Astronomers from an international research team, including scientists from the Department of Physics at the University of Hong Kong (HKU), have found the clearest evidence yet that some fast radio bursts originate in binary star systems. Fast radio bursts, or FRBs, are extremely powerful bursts of radio waves that last only milliseconds and come from distant galaxies. Until now, it was widely assumed that these signals came from single, isolated stars.
The new findings show that at least some FRB sources are part of stellar pairs, with two stars orbiting each other. The discovery upends 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 Spherical Telescope (FAST) in Guizhou, widely known as the “China Sky Eye”. Observing a repeating FRB roughly 2.5 billion light-years from Earth, scientists detected a unique signal that indicated the presence of a nearby companion star. The results, published in Science, are based on nearly 20 months of detailed follow-up.
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 around the FRB source. During the 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.
Scientists believe that this eruption was caused by a coronal mass ejection (CME) from a companion star. Such an eruption would release a plume of dense, magnetized plasma and temporarily alter the space around the FRB source as it passes through the line of sight.
“This finding provides a definitive clue to the origin of at least some recurring FRBs,” said Professor Bing ZHANG, Senior Professor of Astrophysics in the Department of Physics and Founding Director of the Hong Kong Institute of Astronomy and Astrophysics at HKU and 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 fast repeat radio matters
Fast radio bursts release enormous amounts of energy in a very short time, even if they last only milliseconds. Most FRBs have only been detected once, making them difficult to study. However, a smaller group repeats, giving astronomers rare opportunities to track changes over time and detect patterns.
Since 2020, FAST has been closely monitoring recurring FRBs through the dedicated FRB Key Science Program led by Professor Bing Zhang. One of these sources, known as FRB 220529A, became the focus of the new discovery.
“FRB 220529A has been observed for months and initially appeared inconspicuous,” said Professor Bing Zhang. “Then, after long-term observation for 17 months, something really exciting happened.
Monitoring sudden signal shift
FRBs are known for having nearly 100% linear polarization. As the radio waves pass through the magnetized plasma, their polarization angle shifts as a function of frequency, a process called Faraday rotation. This effect is measured using a value known as rotation rate (RM).
“Toward the end of 2023, we saw a sudden increase in RM of more than a hundred,” said Dr. Ye LI of Purple Mountain Observatory and University of Science and Technology of China, first author of the paper.
“The RM then dropped rapidly within two weeks and returned to the previous level. We call this the ‘RM flare’.”
This brief but extreme change is consistent with a dense plume of magnetized plasma crossing the path between the FRB and Earth.
“One natural explanation is that a nearby companion star ejected this plasma,” Professor Bing Zhang explained.
“Such a model works well for the interpretation of observations,” said Professor Yuanpei YANG, a professor at Yunnan University and co-author of the paper. “The requested plasma cluster 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 great distance, its presence has become clear thanks to ongoing radio observations with FAST and Australia’s Parkes Telescope.
A wider picture of fast radio clusters
“This discovery was made possible by persistent observations using the world’s best telescopes and the tireless work of our dedicated research team,” said Professor Xuefeng WU of the Purple Mountain Observatory and the University of Science and Technology of China, lead author of the correspondence.
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 binaries help create the conditions that allow some of these sources to emit recurrent bursts more often. Continued long-term monitoring may help scientists determine how common binaries are among FRB sources.
Cooperation and support
Scientists from HKU, Purple Mountain Observatory, Yunnan University, National Astronomical Observatories of Chinese Academy of Sciences and other institutions participated in the research. Professor Xuefeng Wu (Purple Mountain Observatory), Professors Peng Jiang and Weiwei Zhu (National Astronomical Observatory), and Professor Bing Zhang from the Department of Physics of HKU served as co-corresponding authors.
Funding came from the National Natural Science Foundation of China, along with other national and international grants. Observing time was provided by the FAST FRB Key Science Project (W.-W. Zhu and B. Zhang as Co-PI), the FAST DDT program (coordinated by X.-F. Wu and P. Jiang), and separate FAST and Parkes PI projects (PI: Y. Li and SB Zhang).
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