Figure 1. Artist's impression of the Einstein Probe satellite catching an intermediate black hole, tearing apart a white dwarf, and producing a relativistic jet. Image credit: Einstein Probe Science Center, National Astronomical Observatories, CAS / Sci Visual.
An unprecedented high-energy cosmic event is offering new insights into extreme astrophysical processes.
On 2 July, 2025, the China-led Einstein Probe (EP) space telescope detected an exceptionally bright X-ray source whose brightness varied rapidly during a routine sky survey. Its unusual signal immediately set it apart from ordinary cosmic sources, triggering rapid follow-up observations by telescopes worldwide.
This research was coordinated by the EP Science Center of the National Astronomical Observatories, Chinese Academy of Sciences (NAOC), with participation from multiple research institutions in China and abroad. Astrophysicists from the Department of Physics at The University of Hong Kong (HKU), who are integral members of the EP scientific team, worked together with the broader collaboration to interpret the event, proposing that it may mark the moment when an intermediate-mass black hole tears apart and consumes a white dwarf star. If confirmed, this would be the first observational evidence of such an extreme black hole "feeding" process. The findings have been published as a cover article in Science Bulletin.
A Cosmic Event that Broke the Usual Pattern
This discovery was made possible by EP's two unique and complementary X-ray instruments. During a routine sky survey on 2 July 2025, the Wide-field X-ray Telescope (WXT), which uses advanced lobster-eye micro-pore optics and offers a vast field of view with high sensitivity, detected a transient X-ray source exhibiting violent variability, later designated EP250702a (also known as GRB 250702B). Almost at the same time, NASA's Fermi Gamma-ray Space Telescope recorded a series of gamma-ray bursts from the same region of the sky.
The significance of the event became clear only after scientists examined WXT's earlier observations. The telescope had already detected persistent X-ray emission from the exact location about a day before the gamma-ray bursts appeared—a sequence rarely seen in high-energy cosmic explosions. About 15 hours after the initial signal, the source erupted into a series of intense X-ray flares, reaching a peak luminosity of around 3 × 10⁴⁹ erg s⁻¹ and placing it among the brightest instantaneous outburst events ever observed in the Universe.
"This early X-ray signal is crucial," said Dr Dongyue LI, first author of the paper from the National Astronomical Observatories of China. "It tells us this was not an ordinary gamma-ray burst."
Thanks to the precise coordinates provided by WXT, several large telescopes worldwide rapidly followed up, successfully pinpointing the celestial object across multiple wavelengths and confirming its location in the outskirts of a distant galaxy. Subsequently, EP's other instrument—the Follow-up X-ray Telescope (FXT) took over, tracking the source's dramatic evolution. Over about 20 days, its brightness dropped by more than a hundred thousand times, while its X-ray emission shifted from higher-energy ("hard") to lower-energy ("soft") states.
By combining data from the EP with follow-up observations across the electromagnetic spectrum, scientists found that EP250702a exhibited a set of unusual features that existing models could not fully explain. Its X-ray emission appeared before the gamma-ray burst, was extraordinarily bright, evolved on a remarkably fast timescale, and occurred in the outskirts of its host galaxy rather than at its centre—a pattern rarely seen in known high-energy cosmic events. Among the many theoretical scenarios considered, one explanation finally stood out: an intermediate-mass black hole tearing apart a white dwarf star.
HKU Astrophysicists Provide Key Model Support
The HKU team played vital roles in data interpretation and theoretical modelling, leading to the identification of the underlying physical mechanism of the phenomenon. The team of Professor Lixin DAI from the Department of Physics and the Hong Kong Institute of Astronomy and Astrophysics (HKIAA) at HKU provided the crucial theoretical judgement that led to the focus on this model. As a co-corresponding author, she explained, "The white dwarf–intermediate-mass black hole model can most naturally explain its rapid evolution and extreme energy output."
Dr Jinhong CHEN, a co-first author of the paper and a postdoctoral fellow in the HKU Department of Physics, conducted in-depth numerical simulations to analyse the model. "Our computational simulations show that the combination of the tidal forces of an intermediate-mass black hole, combined with the extreme density of a white dwarf, can produce jet energies and evolutionary timescales that are highly consistent with the observational data," he said.
Professor Bing ZHANG, Director of HKIAA at HKU and a co-author of the paper, stated, "Hong Kong possesses an internationalised research vision and technical expertise in astronomy. The HKU team's deep involvement and contribution to this significant discovery fully demonstrate the unique value and critical role of Hong Kong's scientific research capabilities in the forefront of global scientific exploration."
Professor Lixin Dai added, "The robust discussion among international teams, each with their competing models to explain this event, is precisely what highlights its immense scientific value."
"The mission of the Einstein Probe is to capture unpredictable and extreme transient phenomena in the universe," said Professor Weimin YUAN from the National Astronomical Observatories of China, Principal Scientist of the Einstein Probe mission. "The discovery of EP250702a fully demonstrates our capability to be the first to capture the Universe's most extreme moments and further exemplifies China's ability to make decisive contributions to international astronomical exploration."
If ultimately confirmed, this event would provide the first clear, direct evidence of an intermediate-mass black hole tearing apart a white dwarf and producing a relativistic jet. Such a discovery would help shed light on the long-missing population of intermediate-mass black holes and open new avenues for studying how black holes grow, the ultimate fate of compact stars, and the emerging field of multi-messenger astronomy.
Project and Team Collaboration
The collaborative team for this paper includes over 40 universities and research institutions, such as The University of Hong Kong, the National Astronomical Observatories of China, Anhui Normal University, Sun Yat-sen University, and the University of Science and Technology of China.
Co-first authors of the paper are Dr Dongyue Li and Associate Researcher Wenda Zhang (National Astronomical Observatories of China), Dr Jun Yang (Zhengzhou University), and Dr Jin-Hong Chen (The University of Hong Kong).
Co-corresponding authors are Associate Researcher Wenda Zhang, Researcher Weimin Yuan, and Researcher Chichuan Jin (National Astronomical Observatories of China); Professor Lixin Dai (The University of Hong Kong); and Researcher Chen Zhang (National Astronomical Observatories of China).
The Einstein Probe satellite project is led by the Chinese Academy of Sciences. The mission also reflects extensive international collaboration, with key partners including the European Space Agency (ESA), the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany, and the French National Centre for Space Studies (CNES). The research brought together more than 300 scientists from over 40 universities and research institutions in China and around the world, highlighting the strength of open science and large-scale international collaboration in tackling frontier scientific challenges.
For more details, please refer to the journal paper: https://doi.org/10.1016/j.scib.2025.12.050
