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An international team of astronomers, including researcher from the Department of Physics at The University of Hong Kong (HKU), has uncovered the first decisive evidence that at least some fast radio burst (FRB) sources—brief but powerful flashes of radio waves from distant galaxies—reside in binary stellar systems. This means the FRB source is not an isolated star, as previously assumed, but part of a binary stellar system in which two stars orbit each other.

Using the Five-hundred-meter Aperture Spherical Telescope (FAST) located in Guizhou, also known as the “China Sky Eye”, the team detected a distinctive signal that reveals the presence of a nearby companion star orbiting the FRB source. The discovery, published in Science, is based on nearly 20 months of monitoring an active repeating FRB located about 2.5 billion light-years away.

A rare signal: the RM flare

Changes in the polarisation properties of radio waves can reveal the environment around an FRB source. The team observed a rare phenomenon known as an ‘RM flare’—a sudden and dramatic change in the polarisation properties of the radio signal, likely caused by a coronal mass ejection (CME) from a companion star that contaminates the environment of the FRB source.

‘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.’

Monitoring repeating FRBs with FAST

Fast radio bursts are millisecond-long but extraordinarily bright radio flashes from beyond our Milky Way galaxy. While most FRBs are observed only once, a small fraction repeat, offering rare opportunities for long-term study and making it possible to detect unusual changes over time. These repeating sources have been closely monitored by FAST since 2020 through a dedicated FRB Key Science Programme co-led by Professor Bing Zhang.

FRB 220529A was one of the active repeating FRBs continuously monitored with FAST.

‘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.’

Tracing the signal through space

FRBs are known for their near 100% linear polarisation. As radio waves travel through a magnetised plasma, their polarisation angle rotates with frequency—an effect known as Faraday rotation, measured by 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”.’

Such a short-lived RM change is consistent with a dense magnetised plasma briefly crossing the line of sight.

‘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 cannot be directly observed at this distance, its presence was revealed through continuous radio observations with FAST and Australia’s Parkes telescope.

‘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 discovery also supports a recent unified physical picture proposed by Professor Bing Zhang and his collaborator, in which all FRBs originate from magnetars, with interactions in binary systems enabling a preferred geometry that allows more frequent, repeating bursts. Continued long-term monitoring of repeating FRBs may reveal how common binary systems are among these mysterious sources.

Collaboration and Support

The research was carried out jointly by HKU, Purple Mountain Observatory, Yunnan University, the National Astronomical Observatories of the Chinese Academy of Sciences, and other collaborating 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.

The project received support from the National Natural Science Foundation of China and other national and international grants from the collaborators. 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), as well as FAST and Parkes PI projects (PIs: Y. Li and S. B. Zhang).

For related research papers, please refer to the following link: https://www.science.org/doi/10.1126/science.adq3225

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