dwarf is the dense core left behind when a star like the Sun runs out of fuel. On its own, a white dwarf slowly cools and fades. In a binary system, things can get much more active.

If a white dwarf has a nearby companion star, gravity can pull gas from the companion. This gas forms a disk as it spirals toward the white dwarf. Astronomers call this process accretion.

The disk heats up and shines in visible and ultraviolet light. In some cases, gas falling onto the white dwarf produces X-rays as well.

These systems are known to be variable. They can brighten suddenly and then settle down again. Until now, their behavior at millimeter wavelengths has not been well studied.

Two Systems, One Shared Pattern

One of the detected flares came from a system classified as a cataclysmic variable. In this type of system, a white dwarf pulls material from a smaller companion star. This particular system has an orbital period of about nine days.

The second flare came from a symbiotic system. Here, the white dwarf orbits a much larger star, a red giant, and accretes material from the giant’s stellar wind.

Despite these differences, the millimeter flares from both systems looked surprisingly similar. They lasted about a day and reached comparable brightness levels.

This suggests that the same basic process may be operating in very different environments.

Signs of Magnetic Activity

Because the telescope observed at multiple frequencies, the researchers could study how the brightness changed with wavelength. This pattern, known as the spectrum, gives clues about the type of emission involved.

The two flares showed different spectral shapes, but both were consistent with nonthermal emission. In simple terms, this means the light was not coming from hot material alone.

One strong candidate is synchrotron radiation. This type of emission is produced when fast-moving electrons spiral around magnetic field lines.

The team also looked for polarization, another possible sign of synchrotron radiation. They did not detect clear polarization, but the limits were not strong enough to rule it out.

In messy, turbulent regions, magnetic fields can be tangled. That can reduce the overall polarization even if synchrotron emission is present.

Magnetic Reconnection as the Trigger

The most likely explanation for the flares is magnetic reconnection in the accretion disks. This happens when magnetic field lines suddenly snap and reconnect, releasing stored energy.

A familiar example is a solar flare on the Sun. In accretion disks, similar processes can occur, but on much larger scales.

As the disk material moves and twists, magnetic fields can build up stress. When that stress is released, particles are accelerated and energy is radiated across the spectrum.

The day-long duration of the observed flares fits with events that involve large regions of the disk, rather than small, brief flashes.

How Common Are These Events?

Earlier searches for millimeter transients mostly avoided the Galactic Plane. Those studies found flares from nearby stars and a few distant galaxies, but not much from compact binaries in the Milky Way.

This new survey looks directly into the crowded heart of the Galaxy. That increases the chance of finding rare or previously missed events.

The fact that two accreting white dwarf flares were detected suggests that such events may not be extremely rare. Still, the study was designed to find only the strongest and clearest signals.

Shorter or weaker flares could easily slip below the detection threshold.

What Remains Uncertain

The results also come with limits. The survey cadence was best suited for events lasting about a day. Very fast flares or very slow ones are harder to catch.

At the highest observing frequency, the data were noisier. This made it difficult to track detailed changes in the spectrum over time.

With only two detections, it is not yet possible to estimate how often these flares occur across the Galaxy. More data will be needed.

A Growing Role for Millimeter Astronomy

Even with these uncertainties, the study shows the growing power of millimeter-wave surveys. Telescopes built for studying the early universe are now revealing new kinds of activity much closer to home.

By adding millimeter observations to optical, X-ray, and radio data, astronomers can build a more complete picture of how compact systems behave.

For accreting white dwarfs, this opens a new window on magnetic processes that are otherwise hard to isolate.

What Comes Next

The South Pole Telescope will continue surveying the Galactic Plane in the coming years. With more data, researchers expect to find more transient events and a wider variety of sources.

Future surveys may lower detection thresholds and explore different timescales. That could reveal a hidden population of millimeter flares that have gone unnoticed until now.

In the crowded center of the Milky Way, even brief flashes of light can carry valuable information. At millimeter wavelengths, the Galaxy is starting to tell new stories.

The research was published in The Astrophysical Journal on January, 2026.