Astronomers have traced a mysterious type of repeating cosmic signal to an unusual pair of stars, providing the strongest evidence yet for the source of one of astronomy’s most puzzling phenomena.
The discovery was made by an international research team led by scientists at the University of Sydney using CSIRO’s ASKAP radio telescope. Their findings identify the origin of a rare class of objects known as long-period radio transients, mysterious bursts of radio waves that have puzzled astronomers since they were first detected in only a handful of locations across the Milky Way.
The results were published in Nature Astronomy.
Lead author Kovi Rose, a PhD student in the University of Sydney’s School of Physics and CSIRO, said the team was finally able to connect one of these enigmatic signals to a specific type of stellar system.
“For the first time we have pinpointed the origin of these signals, confirming the source to be a ‘cataclysmic variable’, or an accreting white dwarf star,” said Mr. Rose.
“Long-period radio transients have puzzled astronomers for years,” Mr. Rose said. “We’ve only found about a dozen, and their origins have been unclear. Now, we’ve been able to show that the source for one of these transients comes from a white dwarf actively pulling material from a companion star.”
Rare White Dwarf System Revealed
The newly identified system, known as ASKAP J1745−5051, consists of a white dwarf and a red dwarf locked in an extremely close orbit. A white dwarf is the dense remnant of a dead star, roughly the size of Earth but with a mass comparable to that of the Sun. Its companion is a much larger but less dense red dwarf star containing about one-tenth the Sun’s mass.
The two stars circle one another in just over an hour.
As the white dwarf pulls gas from its companion, the material heats up and emits X-rays. At the same time, interactions between the stars’ magnetic fields generate powerful radio bursts. Together, these emissions repeat on a regular cycle every 1.4 hours.
“These emissions are all tied to the orbital motion of the system,” Mr. Rose said. “But interestingly, the radio and X-ray signals don’t peak at the same time, which tells us they’re being produced in different regions of the system.”
The researchers found that the radio waves are likely produced where the stars’ magnetic fields collide and interact with the stream of charged material flowing toward the white dwarf. This process creates tightly focused bursts of radiation that sweep through space.
Solving the Long-Period Radio Transient Mystery
When long-period radio transients were first discovered, many astronomers suspected they might be unusually slow-spinning neutron stars known as pulsars. However, existing models suggest neutron stars rotating that slowly should not be capable of producing these signals.
The new findings support a different explanation. At least some long-period radio transients appear to originate in binary star systems involving white dwarfs.
“Some similar objects had been linked to binary systems before, but this is the first one where we can clearly see both stars and the accretion process in action,” said Professor Murphy, Head of School at the University of Sydney School of Physics and Chief Investigator at the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).
The system is also only the second known long-period radio transient found to produce regular X-rays. It is the first example in which scientists have confirmed exactly what is causing the periodic behavior.
A Cosmic Rosetta Stone
Researchers believe ASKAP J1745−5051 could become an important reference object for understanding other mysterious radio transients.
The system was discovered using ASKAP, a radio telescope owned and operated by CSIRO, Australia’s national science agency. ASKAP combines a wide field of view, high resolution, and exceptional sensitivity, making it particularly effective at detecting unusual signals that might otherwise go unnoticed.
“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,” said Mr. Rose, referring to the archaeological object discovered in Egypt that helped translate ancient hieroglyphics.
Beyond helping explain mysterious radio signals, the system also offers scientists a rare opportunity to study extreme physical conditions that cannot be recreated in laboratories on Earth.
“These systems are natural laboratories,” Mr. Rose said. “They allow us to test our understanding of how matter behaves in strong magnetic fields and under intense gravitational forces.”
Future Observations Planned
The team plans to continue studying the system using radio, optical, and X-ray telescopes. By combining observations across different wavelengths, they hope to better understand how these signals are produced and whether similar mechanisms can explain the broader population of long-period radio transients.
“Each new discovery is helping us piece together the bigger picture,” Mr. Rose said. “We’re only just beginning to understand this new class of cosmic events.”
The international collaboration included researchers from Australia, the United States, China, Canada, Spain, and Israel. Observations were carried out using CSIRO’s ASKAP and Australia Telescope Compact Array in Australia, the MeerKAT radio telescope in South Africa, the SOAR and Magellan optical telescopes in Chile, and the space-based Swift (UV/X-ray) and Einstein Probe (X-ray) observatories.
The authors reported no competing interests. Funding for the research was provided by the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav), NASA, the Alfred P. Sloan Foundation, the Professor Harry Messel Research Fellowship in Physics Endowment, the European Research Council, and the China Scholarship Council.