Repeating radio signals lead UMW astronomers to a rare binary white dwarf star system

A strange radio signal from about 3,000 light-years away has given astronomers their clearest evidence yet that at least some long-period radio transients come from tightly packed binary stars.

The object, ASKAP J1745-5051, emits powerful radio bursts roughly every 80 minutes. That rhythm once made it look like a slow-moving cousin of a pulsar. But the new observations point somewhere else: a compact system containing a strongly magnetized white dwarf and a low-mass companion.

The work involved an international team led by astrophysicist David Kaplan at the University of Wisconsin-Milwaukee and Kovi Rose from the University of Sydney.

Long-period radio transients have puzzled astronomers since their discovery a few years ago. Unlike pulsars, which can pulse every few seconds, these objects can remain quiet for tens of minutes or even hours before producing another burst.

“What they found was something that emitted pulses once about every 20 minutes,” Kaplan said. “The slowest ones pulse every six hours.”

The team first discovered ASKAP J1745-5051 with the Australian SKA Pathfinder radio telescope during a search for circularly polarized radio sources.

Follow-up observations with MeerKAT refined its location. Astronomers then identified an optical counterpart in Gaia data and studied it with the SOAR and Magellan telescopes.

Those spectra showed strong hydrogen and helium emission lines, along with a blue excess. That combination is characteristic of magnetic cataclysmic variables, compact binary systems that contain a strongly magnetized white dwarf and a normal companion star.

The system’s measured orbital period was 1.368 ± 0.053 hours. That closely matched the repeating radio signal measured over nearly two years with ASKAP and the Australia Telescope Compact Array.

“The pulsing that we see is not coming from a spin. We think it’s coming from an orbit,” Kaplan explained.

That distinction matters. Pulsars produce regular signals because of rotation. ASKAP J1745-5051 appears to pulse because two stars circle one another in an extremely tight orbit.

Kaplan said the stars must be both small and close together to complete an orbit in about 80 minutes. Material may be spilling from one star onto the other, producing the signatures seen across several wavelengths.

The white dwarf likely orbits with a very low-mass red dwarf or brown dwarf companion. If the companion fills its Roche lobe, the researchers estimate its mass at about 0.096 times the Sun’s mass and its radius at about 0.132 times the Sun’s radius.

ASKAP J1745-5051 does more than pulse on schedule. Its bursts are strongly polarized, sometimes switch off for hours, and change in frequency in ways not previously seen in most long-period radio transients.

The team also detected narrow frequency structures known as modulation lanes. Similar patterns are commonly seen in radio emission from the Jupiter-Io system but had not been detected in another binary system.

The researchers argue that local plasma, likely tied to accretion, may act as an interference screen for the radio beam. Optical spectra also point to dense plasma and changing accretion conditions.

X-ray and ultraviolet observations add another piece. ASKAP J1745-5051 showed variable X-ray emission that changed by more than an order of magnitude. The X-rays also repeated at nearly the same period as the radio bursts.

The measured X-ray period was 1.32 ± 0.13 hours. That agreement with the orbital period supports the idea that the system’s radio and X-ray behavior is tied to the orbit.

The radio source is also unusually bright. Depending on the uncertain distance, its radio luminosity is greater than that of about 99% of known radio stars and roughly 100 times brighter than known cataclysmic variables at the lower distance limit.

That makes ordinary magnetic activity from the companion an unlikely explanation.

The object resembles magnetic cataclysmic variables in its optical spectrum, yet it also behaves like a long-period radio transient.

That overlap could make it especially useful.

“This system gives us a way to decode these signals. It could help us determine whether other long-period transients are more like pulsars or like white dwarf systems, acting like a stellar Rosetta stone,” lead author Kovi Rose said.

The discovery also supports a broader conclusion: accreting cataclysmic variables make up at least part of the emerging population of long-period radio transients.

That does not mean every long-period transient has the same origin.

“That’s the fundamental question. And the answer is, we don’t know,” Kaplan said.

Other objects with similar radio behavior do not clearly look like white dwarf systems. Some long-period transients also turn on and off intermittently, adding to the uncertainty.

“There’s a big effort to try and discover more of these and discover them more systematically, so we can try to understand which properties are accidents and which properties are required, and to study them over time as well,” Kaplan said. “(These objects all) walk like a duck, but do they talk like a duck? We don’t know if they’re both ducks or maybe one’s a duck and the other’s a goose.”

ASKAP J1745-5051 gives astronomers a concrete system in which they can test how magnetic fields, accretion, orbital motion and plasma combine to create long-period radio bursts.

Future optical photometry, spectropolarimetry, radio observations and X-ray measurements could help determine the white dwarf’s spin, better define the companion star and clarify how the radio emission is generated.

Those observations may also show whether ASKAP J1745-5051 represents a common pathway for long-period radio transients or only one branch of a more varied population.

“We’ve been studying the sky for thousands of years, but we’re still finding these new and exciting objects that we don’t understand. Sometimes they lead to new pieces of fundamental physics that turn out to be really important,” Kaplan said.

These resources explore competing explanations for long-period radio transients, including interacting white dwarf binaries, magnetic plasma processes and sources that remain difficult to classify.

A binary model of long-period radio transients and white dwarf pulsars
Researchers modeled the long-lived transient GPM J1839−10 as a binary system and found its radio behavior can be explained within the same framework used for white dwarf pulsars, strengthening the connection between these object classes. (Nature Astronomy, 2026)

Unraveling the Emission Mechanism Powering Long Period Radio Transients from Interacting White Dwarf Binaries via Kinetic Plasma Simulations
Kinetic plasma simulations show that electron cyclotron maser instability could generate coherent radio pulses in interacting white dwarf and M-dwarf binaries, offering a physical mechanism for some long-period transients. (The Astrophysical Journal Letters, 2026)

Detection of X-ray emission from a bright long-period radio transient
ASKAP J1832−0911 produces synchronized radio and X-ray pulses every 44.2 minutes, revealing that some long-period transients are far more energetic than previously known while leaving both magnetar and white dwarf explanations challenging. (Nature, 2025)

Sporadic radio pulses from a white dwarf binary at the orbital period
Observations of ILT J1101+5521 showed that its 125.5-minute radio period matches the orbit of a white dwarf and M-dwarf binary, demonstrating that orbital motion can control long-period radio emission. (Nature Astronomy, 2025)

Spectroscopic detection of a 2.9-hour orbit in a long-period radio transient
Optical spectroscopy of GLEAM-X J0704−37 identified a white dwarf and M-dwarf binary whose 2.9-hour orbit nearly matches its radio period, providing further evidence that some longer-period transients originate in compact binaries. (Astronomy & Astrophysics, 2025)

Research findings are available online in the journal Nature Astronomy.

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