An unusual luminous flash caught the attention of astronomers in July 2025. During this event, an X-ray source changed its luminosity dramatically in just a few hours.
This event was spotted by the Einstein Probe satellite, developed under Chinese leadership, during a routine survey. Thanks to its rapid detection of variations, a worldwide alert was triggered. Subsequently, telescopes around the world pointed their instruments toward this region of the sky, forming an international collaboration to study the phenomenon. The data were analyzed by a team led by the National Astronomical Observatories of China, with significant contributions from the University of Hong Kong.
Artist's impression of the Einstein Probe satellite capturing an intermediate black hole tearing apart a white dwarf and producing a relativistic jet.
Credit: Einstein Probe Science Center, National Astronomical Observatories, CAS / Sci Visual The observations revealed astonishing features. The X-ray emission began before gamma-ray bursts were recorded, an inverse sequence to what is usually observed. The source was located at the edge of a distant galaxy, not at its center. Its luminosity reached an extreme peak in a few hours, then declined over about twenty days, with a noticeable change in the X-ray spectrum.
To explain these anomalies, scientists considered several scenarios. The model that best fits the data involves an intermediate-mass black hole tearing apart a white dwarf. When the dense star gets too close to the black hole, tidal forces disrupt it, releasing an immense amount of energy. This event could produce a jet of matter at a speed close to that of light.
The University of Hong Kong team played an important role in developing numerical simulations. These calculations showed that the interaction between an intermediate-mass black hole and a white dwarf can generate the observed energy and time evolution.
If this interpretation is confirmed, it would provide the first direct evidence of intermediate-mass black holes in action. These objects, whose mass is between that of stellar black holes and supermassive ones, are rare and difficult to detect. This discovery would open new perspectives for understanding black hole growth and the fate of compact stars. The results of this research are presented in
Science Bulletin.