Ten minutes of X-rays after a 0.4-second flash: the hidden phase of a star collision

When two dead stars collide, astronomers usually catch only the brightest instant — a gamma-ray flash lasting less than a second. Now a team including Hong Kong PhD student Yi-Han Iris Yin has found what came after: a soft X-ray glow that kept shining for nearly ten minutes after the gamma rays faded. The event, catalogued as EP250704a in X-rays and GRB 250704B in gamma rays, was captured from its very onset by the China-led Einstein Probe, together with the SVOM and HXMT telescopes — the first time the earliest X-ray emission of such a merger was ever seen. Yin led the analysis of the high-energy signal and is a co-corresponding author on the international collaboration, which spans the University of Hong Kong's HKIAA, Nanjing University, the University of Rome, and the Chinese Academy of Sciences. Their findings appear as a cover article in Science Bulletin. The lingering X-rays point to a central engine that kept releasing energy long after the flash — and suggest similar afterglows in past mergers may have gone unnoticed, giving astronomers a whole new way to study the collisions that forge the universe's heavy elements.