Einstein Probe Detects Unprecedented Cosmic Event: EP250702a
On July 2, 2025, the China-led Einstein Probe (EP) space telescope made a remarkable discovery during a routine sky survey. It detected an exceptionally bright X-ray source, designated EP250702a (also known as GRB 250702B). This cosmic event, observed by EP's Wide-field X-ray Telescope (WXT), exhibited rapid brightness variations.
Simultaneously, NASA's Fermi Gamma-ray Space Telescope recorded gamma-ray bursts originating from the same celestial region. What made this event particularly unusual was that EP's WXT had detected persistent X-ray emission from the exact location approximately a day before the gamma-ray bursts. This sequence of events is considered highly atypical for high-energy cosmic explosions. Approximately 15 hours after the initial signal, the source produced a series of intense X-ray flares, reaching a peak luminosity of about 3 × 10⁴⁹ erg s⁻¹.
Global Follow-up Unveils Unusual Characteristics
Prompt follow-up observations were swiftly initiated by telescopes across the globe, successfully pinpointing the celestial object. The source was located in the outskirts of a distant galaxy. EP's Follow-up X-ray Telescope (FXT) then meticulously tracked the source's evolution for approximately 20 days.
During this tracking period, the source's brightness dramatically decreased by over a hundred thousand times. Concurrently, its X-ray emission transitioned from higher-energy to lower-energy states.
Data from the Einstein Probe, combined with observations spanning the entire electromagnetic spectrum, revealed a unique set of characteristics for EP250702a, setting it apart from known high-energy cosmic events.
These unusual characteristics include:
- The X-ray emission occurred before the gamma-ray burst.
- The event was extraordinarily bright.
- It evolved on a remarkably fast timescale.
- It was located in the outskirts of its host galaxy, a departure from typical patterns of known high-energy cosmic events.
Intermediate-Mass Black Hole Devouring a White Dwarf: A Leading Hypothesis
Among the various theoretical scenarios considered, one explanation emerged as the most consistent with the observed data: an intermediate-mass black hole tearing apart and consuming a white dwarf star. If confirmed, this would represent the first observational evidence of such an extreme black hole feeding process.
Astrophysicists from The University of Hong Kong (HKU), who are integral members of the EP scientific team, played a crucial role in the data interpretation and theoretical modeling. Professor Lixin Dai's team provided theoretical judgment that guided the focus towards this specific model, asserting its ability to explain both the rapid evolution and the extreme energy output observed.
Further strengthening this hypothesis, Dr. Jinhong Chen conducted numerical simulations. These simulations indicated that the powerful tidal forces exerted by an intermediate-mass black hole, combined with the extreme density of a white dwarf, could indeed produce jet energies and evolutionary timescales that align perfectly with the observed data.
Unlocking New Chapters in Black Hole Astronomy
If this event is ultimately confirmed, it 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 groundbreaking discovery holds immense scientific significance and could:
- Contribute significantly to understanding the long-missing population of intermediate-mass black holes.
- Open new avenues for studying black hole growth and evolution.
- Offer invaluable insights into the ultimate fate of compact stars.
- Advance the emerging and vital field of multi-messenger astronomy.
The research was coordinated by the EP Science Center of the National Astronomical Observatories, Chinese Academy of Sciences (NAOC). The collaborative team for this paper included over 40 universities and research institutions internationally, reflecting widespread participation. The findings were published as a cover article in Science Bulletin. The Einstein Probe satellite project itself, led by the Chinese Academy of Sciences, is a testament to 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).