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Figure 1. Dual binding patterns of MBD proteins in mammals and in C. elegans

HKU Biologists Discover Alternative Systems that Help Cells Control Genes

In mammals (left), previous studies show that most MBD3 binding sites overlap with MBD2 and the Nucleosome Remodeling and Deacetylase (NuRD) complex, while MBD2 also independently occupies additional regions enriched with DNA methylation (5-methylcytosine, 5mC).   In Caenorhabditis elegans (C. elegans) (right), which lacks DNA methylation, MBD-2 (Cel-MBD-2) displays a dual binding pattern as in mammalian MBD2, despite lacking a methyl-binding domain. The majority of CelMBD-2 binds to genomic regions enriched with repressive histone modifications, such as H3K27me3 and H3K9me2/3, independently of NuRD. This model illustrates how MBD-2 can be guided by different epigenetic signals when DNA methylation is absent. (Image credit: Tsui, et al., 2026. Nature Communications) Researchers at the School of Biological Sciences of The University of Hong Kong (HKU) have uncovered how eukaryotic cells can control gene activity even after losing one of their major gene-regulatory systems during evolution. By studying a microscopic soil-living roundworm, the team revealed how an alternative, conserved epigenetic mechanism can take over when a common one is missing. The study has just been published in the interdisciplinary scientific journal Nature Communications. The findings provide new insights into how gene regulation adapts through evolution and may help scientists better understand disease mechanisms involving massive gene dysregulation, such as cancers, neurological disorders and autoimmune diseases. Cells must carefully control which genes are turned on and which are turned off throughout development to function properly. While the DNA sequence provides the genetic blueprint, gene expression is also regulated by epigenetic mechanisms — regulatory systems that influence when genes are turned on without changing the genetic code. This allows different cell types, such as nerve cells and muscle cells, to share the same DNA while behaving very differently. One common way cells control gene activity is through DNA methylation, in which a small chemical label, the methyl group, is added to cytosine, a specific base on DNA, forming 5-methylcytosine (5mC), to signal that certain genes should be kept switched off. 5mC is a key epigenetic mark in many animals and plants. However, some organisms, including the microscopic roundworm C. elegans, have lost DNA methylation multiple times during evolution. For a long time, scientists did not fully understand how these organisms could still regulate their genes properly without this major epigenetic system.   In this study, Dr Emily Hok Ning TSUI, a Postdoctoral Fellow at the School of Biological Sciences at HKU, working in the laboratories of Professor Karen Wing Yee YUEN and Professor Chaogu ZHENG, together with Dr Charmaine Yan Yu WONG, also in the YUEN Lab, showed that when DNA methylation is absent, cells can switch to an alternative epigenetic mechanism. Instead of relying on chemical labels on DNA, cells use various histone modifications — different posttranslational chemical marks on histone proteins, the packaging proteins in which DNA is wrapped around inside the cell.   The researchers focused on a protein called MBD-2 (methyl-CpG-binding domain protein 2), which in many animals recognises 5mC-marked DNA and helps silence or activate genes. Surprisingly, even though C. elegans lacks DNA methylation and 5mC, its version of MBD-2 remains essential.   The HKU team found that in C. elegans, MBD-2 no longer reads DNA methylation signals. Instead, it is localised to genes in association with specific repressive histone marks, particularly H3K27me3, a histone modification known to be associated with gene silencing.   When MBD-2 was deleted, the worms became infertile and developed severe physical defects. A large number of genes were no longer properly regulated, demonstrating that MBD-2 remains a key regulator of gene activity, even in the absence of DNA methylation.   These findings reveal that epigenetic regulation is highly adaptable. When one gene-control system is lost, organisms can adapt to read different signals and maintain precise control in gene expression.   “While scientists already know histone modifications and DNA methylation are highly interconnected and crosstalk with each other, this study in C. elegans showcases the functional conservation of the gene-regulatory NuRD complex on one hand, but also the plasticity and adaptability of epigenetic mechanisms in eukaryotes on the other hand,” said Professor Karen YUEN.   This work may help scientists better understand the causes of human diseases, such as cancers, autism and inflammation, in which aberrant DNA methylation disrupts the regulation of many genes at the same time. Understanding how different epigenetic mechanisms can compensate for one another may also facilitate the development of alternative therapeutic approaches.   For more details, please refer to the journal paper: https://www.nature.com/articles/s41467-026-68592-0   Figure 2. The research team at the School of Biological Sciences. From the left: Professor Chaogu ZHENG, Dr Emily Hok Ning TSUI, Dr Charmaine Yan Yu WONG and Professor Karen Wing Yee YUEN. 

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Image credit: S. Tian and Z. Yao

HKU and UCLA Scientists Uncover the Mechanism powering “Space Battery” above Auroral Regions

The dazzling lights of the aurora are created when high-energy particles from space collide with Earth’s atmosphere. While scientists have long understood this process, one big mystery remained: What powers the electric fields that accelerate these particles in the first place? A new study co-led by the Department of Earth and Planetary Sciences at The University of Hong Kong (HKU) and the Department of Atmospheric and Oceanic Sciences at the University of California, Los Angeles (UCLA) now provided an answer. Published in Nature Communications, the research reveals that Alfvén waves — plasma waves travelling along Earth’s magnetic field lines — act like an invisible power source, fueling the stunning auroral displays we see in the sky. By analysing how charged particles move and gain energy in different regions of space, the researchers demonstrated that these waves act as a natural accelerator, supplying energy that drives charged particles down into the atmosphere and produces the glowing auroral lights. To confirm their findings, the team analysed data collected by multiple satellites orbiting Earth, including NASA's Van Allen Probes and the THEMIS mission. The data provided solid evidence that Alfvén waves continuously transfer energy to the auroral acceleration region, maintaining the electric fields that would otherwise dissipate. “This discovery not only provides a definitive answer to the physics of Earth’s aurora, but also offers a universal model applicable to other planets in our solar system and beyond,” said Professor Zhonghua YAO of the Department of Earth and Planetary Sciences at HKU. Professor Yao leads a dedicated team in space and planetary science at HKU, which has established a reputation for high-impact research on planetary auroras. With deep expertise in the magnetospheric dynamics of planets like Jupiter and Saturn, the HKU team brought a critical planetary perspective to the study. “Our team at HKU has long focused on the auroral processes of giant planets. By applying this knowledge to the high-resolution data available near Earth, we have bridged the gap between Earth science and planetary exploration.” Professor Yao added. The research represents a model of interdisciplinary collaboration. The UCLA team, led by Dr Sheng TIAN, contributed extensive expertise in Earth’s auroral physics, while the HKU team provided the broader context of planetary space physics. The full research paper can be read here.   Caption: Comparative schematic of auroral acceleration processes on Earth and Jupiter. The electron spectrum for the Earth was from DMSP F19 spacecraft, and the one for Jupiter was from Juno spacecraft. Both spectra exhibit a similar inverted V-shaped structure, indicating the presence of stable electric potential drops above the auroral regions. This similarity points to a common auroral acceleration mechanism across planets and illustrates how insights from planetary aurorae help interpret high-resolution observations near Earth. Image credit: S. Tian and Z. Yao  

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Internationally Renowned Mathematician Professor Van H. Vu Joins HKU

The Department of Mathematics at The University of Hong Kong has welcomed Professor Van Ha VU, a world-leading mathematician whose research has shaped modern combinatorics, probability, and random matrix theory. Professor Vu received his PhD from Yale University in 1998 and was previously a full professor there. Before that, he held academic positions at the University of California, San Diego and Rutgers University. He is internationally recognised for solving several landmark problems in mathematics, including the Erdős–Folkman problem in number theory with Endre Szemerédi, the Shamir conjecture in random graph theory with Anders Johansson and Jeff Kahn, and together with Terence Tao, the circular law conjecture and four-moment theorem in random matrix theory. Random matrix theory plays a foundational role in quantum physics, complex systems, and artificial intelligence, where large random matrices are used to model quantum behaviour, analyse massive datasets, and understand the stability and performance of modern algorithms. Professor Vu’s work has helped establish the theoretical framework underlying these calculations. His honours include the George Pólya Prize in 2008, the Delbert Ray Fulkerson Prize in 2012,  and an invitation to speak at the International Congress of Mathematicians in 2014, reflecting his standing among the world’s leading mathematicians. At HKU, Professor Vu will further strengthen the University’s research capacity in pure and applied mathematics, foster international collaboration, and contribute to the training of the next generation of mathematical scientists at a time when deep theoretical insight is increasingly vital to science and technology.  

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Figure 1. Dual binding patterns of MBD proteins in mammals and in C. elegans

HKU Biologists Discover Alternative Systems that Help Cells Control Genes

In mammals (left), previous studies show that most MBD3 binding sites overlap with MBD2 and the Nucleosome Remodeling and Deacetylase (NuRD) complex, while MBD2 also independently occupies additional regions enriched with DNA methylation (5-methylcytosine, 5mC).   In Caenorhabditis elegans (C. elegans) (right), which lacks DNA methylation, MBD-2 (Cel-MBD-2) displays a dual binding pattern as in mammalian MBD2, despite lacking a methyl-binding domain. The majority of CelMBD-2 binds to genomic regions enriched with repressive histone modifications, such as H3K27me3 and H3K9me2/3, independently of NuRD. This model illustrates how MBD-2 can be guided by different epigenetic signals when DNA methylation is absent. (Image credit: Tsui, et al., 2026. Nature Communications) Researchers at the School of Biological Sciences of The University of Hong Kong (HKU) have uncovered how eukaryotic cells can control gene activity even after losing one of their major gene-regulatory systems during evolution. By studying a microscopic soil-living roundworm, the team revealed how an alternative, conserved epigenetic mechanism can take over when a common one is missing. The study has just been published in the interdisciplinary scientific journal Nature Communications. The findings provide new insights into how gene regulation adapts through evolution and may help scientists better understand disease mechanisms involving massive gene dysregulation, such as cancers, neurological disorders and autoimmune diseases. Cells must carefully control which genes are turned on and which are turned off throughout development to function properly. While the DNA sequence provides the genetic blueprint, gene expression is also regulated by epigenetic mechanisms — regulatory systems that influence when genes are turned on without changing the genetic code. This allows different cell types, such as nerve cells and muscle cells, to share the same DNA while behaving very differently. One common way cells control gene activity is through DNA methylation, in which a small chemical label, the methyl group, is added to cytosine, a specific base on DNA, forming 5-methylcytosine (5mC), to signal that certain genes should be kept switched off. 5mC is a key epigenetic mark in many animals and plants. However, some organisms, including the microscopic roundworm C. elegans, have lost DNA methylation multiple times during evolution. For a long time, scientists did not fully understand how these organisms could still regulate their genes properly without this major epigenetic system.   In this study, Dr Emily Hok Ning TSUI, a Postdoctoral Fellow at the School of Biological Sciences at HKU, working in the laboratories of Professor Karen Wing Yee YUEN and Professor Chaogu ZHENG, together with Dr Charmaine Yan Yu WONG, also in the YUEN Lab, showed that when DNA methylation is absent, cells can switch to an alternative epigenetic mechanism. Instead of relying on chemical labels on DNA, cells use various histone modifications — different posttranslational chemical marks on histone proteins, the packaging proteins in which DNA is wrapped around inside the cell.   The researchers focused on a protein called MBD-2 (methyl-CpG-binding domain protein 2), which in many animals recognises 5mC-marked DNA and helps silence or activate genes. Surprisingly, even though C. elegans lacks DNA methylation and 5mC, its version of MBD-2 remains essential.   The HKU team found that in C. elegans, MBD-2 no longer reads DNA methylation signals. Instead, it is localised to genes in association with specific repressive histone marks, particularly H3K27me3, a histone modification known to be associated with gene silencing.   When MBD-2 was deleted, the worms became infertile and developed severe physical defects. A large number of genes were no longer properly regulated, demonstrating that MBD-2 remains a key regulator of gene activity, even in the absence of DNA methylation.   These findings reveal that epigenetic regulation is highly adaptable. When one gene-control system is lost, organisms can adapt to read different signals and maintain precise control in gene expression.   “While scientists already know histone modifications and DNA methylation are highly interconnected and crosstalk with each other, this study in C. elegans showcases the functional conservation of the gene-regulatory NuRD complex on one hand, but also the plasticity and adaptability of epigenetic mechanisms in eukaryotes on the other hand,” said Professor Karen YUEN.   This work may help scientists better understand the causes of human diseases, such as cancers, autism and inflammation, in which aberrant DNA methylation disrupts the regulation of many genes at the same time. Understanding how different epigenetic mechanisms can compensate for one another may also facilitate the development of alternative therapeutic approaches.   For more details, please refer to the journal paper: https://www.nature.com/articles/s41467-026-68592-0   Figure 2. The research team at the School of Biological Sciences. From the left: Professor Chaogu ZHENG, Dr Emily Hok Ning TSUI, Dr Charmaine Yan Yu WONG and Professor Karen Wing Yee YUEN. 

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Image credit: S. Tian and Z. Yao

HKU and UCLA Scientists Uncover the Mechanism powering “Space Battery” above Auroral Regions

The dazzling lights of the aurora are created when high-energy particles from space collide with Earth’s atmosphere. While scientists have long understood this process, one big mystery remained: What powers the electric fields that accelerate these particles in the first place? A new study co-led by the Department of Earth and Planetary Sciences at The University of Hong Kong (HKU) and the Department of Atmospheric and Oceanic Sciences at the University of California, Los Angeles (UCLA) now provided an answer. Published in Nature Communications, the research reveals that Alfvén waves — plasma waves travelling along Earth’s magnetic field lines — act like an invisible power source, fueling the stunning auroral displays we see in the sky. By analysing how charged particles move and gain energy in different regions of space, the researchers demonstrated that these waves act as a natural accelerator, supplying energy that drives charged particles down into the atmosphere and produces the glowing auroral lights. To confirm their findings, the team analysed data collected by multiple satellites orbiting Earth, including NASA's Van Allen Probes and the THEMIS mission. The data provided solid evidence that Alfvén waves continuously transfer energy to the auroral acceleration region, maintaining the electric fields that would otherwise dissipate. “This discovery not only provides a definitive answer to the physics of Earth’s aurora, but also offers a universal model applicable to other planets in our solar system and beyond,” said Professor Zhonghua YAO of the Department of Earth and Planetary Sciences at HKU. Professor Yao leads a dedicated team in space and planetary science at HKU, which has established a reputation for high-impact research on planetary auroras. With deep expertise in the magnetospheric dynamics of planets like Jupiter and Saturn, the HKU team brought a critical planetary perspective to the study. “Our team at HKU has long focused on the auroral processes of giant planets. By applying this knowledge to the high-resolution data available near Earth, we have bridged the gap between Earth science and planetary exploration.” Professor Yao added. The research represents a model of interdisciplinary collaboration. The UCLA team, led by Dr Sheng TIAN, contributed extensive expertise in Earth’s auroral physics, while the HKU team provided the broader context of planetary space physics. The full research paper can be read here.   Caption: Comparative schematic of auroral acceleration processes on Earth and Jupiter. The electron spectrum for the Earth was from DMSP F19 spacecraft, and the one for Jupiter was from Juno spacecraft. Both spectra exhibit a similar inverted V-shaped structure, indicating the presence of stable electric potential drops above the auroral regions. This similarity points to a common auroral acceleration mechanism across planets and illustrates how insights from planetary aurorae help interpret high-resolution observations near Earth. Image credit: S. Tian and Z. Yao  

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Internationally Renowned Mathematician Professor Van H. Vu Joins HKU

The Department of Mathematics at The University of Hong Kong has welcomed Professor Van Ha VU, a world-leading mathematician whose research has shaped modern combinatorics, probability, and random matrix theory. Professor Vu received his PhD from Yale University in 1998 and was previously a full professor there. Before that, he held academic positions at the University of California, San Diego and Rutgers University. He is internationally recognised for solving several landmark problems in mathematics, including the Erdős–Folkman problem in number theory with Endre Szemerédi, the Shamir conjecture in random graph theory with Anders Johansson and Jeff Kahn, and together with Terence Tao, the circular law conjecture and four-moment theorem in random matrix theory. Random matrix theory plays a foundational role in quantum physics, complex systems, and artificial intelligence, where large random matrices are used to model quantum behaviour, analyse massive datasets, and understand the stability and performance of modern algorithms. Professor Vu’s work has helped establish the theoretical framework underlying these calculations. His honours include the George Pólya Prize in 2008, the Delbert Ray Fulkerson Prize in 2012,  and an invitation to speak at the International Congress of Mathematicians in 2014, reflecting his standing among the world’s leading mathematicians. At HKU, Professor Vu will further strengthen the University’s research capacity in pure and applied mathematics, foster international collaboration, and contribute to the training of the next generation of mathematical scientists at a time when deep theoretical insight is increasingly vital to science and technology.  

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Distinguished guests, led by the Vice-President and Pro-Vice-Chancellor Professor Peng Gong, and 17 CAS Academicians, officiated the celebrations with a ceremonial balloon launch.

DEPS at 30: A Community Shaped by Earth, Bound by Curiosity

In 2025, the Department of Earth and Planetary Sciences (DEPS) celebrated a milestone that was as meaningful as it was hard-earned: its 30th anniversary. The occasion marked not only three decades of teaching and research, but 30 years of building a community shaped by curiosity, care, and a shared fascination with the planet we call home. Where the Story Begins Every department begins with a founding date; far fewer begin with a sense of adventure. When the Department first opened its doors in 1995, it was a modest beginning powered by conviction. Led by founding Chair Professor John MALPAS and supported generously by the Hui family, the team set out to build something Hong Kong had long needed: a home for geoscience expertise, and a training ground to understand the city’s complex landscape. In those early years, learning unfolded far beyond lecture rooms. Staff and students traced coastlines and hillsides, read stories written in stone and strata, and learned to see the Earth not as a static object but as a living archive of time. Field boots, folded maps and shared laughter became part of daily life, underpinned by a simple belief—that to understand the Earth is also to learn how to care for it. From the outset, the Department embraced a clear dual mission: to master the fundamentals of geology while responding to the applied challenges of an increasingly urbanised world. “This balance between pure and practical science has shaped three decades of discovery, innovation and impact,” said Professor Zhonghui LIU, Interim Head of DEPS. A Homecoming Written in Memory  “We’ve gone from tracing Hong Kong’s bedrock to studying planetary formation and climate futures,” Professor Liu reflected. “That’s why our celebration theme, ‘Beyond the Earth,’ feels so fitting. It captures not just our new scientific direction in space research, but the way this community has always tried to reach a little further than anyone expected.” The celebrations began on 28 November, like a long-awaited reunion. Alumni streamed back, some travelling hours, others crossing continents, to find their teachers, their friends, and, in many ways, a younger version of themselves. There were embraces held a moment longer, and jokes that resurfaced effortlessly. Nearly 50 alumni shared stories of their first field trips, their toughest assignments, and the sense of discovery that shaped their careers. What filled the room was not simply nostalgia. It was a quiet acknowledgement that the Department had not only given them knowledge, but a place to belong. Ideas by Day, a Community by Night The following day, 29 November, brought a a shift toward the future. At the anniversary symposium, 12 distinguished speakers reflected on the Earth beneath us and the worlds beyond, inviting more than 100 participants to imagine the possibilities of the coming decades. The keynote by Mr Tony Ying Kit HO, JP, added a thoughtful public dimension, grounding the scientific excitement in civic purpose. As evening settled, Loke Yew Hall glowed under evening lights as over 150 guests gathered for the Celebration Ceremony. The presence of 17 academicians of the Chinese Academy of Sciences (CAS) made the room feel both intimate and momentous, a reminder of how far DEPS’s reputation has travelled. Vice-President and Pro-Vice-Chancellor Professor Peng GONG and Dean of Science Professor Qiang ZHOU offered warm reflections —not only marking achievements, but honouring the people who made them possible. The Celebration Dinner that followed was alive with clinking glasses, renewed friendships, and the gentle hum of gratitude. More than 200 guests, including our staunch supporter Ms Sylvia HUI, came together with an ease that only a well-knit community can create.   The Road Ahead The final morning, 30 November, captured the Department’s soul. In one room, Mok Sau-King Professor and Chair Professor Guochun ZHAO, joined by 18 CAS academicians, led deep discussions about the Department’s next strategic chapter: new research frontiers, collaborations, and responsibilities. It was the kind of quiet, serious work that shapes decades to come. Out on the calm waters of the Tolo Channel, a boat glided under the guidance of retired academic Professor L S CHAN. Alumni pointed toward familiar rock formations as the coastline passed like an old friend. The tranquility offered a space for quiet reflection, a chance for all to remember what first drew them to the field. As DEPS steps into its next decade, it carries a rare kind of warmth: a community built on curiosity, lifted by generosity, and sustained by the belief that understanding our world is a shared human calling.  

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Tumour suppression in vivo — In animal models, LS-170 treatment significantly reduced tumour volume, demonstrating its strong anti-cancer potential. (Image adapted from the relevant journal.)

HKU Chemists Develop First-in-Class Inhibitor Targeting a Key Epigenetic Regulator A New Strategy to Beat Lung Cancer

A research team led by Professor Xiang David Li from the Department of Chemistry at The University of Hong Kong (HKU), in collaboration with researchers from the Shenzhen Bay Laboratory and Tsinghua University, has made a breakthrough in epigenetic drug discovery. The team has successfully developed a first-in-class chemical inhibitor that precisely and selectively targets the ATAC complex, a critical cellular “switch operator” that activates tumour-promoting genes, opening a novel therapeutic avenue for non-small cell lung cancer (NSCLC). The findings were recently published in the top-tier journal Nature Chemical Biology, and multiple international patent applications have been filed. Histone Modifications as Genetic Switches in Our Cells Inside human cells, DNA is wrapped around protein structures called histones to form chromatin. Chemical modifications on histones function like genetic “switches”, determining whether genes are turned on or remain silent.  Among these modifications, histone acetylation is one of the most important “on” switches that activate gene expression. This modification is catalysed by enzyme complexes known as histone acetyltransferases (HATs).  The ATAC complex is one such HAT complex and plays a pivotal role in activating genes involved in cell growth and DNA replication. In cancers such as NSCLC, the ATAC complex becomes overactive, inappropriately flipping the “on” switch for numerous cancer-driving genes, fuelling uncontrolled tumour growth and spread. However, selectively inhibiting ATAC without disrupting other essential cellular complexes has remained a challenge in drug development. Precisely Targeting a Unique Component of ATAC Previous drug-development efforts focused on inhibiting GCN5, the catalytic subunit responsible for histone acetylation within ATAC. Nevertheless, GCN5 is also shared by several other HAT complexes, meaning that blocking it would inadvertently interfere with normal cellular functions and lead to significant side effects. To address this challenge, Professor Li’s team devised an innovative strategy targeting YEATS2, a protein subunit specific to the ATAC complex.  Using structure-guided design, the researchers developed a potent and highly selective inhibitor of YEATS2, named LS-170. This inhibitor specifically binds to the acetyl-lysine recognition domain of YEATS2, preventing it from anchoring the ATAC complex to chromatin. Consequently, the complex is displaced from its target genomic regions, leading to a significant reduction in local histone acetylation and the “off” switching of oncogenes in NSCLC. Strong Suppression of Tumour Growth and Metastasis In NSCLC cell lines and animal models, LS-170 demonstrated strong efficacy in suppressing tumour growth and metastasis. Notably, the YEATS2 gene is frequently amplified in multiple solid tumours—including lung, ovarian, and pancreatic cancers—suggesting that this targeted strategy may hold broader therapeutic potential beyond lung cancer. This study represents the first chemical approach to precisely decode the function of a specific HAT complex, revealing ATAC's distinct role in maintaining gene expression programs in cancer. It also offers new insights for developing other complex-specific epigenetic drugs for human diseases. “In this work, we didn’t just create a potent and highly specific inhibitor that can suppress tumours, we also uncovered a novel strategy to target just one epigenetic complex out of several that share the same enzyme core. This approach opens up exciting possibilities for developing highly selective, complex-specific drugs that could potentially revolutionise treatments for human diseases,” said Professor Xiang Li, one of the corresponding authors of the paper. About the Research Team: The interdisciplinary collaboration was led by Professor Xiang David LI (HKU Chemistry), together with Professor Weiping WANG (HKU Pharmacology and Pharmacy), Researcher Xin LI (Shenzhen Bay Laboratory), and Professor Haitao LI (Tsinghua University). Co-first authors included Dr Sha LIU, Dr Yin Qiao WU, Dr Jinzhao LIU, and Dr Xinyi YAO.   For more details, please refer to the journal paper: https://www.nature.com/articles/s41589-025-02132-7  

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Image credit: Y. Liu, X. Yang, Y.F. Liang, W.L. Zhang and Y. Li (PMO).

HKU Astronomer Uses “China Sky Eye” to Reveal Binary Origin of Fast Radio Bursts

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 a factor of twenty,’ 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.  Image caption: Artist’s impression illustrating a binary-origin scenario for fast radio bursts. A magnetised plasma cloud, generated by a coronal mass ejection from the companion star, crosses the line of sight to the FRB source, causing a sharp and transient variation in the rotation measure.  

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