An unusual high-energy outburst detected by China’s Einstein Probe space telescope is offering astronomers a rare glimpse into the violent process of a black hole consuming a white dwarf star. The event, designated EP250702a, as well known as GRB 250702B, was first observed on July 2, 2025, and triggered a global observing campaign involving telescopes across multiple wavelengths.
The Einstein Probe’s Wide-field X-ray Telescope initially detected a rapidly varying X-ray source, which stood out from typical cosmic objects. Almost simultaneously, NASA’s Fermi Gamma-ray Space Telescope recorded a sequence of gamma-ray bursts originating from the same region of the sky. Further analysis revealed that the Einstein Probe had already registered steady X-ray emissions from the location approximately a day before the gamma-ray bursts, a sequence rarely observed in high-energy transient events.
Within 15 hours of the initial signal, the source erupted in powerful X-ray flares, reaching a peak luminosity of roughly 3 x 1049 erg per second – placing it among the most luminous outbursts ever recorded. The precise coordinates provided by the Einstein Probe enabled ground-based and space-based observatories worldwide to quickly join the follow-up effort. Observations revealed the source to be located in the outskirts of a distant galaxy, rather than at its center, distinguishing it from typical high-energy explosions.
Over a period of roughly 20 days, the Einstein Probe’s Follow-up X-ray Telescope tracked the source as its brightness diminished by a factor of over 100,000, and its emissions shifted from higher-energy, “hard” X-rays to lower-energy, “soft” X-rays. Scientists found that the characteristics of EP250702a – including its early, intense X-ray signal, rapid evolution, and off-center galactic location – did not align neatly with existing models of gamma-ray bursts or known tidal disruption events.
Researchers from The University of Hong Kong and the Hong Kong Institute of Astronomy and Astrophysics, collaborating within a larger international team, proposed that an intermediate-mass black hole tearing apart a white dwarf star could explain the observed phenomena. “The white dwarf intermediate-mass black hole model most naturally explains the observations,” stated Professor Lixin Dai, a co-corresponding author from HKU.
Dr. Jinhong Chen, a postdoctoral fellow at HKU and co-first author of the research, conducted detailed numerical simulations to support this hypothesis. “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. The simulations suggest that a relativistic jet launched during the disruption process generated the observed high-energy emissions.
Professor Bing Zhang, Director of the Hong Kong Institute of Astronomy and Astrophysics, emphasized the significance of Hong Kong’s role in the discovery, highlighting the region’s growing scientific capabilities. Professor Dai added that the intense debate among international teams proposing competing explanations underscores the event’s scientific impact.
Einstein Probe mission scientist Professor Weimin Yuan of the National Astronomical Observatories of China described the event as a demonstration of the satellite’s core purpose: to capture unpredictable and extreme transient phenomena in the universe. “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,” he said.
If confirmed, EP250702a would represent the first direct evidence of an intermediate-mass black hole disrupting a white dwarf and generating a relativistic jet. Such a finding would help to fill a gap in the understanding of black holes, illuminating the elusive intermediate-mass population between stellar-mass and supermassive objects. It would also provide new avenues for studying black hole growth, the ultimate fate of compact stars, and the connections between high-energy photons, gravitational waves, and other cosmic messengers.
The research involved a collaboration of over 300 scientists from more than 40 universities and research institutes worldwide, including The University of Hong Kong, the National Astronomical Observatories of China, and international partners such as the European Space Agency, the Max Planck Institute for Extraterrestrial Physics in Germany, and the French National Centre for Space Studies.
