The new findings show that at least some FRB sources are part of stellar pairs, with two stars orbiting one another. This discovery reshapes long-standing assumptions about where these mysterious signals come from and how they are produced.

The team made the breakthrough using the Five-hundred-meter Aperture Spherical Telescope (FAST) in Guizhou, widely known as the "China Sky Eye." While observing a repeating FRB roughly 2.5 billion light-years from Earth, researchers detected a unique signal that pointed to the presence of a nearby companion star. The results, published in Science, are based on nearly 20 months of detailed monitoring.

A Rare Signal Points to a Companion Star

Radio waves carry clues about the space they travel through, including changes in their polarization. By studying these changes, astronomers can learn about the environment surrounding an FRB source. During their observations, the team detected an unusual event known as an 'RM flare'. This involves a sudden and dramatic shift in the polarization properties of the radio signal.

The researchers believe this flare was caused by a coronal mass ejection (CME) from a companion star. Such an eruption would release a cloud of dense, magnetized plasma, temporarily altering the space around the FRB source as it passed through the line of sight.

"This finding provides a definitive clue to the origin of at least some repeating FRBs," said Professor Bing ZHANG, Chair Professor of Astrophysics of the Department of Physics and Founding Director of the Hong Kong Institute for Astronomy and Astrophysics at HKU, and a corresponding author of the paper. "The evidence strongly supports a binary system containing a magnetar -- a neutron star with an extremely strong magnetic field, and a star like our Sun."

Why Repeating Fast Radio Bursts Matter

Fast radio bursts release enormous amounts of energy in a very short time, even though they last only milliseconds. Most FRBs have been detected just once, making them difficult to study. A smaller group, however, repeats, giving astronomers rare opportunities to track changes over time and uncover patterns.

Since 2020, FAST has been closely monitoring repeating FRBs through a dedicated FRB Key Science Program co-led by Professor Bing Zhang. One of these sources, known as FRB 220529A, became central to the new discovery.

"FRB 220529A was monitored for months and initially appeared unremarkable," said Professor Bing Zhang. "Then, after a long-term observation for 17 months, something truly exciting happened."

Tracking a Sudden Shift in the Signal

FRBs are known for having nearly 100% linear polarization. As radio waves pass through magnetized plasma, the angle of their polarization shifts depending on frequency, a process called Faraday rotation. This effect is measured using a value known as the rotation measure (RM).

"Near the end of 2023, we detected an abrupt RM increase by more than a factor of a hundred," said Dr. Ye LI of Purple Mountain Observatory and the University of Science and Technology of China, the paper's first author.

"The RM then rapidly declined over two weeks, returning to its previous level. We call this an 'RM flare'."

This brief but extreme change is consistent with a dense cloud of magnetized plasma crossing the path between the FRB and Earth.

"One natural explanation is that a nearby companion star ejected this plasma," explained Professor Bing Zhang.

"Such a model works well to interpret the observations," said Professor Yuanpei YANG, a professor from Yunnan University and a co-first author of the paper. "The required plasma clump is consistent with CMEs launched by the Sun and other stars in the Milky Way."

Although the companion star itself cannot be directly seen at such a vast distance, its presence became clear through continued radio observations using FAST and Australia's Parkes telescope.

A Broader Picture of Fast Radio Bursts

"This discovery was made possible by the persevering observations using the world's best telescopes and the tireless work of our dedicated research team," said Professor Xuefeng WU of Purple Mountain Observatory and the University of Science and Technology of China, the lead corresponding author.

The findings also support a broader theoretical framework proposed by Professor Bing Zhang and his collaborator. In this model, all FRBs are produced by magnetars, while interactions within binary systems help create conditions that allow some of these sources to emit repeating bursts more frequently. Continued long-term monitoring may help scientists determine how common binary systems are among FRB sources.

Collaboration and Support

The research involved scientists from HKU, Purple Mountain Observatory, Yunnan University, the National Astronomical Observatories of the Chinese Academy of Sciences, and other institutions. Professor Xuefeng Wu (Purple Mountain Observatory), Professors Peng Jiang and Weiwei Zhu (National Astronomical Observatories), and Professor Bing Zhang of the Department of Physics at HKU served as co-corresponding authors.

Funding came from the National Natural Science Foundation of China along with additional national and international grants. Observing time was provided by the FAST FRB Key Science Project (W.-W. Zhu and B. Zhang as Co-PIs), a FAST DDT program (coordinated by X.-F. Wu and P. Jiang), and separate FAST and Parkes PI projects (PIs: Y. Li and S. B. Zhang).