A study in Nature Astronomy has traced a puzzling repeating radio signal to a white dwarf binary, giving astronomers one of their clearest identifications yet for a rare class of slow cosmic pulses. The system, called ASKAP J174508.9-505149, contains a dense stellar remnant that is pulling material from a small companion star.
The discovery matters because these signals have been difficult to classify. They repeat over minutes or hours, which is far slower than the clocklike pulses usually linked to rapidly spinning neutron stars. By combining radio, optical, ultraviolet and X-ray observations, researchers tied this source to a compact pair of stars in a tight orbit that lasts about 1.3 hours.
Lead author Kovi Rose, affiliated with the University of Sydney and CSIRO, found the object during a search through radio survey data. The result gives astronomers a real system they can use to test ideas about other long-period radio transients scattered across the Milky Way.
Signals that broke the usual rhythm
Radio astronomers are used to cosmic objects that blink with astonishing precision. Pulsars, the dense remains of exploded stars, can spin many times per second and send radio beams across space like lighthouse flashes. Those sources fit well into existing models.
Long-period radio transients have a stranger tempo. According to the Nature Astronomy study abstract, “Long-period radio transients (LPTs) are coherent bursts of polarized radio emission that repeat periodically on timescales of minutes to hours.” Only a small number are known and each one adds another clue to a young astronomical puzzle.
The timing has been the central problem. A signal that repeats every few minutes or more than an hour requires a mechanism that can keep a steady rhythm while producing bright radio bursts. That pushed astronomers toward unusual objects with strong magnetic fields, close binary orbits, or both.
Two broad possibilities had drawn attention. One involved a very slowly rotating magnetar, a neutron star with an extreme magnetic field. Another involved a white dwarf in a compact binary, where the orbital dance of two stars might control the pulse cycle.
ASKAP finds the strange source
The break came through CSIRO’s ASKAP radio telescope in Western Australia. ASKAP is designed to scan large areas of the sky, making it well suited for spotting unusual radio objects that appear in survey data.
Rose was searching for circularly polarized radio sources in the Rapid ASKAP Continuum Survey when one object stood out. Its radio behavior matched the broader family of long-period radio transients, yet its identity was still hidden. Follow-up observations were needed to move from a mysterious radio point to a physical star system.
Those observations brought in other facilities and other wavelengths of light. Radio measurements helped refine the source position. Optical spectroscopy then split the system’s light into its colors, revealing the chemical fingerprints of hydrogen and helium. Ultraviolet and X-ray observations added another layer of evidence.
Together, the data pointed to a cataclysmic variable. That phrase describes a binary system in which a white dwarf pulls gas from a companion star. In this case, the companion is a red dwarf, a small cool star with much less mass than the Sun.
A white dwarf stealing gas
A white dwarf is the exposed core left behind after a Sun-like star exhausts its fuel and sheds its outer layers. It can pack a mass close to the Sun’s into a sphere roughly the size of Earth. That gives it intense gravity at the surface.
In ASKAP J174508.9-505149, the white dwarf sits close to a red dwarf companion. The two stars circle each other in about 1.3 hours, a remarkably short orbit for this kind of system. Earlier white-dwarf examples connected with similar radio behavior had orbital periods of roughly 2 to 4 hours.
At such close range, the white dwarf can strip gas from its partner. The material forms a stream and spirals inward. As it falls deeper into the white dwarf’s gravity, it becomes compressed and heated to extreme temperatures.
This process is called accretion. It is one of the most important engines in high-energy astronomy, because falling material can convert gravitational energy into radiation. Around compact stars, that radiation can emerge across the spectrum, from visible light to X-rays.
The Nature Astronomy abstract describes the key identification directly: “Here we report our discovery and classification of the LPT ASKAP J174508.9-505149 as an accreting white dwarf binary.” That classification gave the radio signal a physical home.
X-rays expose the feeding cycle
The X-ray evidence made the case especially strong. Hot gas near a white dwarf can shine in X-rays as it falls onto the compact star. If the radio signal and the X-rays share the same rhythm, the binary orbit becomes difficult to ignore.
For ASKAP J174508.9-505149, the X-ray brightness rises and falls on the same roughly 1.3-hour cycle as the radio bursts. The X-ray strength also changes dramatically, varying by more than a factor of ten. That behavior suggests an uneven flow of material rather than a smooth stream.
Orbitally modulated X-rays give researchers a way to connect the radio pulses with the geometry of the system. As the two stars circle each other, the hot material and magnetic regions around the white dwarf can move in and out of the best viewing angle from Earth.
The radio and X-ray peaks do not line up perfectly. That detail is important because it suggests the two forms of radiation may come from different regions of the binary. The X-rays likely trace very hot accreting gas, while the radio bursts may come from magnetic or plasma processes elsewhere in the system.
Only a few long-period radio transients have been detected in X-rays. That makes this object especially valuable. It offers a rare chance to compare radio pulses with high-energy emission from the same source.
Radio stripes echo Jupiter
The radio signal contains another surprise. Its brightness breaks into fine, evenly spaced bands across the radio spectrum. Similar banding is known from radio emission linked to Jupiter and its moon Io.
Finding such Jupiter-like radio stripes in a distant stellar system gives astronomers a new clue about the plasma around the source. Plasma is gas so energized that electrons have been separated from atoms. It can shape, scatter and filter radio waves as they travel outward.
In this system, charged gas between the emitting region and Earth may be imprinting the striped pattern on the signal. The stripes could record how the radio waves pass through turbulent or structured plasma near the binary.
The radio bursts also drift in frequency and sometimes switch off for hours. That stop-start behavior shows that the emission process can change quickly. The system remains bright in radio compared with most known radio stars, which points to an efficient emission mechanism tied to the white dwarf binary.
Polarized radio bursts add another piece of the puzzle. Polarization describes the orientation of the radio waves. Strong polarization often points to ordered magnetic fields, which are expected to play a major role around compact stellar remnants.
A Rosetta stone for slow cosmic pulses
With ASKAP J174508.9-505149, astronomers now have a strongly identified example of a long-period radio transient powered by an accreting white dwarf binary. That gives the field a benchmark object. Other slow radio sources can now be compared against it in detail.
The study does not require every long-period radio transient to share the same origin. Some may still involve neutron stars or other compact objects. The new result shows that at least some of these slow cosmic signals can arise from white dwarf binaries with active mass transfer.
The paper’s abstract states, “Our results strengthen the link between at least some LPTs and white dwarf binaries.” That cautious wording matters. Astronomy often advances by building a family portrait one system at a time.
The University of Sydney and CSIRO-led work also highlights the power of all-sky radio surveys. ASKAP can reveal the odd sources. Follow-up telescopes can then test whether those sources are isolated stars, compact binaries, or something even rarer.
For readers, the picture is vivid. A dead star, compressed to Earth’s size, is siphoning gas from a small stellar neighbor. Their tight orbit acts like a cosmic metronome. Every cycle, the system can flare in radio waves and X-rays, broadcasting a signal that finally gave away its source.
For astronomers, the next step is comparison. If other long-period radio transients show matching optical lines, X-ray cycles, or radio structures, they may join this white dwarf family. If they differ, the sky may hold several kinds of slow cosmic engines waiting to be sorted out.
