What are white dwarfs and cataclysmic variable binary systems?

White dwarfs are ultra-dense stellar remnants—the exposed cores of dead stars compressed to Earth size but retaining solar mass. In cataclysmic variable systems, a white dwarf gravitationally pulls material from a nearby companion star, forming an accretion disk. As material spirals inward and crashes onto the white dwarf's surface, it releases tremendous energy in violent outbursts. These systems represent some of the most extreme stellar environments in the universe.

Why is the discovery of millimeter-wavelength flares revolutionary for astronomy?

Millimeter-wavelength flares from white dwarfs were previously unknown and undetected despite decades of observations at other wavelengths. The SPT-3G Galactic Plane Survey's blind discovery (not targeting pre-selected objects) demonstrates that millimeter-wavelength surveys are powerful tools for discovering transient phenomena invisible to optical and X-ray observers, opening an entirely new window on stellar dynamics and accretion physics.

What mechanism produces these millimeter-wavelength flares?

The most likely mechanism is magnetic reconnection in the accretion disk—a process analogous to solar flares but occurring in an environment billions of times more energetic. Magnetic field lines with opposite polarities suddenly reconfigure, explosively releasing stored magnetic energy, heating gas, and accelerating particles to extreme energies. This process produces the intense millimeter-wavelength emission detected by SPT-3G.

How will future observations advance this research?

The SPT-3G survey will continue observing the Galactic Plane yearly, building a longer time-domain record that will reveal additional flares and enable statistical studies of flare rates and properties. Multi-wavelength observations coordinating millimeter data with optical, infrared, and X-ray observations will reveal how energy is distributed across the electromagnetic spectrum. Future facilities like the SKA will enable even more sensitive transient surveys across the radio to millimeter band.

White Dwarf Flares Millimeter Discovery South Pole Telescope

In the heart of our Milky Way, orbiting one another in close embrace, pairs of stars engage in a cosmic dance of theft and violence. A white dwarf—the exposed, ultra-dense core of a dead star, crushed to Earth-size but possessing the mass of the Sun—gravitationally pulls material from its companion star, creating a swirling accretion disk of superhot, infalling gas. This material spirals inward, accelerating to tremendous speeds, heating to millions of degrees, and ultimately striking the white dwarf's surface in a cataclysm of thermonuclear fury. These systems, called cataclysmic variables, have been studied for over a century. Yet on January 20, 2026, researchers announced a discovery that fundamentally expands our understanding of these violent stellar systems: the first-ever detection of energetic millimeter-wavelength flares from accreting white dwarfs, observed by the South Pole Telescope (SPT) during routine monitoring of the Galactic Plane. The discovery, published in The Astrophysical Journal and reported by the Center for AstroPhysical Surveys, is revolutionary for multiple reasons. It represents the first time millimeter-wave transients from white dwarf accretion systems have been discovered in a blind, untargeted survey—as opposed to observations specifically aimed at known candidate sources. It demonstrates that millimeter-wavelength astronomy, long considered primarily a tool for studying the universe's static properties, can also serve as a powerful instrument for detecting rapid, violent transients. And it provides unprecedented insight into the mechanisms by which accretion disks around compact objects release energy catastrophically in brief, energetic bursts. The two flare events detected—each lasting approximately 24 hours and reaching peak luminosities of order 10³¹ ergs per second—are believed to result from magnetic reconnection in the accretion disk, a process analogous to solar flares but occurring in an environment billions of times more energetic.

White Dwarfs and Cataclysmic Variables: Stellar Vampires in Close Orbit

A white dwarf is what remains after a Sun-like star exhausts its nuclear fuel and sheds its outer layers into space. The core—robbed of any internal nuclear burning to support it against gravity—collapses to an astounding density. A teaspoon of white dwarf material would weigh as much as a truck on Earth's surface. The Sun, if crushed into a white dwarf, would squeeze into a sphere the size of Earth. When a white dwarf exists alone in space, it simply cools and fades, eventually becoming a black dwarf—a inert stellar cinder. However, if a white dwarf has a companion star—a low-mass star or evolved star in a close orbit—the situation becomes dramatically more violent. The white dwarf's gravity, far more intense than its companion's, pulls material from the companion's outer atmosphere. This material, following the companion's gravity and initial orbital motion, begins falling toward the white dwarf. However, angular momentum—the tendency of objects in orbit to maintain their rotational motion—prevents the material from falling directly. Instead, it spirals inward, forming an accretion disk of infalling material circling the white dwarf. Within this disk, friction heats the gas to millions of degrees, causing it to emit intense radiation across the electromagnetic spectrum. Eventually, the gas spirals close enough to strike the white dwarf's surface, delivering a cataclysm of energy. These systems, in which a white dwarf accretes material from a companion, are called cataclysmic variables because their brightness varies dramatically and unpredictably as the accretion disk properties change. They represent some of the most extreme and violent stellar systems known, environments where gravity, magnetism, and thermonuclear burning combine in ways rarely found elsewhere in nature.

The South Pole Telescope and Millimeter-Wavelength Astronomy

The SPT-3G Instrument and Unique Capabilities

The South Pole Telescope is a 10-meter diameter radio telescope located at the Amundsen-Scott South Pole Station in Antarctica. The SPT-3G camera, its latest instrument, operates at millimeter and submillimeter wavelengths—electromagnetic radiation with wavelengths of roughly 1-3 millimeters, between microwave and infrared on the electromagnetic spectrum. Millimeter-wavelength observations are valuable because these wavelengths are largely transparent to the dust that obscures visible light, enabling observations of the dusty galactic plane. Additionally, millimeter radiation is particularly effective for studying synchrotron emission—the radiation produced by high-energy electrons spiraling in magnetic fields—a phenomenon that accompanies energetic events and particle acceleration. Originally, the SPT was constructed to measure the cosmic microwave background (CMB)—the faint radiation that fills the universe, left over from the Big Bang. However, the telescope's unprecedented sensitivity and multi-frequency observing capabilities make it ideally suited for other scientific pursuits as well. The SPT-3G camera observes in three frequency bands centered at 95 GHz, 150 GHz, and 220 GHz (corresponding to wavelengths of roughly 3, 2, and 1.4 millimeters, respectively). The combination of high sensitivity, multiple frequency bands, and wide field of view makes SPT-3G a powerful instrument for time-domain surveys—detecting transient, short-lived phenomena across the sky.
The SPT-3G Galactic Plane Survey: A New Time-Domain Frontier

In 2023 and 2024, the South Pole Telescope team initiated the SPT-3G Galactic Plane Survey, the first dedicated high-sensitivity, wide-field, time-domain survey of the Galactic Plane conducted at millimeter wavelengths. The survey observes a region of approximately 100 square degrees centered near the Galactic Center, one of the most complex and crowded regions of the sky. This region contains an extraordinary density of stars, dust, and gas, making it challenging for optical telescopes to observe due to interstellar extinction. However, in millimeter light, the dust becomes partially transparent, enabling observations of phenomena hidden to optical telescopes. The survey strategy consists of roughly 1,500 individual 20-minute observations in the two-year period from 2023 to 2024, with plans for continued observations in coming years. Each position on the sky is observed multiple times, building up a time-domain record of the region. This repeated observational strategy is crucial for transient detection: by observing the same region of sky multiple times, astronomers can identify sources that change brightness between observations, distinguishing genuine transient events from constant sources and instrumental noise. The survey was not specifically designed to discover white dwarf accretion flares; rather, it was a general-purpose time-domain survey aimed at detecting any millimeter-wavelength transients in the Galactic Plane. The discovery of white dwarf flares was thus a serendipitous discovery—exactly the kind of unexpected finding that demonstrates the power of blind surveys to reveal phenomena that targeted observations might miss.
Why Millimeter Wavelengths? A Window into Accretion Dynamics

Cataclysmic variable white dwarfs are known to emit radiation across the electromagnetic spectrum, from the infrared through the ultraviolet to X-rays. However, millimeter-wavelength emission from these systems has traditionally been less studied, as millimeter observations are technically challenging and require sensitive, specialized instruments. The SPT-3G flare detections reveal that millimeter wavelengths are a particularly valuable window into white dwarf accretion disk dynamics. The brightness of the detected flares at millimeter wavelengths—peak flux densities exceeding 50 millijanskys at 150 GHz—is sufficient to be clearly detected above the background noise and confusion from other sources in the Galactic Plane. Moreover, millimeter emission from accreting systems is produced through specific physical processes: synchrotron emission from relativistic electrons in the accretion disk, thermal emission from heated gas, and potentially other mechanisms. By measuring the flares' millimeter properties—their brightness at multiple frequencies, their duration, their rise and decay timescales—astronomers can constrain the physical properties of the accretion disk environment. The fact that these flares were previously unknown suggests that millimeter-wavelength transient surveys can discover phenomena invisible to optical or X-ray surveys, opening a new observational window on accretion physics.

Analysis I: The Two Flare Events and Their Physical Properties

Detection and Characterization: 5-Sigma Significance

The two millimeter-wavelength transient events were identified through automated analysis of the SPT-3G Galactic Plane Survey data. The detection algorithm searches the repeated observations for sources that vary significantly between consecutive observations. The two events exceeded the 5-sigma (5σ) detection threshold—meaning the statistical probability that they represent random noise fluctuations is less than one in 3.5 million. This extraordinarily high confidence threshold is standard in astrophysics for claiming discoveries. Both events were detected in both the 95 GHz and 150 GHz frequency bands, providing multiple independent confirmations of the detection. The peak flux densities at 150 GHz exceeded 50 millijanskys, with luminosities at 150 GHz of order 10³¹ ergs per second. For context, the Sun's total luminosity across all wavelengths is about 4×10³³ ergs per second; these white dwarf flares are thus about 100 times dimmer than the Sun's total output, but this is for emission in a single millimeter-wavelength band. The total luminosity across all wavelengths could be substantially higher. Both events exhibited durations of approximately 24 hours (one day), rising to peak brightness and then declining over this timescale. This timescale is considerably longer than millisecond-duration phenomena like gamma-ray bursts or magnetar flares, but much shorter than the hour-to-week timescales typical of optical outbursts in cataclysmic variables. The day-long duration provides a crucial constraint on the physical size and dynamics of the emitting region.
Association with Known Accreting White Dwarf Systems

A crucial aspect of the discovery is that both transient events could be spatially associated with previously known white dwarf binary systems. The survey team matched the positions of the transient events with catalogs of known accreting white dwarfs and cataclysmic variable stars. Both events were identified as originating from systems already catalogued as accreting white dwarfs in close binary orbits with companion stars. This identification provides essential confirmation that the events arise from the suspected white dwarf accretion disks rather than from other possible sources. The white dwarf systems associated with the flares are not exotic or exceptionally rare; they are known systems that have been studied previously. This fact emphasizes the surprising nature of the discovery: these systems have been observed at many wavelengths over years or decades without detection of millimeter-wavelength flares like those now reported. The millimeter observations have revealed a new aspect of these systems' behavior that was previously unknown or undetected. This underscores how time-domain surveys can reveal phenomena invisible to traditional, non-repeated observations, even of well-known objects.
Spectral Properties and the Frequency Dependence of Flare Emission

The availability of observations in multiple frequency bands (95, 150, and 220 GHz) provides information about how the flare brightness varies with frequency—the "spectrum" of the flare. By measuring the flux density at different frequencies, the team determined how the flare brightness changes as frequency increases. The spectral properties depend on the physical mechanism producing the emission. For synchrotron radiation from relativistic electrons, the spectrum typically exhibits a characteristic shape that depends on the energy distribution of electrons and the strength of magnetic fields. For thermal emission from hot gas, the spectrum depends on temperature. Both flare events exhibited spectral properties consistent with synchrotron emission from an energetic environment, consistent with the hypothesis that the flares originate in the accretion disk where magnetic fields are strong and particles are accelerated to high energies. The spectral measurements thus provide evidence supporting the magnetic reconnection mechanism as the source of the flares.

Analysis II: Magnetic Reconnection and Accretion Disk Dynamics

Magnetic Reconnection: Analog to Solar Flares in Extreme Settings

The leading hypothesis for the origin of the detected millimeter-wavelength flares is magnetic reconnection in the white dwarf accretion disk. Magnetic reconnection is a plasma physics process in which magnetic field lines with opposite polarities approach one another closely enough to break apart and reconnect in a lower-energy configuration. As the field lines reconfigure, the magnetic energy they contained is suddenly released, converted into heat, kinetic energy of bulk motion, and particle acceleration. This process occurs in the Sun's corona, producing solar flares and coronal mass ejections. It occurs in the Earth's magnetosphere during geomagnetic storms. And it appears to occur in the accretion disks of white dwarfs and other compact objects. In the white dwarf accretion disk environment, magnetic reconnection operates under far more extreme conditions than in the Sun. The density is higher, the magnetic field is stronger, the temperature is higher, and the energetics are more violent. Magnetic reconnection in an accretion disk can potentially accelerate particles to extremely high energies and heat the disk to temperatures that produce intense radiation at millimeter wavelengths and shorter wavelengths. A sudden, violent episode of magnetic reconnection—akin to a flare event—could produce the observed brightenings that lasted approximately 24 hours. As the reconnection event proceeds, the disk configuration relaxes to a new equilibrium, and the emission fades.
Rapid Energy Release and Particle Acceleration

The day-long timescale of the flares provides crucial constraints on the physical size and energetics of the emitting region. If the flare begins at a particular location in the accretion disk and propagates outward or couples to the surrounding material, the light travel time provides a minimum timescale for the event. If the flare lasts 24 hours, then information cannot propagate across a region larger than roughly 24 light-hours in size—a distance of several hundred billion kilometers. For context, this is many times larger than the Solar System, but vastly smaller than the distance to nearby stars. This implies that the flare is a large-scale phenomenon within the accretion disk, not a tiny localized event. Alternatively, if the flare involves shock waves or other propagating disturbances, the propagation speed constrains the magnetic field strength and plasma properties. The requirement that a day-long flare occur in the disk of a white dwarf—a system with orbital periods of hours or less—implies that the flare involves large portions of the accretion disk or perhaps resonant instabilities that couple to the entire disk on timescales of order the orbital period.
Implications for Accretion Disk Stability and Variability Mechanisms

The discovery of millimeter-wavelength flares from white dwarf accretion disks reveals that these systems are capable of dramatic, sudden energy release on day-long timescales. Previous studies of cataclysmic variables have identified optical outbursts lasting days to weeks, X-ray flares lasting hours to days, and radio variability on similar timescales. However, simultaneous millimeter-wavelength observations during these outbursts have been rare. The SPT-3G detections suggest that millimeter-wavelength flaring is a common or at least repeatable phenomenon in some cataclysmic variable systems. Understanding what triggers these flares—whether they occur preferentially in certain types of systems, whether they correlate with optical or X-ray activity, and whether they occur periodically or randomly—requires continued monitoring of additional systems. The two events detected in the first two years of SPT-3G Galactic Plane Survey data suggest that continued observations over the next several years will likely reveal additional flares, eventually enabling statistical studies of flare rates, properties, and correlations with other system parameters.

Discussion: Opening New Pathways in Time-Domain Millimeter Astronomy

Blind Surveys as Discovery Engines: A Paradigm Shift in Transient Astronomy

The discovery of millimeter-wavelength flares from white dwarfs emerged not from targeted observations of pre-selected candidate objects but from a blind, wide-field survey designed with no specific expectation of discovering these phenomena. This distinction is scientifically profound. Targeted observations can achieve exquisite depth and sensitivity for specific objects, but they are biased toward systems that theorists predict should be interesting or that optical surveys have previously identified. Blind surveys, by contrast, observe broad regions of sky without prejudgment and are thus capable of discovering phenomena that theorists did not anticipate or that previous surveys in other wavelength regimes missed. The SPT-3G Galactic Plane Survey, despite having only two years of data analyzed to date, has already demonstrated this principle: two white dwarf flares that years or decades of targeted millimeter observations would likely have missed were discovered serendipitously. This success suggests that continued blind surveys with improved sensitivity will uncover entirely new classes of millimeter-wavelength transients. The paradigm is shifting: millimeter-wavelength astronomy, long considered primarily a tool for studying the universe's static properties, is being recognized as equally valuable for discovering rapid, violent transients. Future blind surveys with the SPT-3G and other millimeter-wavelength facilities will likely reveal many more unexpected transient phenomena.
Implications for Compact Binary Physics and Accretion Theory

The detection of these millimeter-wavelength flares has immediate implications for understanding accretion disk physics and the mechanisms by which accretion disks around compact objects release energy. In addition to white dwarfs, accretion disks orbit neutron stars and black holes, spanning a vast range of system parameters and physics regimes. Understanding magnetic reconnection and transient energy release in white dwarf accretion disks provides insights applicable to these other systems as well. The day-long timescale of the flares, the luminosities achieved, and the spectral properties all provide constraints on accretion disk models and magnetohydrodynamic simulations. Theorists can now use these observations to test and refine models of accretion disk instability, magnetic field evolution, and energy release mechanisms. Additionally, the discovery motivates follow-up observations of the white dwarf systems that produced the flares and targeted millimeter observations of other known cataclysmic variables, searching for additional flare events. Multi-wavelength observations coordinating millimeter data with simultaneous optical, infrared, and X-ray observations would provide unprecedented insight into how energy is distributed across the electromagnetic spectrum during these transient events.
Future Prospects: SKA and Next-Generation Millimeter Surveys

The SPT-3G Galactic Plane Survey represents the current state of the art in millimeter-wavelength time-domain surveys, but it is merely the beginning. Future observatories promise dramatically improved capabilities. The Vera C. Rubin Observatory's Legacy Survey of Space and Time, commencing in 2026, will conduct optical time-domain surveys of unprecedented depth and cadence, discovering optical transients at rates orders of magnitude higher than current surveys. Radio surveys with the Karl G. Jansky Very Large Array and the European VLBI Network will probe radio transients. The next generation of millimeter-wavelength facilities—including next-generation instruments on existing telescopes and new telescopes under construction—will achieve sensitivities and survey speeds enabling discovery of fainter, shorter-duration transients. The Square Kilometre Array, under construction and expected to begin operations in the 2030s, will revolutionize transient astronomy at radio wavelengths. These future facilities, coordinated in multi-wavelength survey campaigns, will enable astronomers to discover and characterize millimeter-wavelength transients with unprecedented frequency and detail. The SPT-3G discoveries of white dwarf flares serve as a proof-of-concept demonstrating the scientific value of dedicated millimeter time-domain surveys and motivating the development of next-generation instruments optimized for this emerging field.

Conclusion: A New Window on Stellar Violence and Magnetic Fury

The South Pole Telescope's detection of energetic millimeter-wavelength flares from accreting white dwarf binary systems marks a milestone in time-domain astronomy. For the first time, millimeter-wavelength surveys have revealed transient phenomena from these systems, phenomena that persisted undetected despite decades of observations at other wavelengths. The discovery demonstrates that blind, wide-field millimeter surveys can serve as powerful discovery engines for transient phenomena, revealing unexpected phenomena that targeted observations at other wavelengths miss. The flares themselves provide unprecedented insight into magnetic reconnection and rapid energy release in the extreme environments of white dwarf accretion disks, analogous to solar flares but occurring under conditions billions of times more energetic. The day-long durations, multi-frequency detections, and large luminosities all constrain theoretical models of accretion disk instability and magnetic field dynamics. As the SPT-3G Galactic Plane Survey continues to observe the Galactic Plane in coming years, and as future millimeter-wavelength surveys improve in sensitivity and survey speed, the expectation is that many more transient phenomena—both from white dwarfs and from other astrophysical sources—will be discovered. Millimeter-wave astronomy is transitioning from a field primarily concerned with measuring the universe's static properties to a field equally engaged in capturing the universe in motion, revealing brief, violent phenomena that reshape our understanding of extreme astrophysics.