What made this event different?

The key was speed. The team, led by professor Eleonora Troja of Tor Vergata University of Rome, had searched for a connection between fast X-ray transients and neutron star mergers for several years. Earlier candidate events faded before yielding a decisive answer. This time, Passaleva reacted within minutes, while the explosion's light was still bright enough to study in detail.

"I was traveling home by train," Passaleva recalls, "and all of a sudden I was rushing against time to commandeer one of the largest telescopes in the world from my laptop."

After the Einstein Probe alert, the team quickly arranged observations with other telescopes, including the European Southern Observatory's VLT and the radio-sensitive Very Large Array, to study the explosion's aftermath.

How did they measure the distance?

Using the VLT's X-Shooter instrument, the researchers split the light into its component wavelengths and identified clear absorption patterns. The positions of those patterns revealed the burst's redshift — a measure of how much the expansion of the universe has stretched the light, which tells astronomers the distance to its source.

The measured redshift of z=0.6610 means the explosion occurred long before our sun and its planets formed. The burst's light traveled for more than 6 billion years before reaching Earth.

The team then used deep images from the VLT's FORS2 instrument to search for a bright supernova — the exploding star expected to accompany a long X-ray flash caused by the collapse of a massive star. None appeared. The distance measurement, the absence of a supernova, and the burst's properties together provided strong evidence for a neutron star merger.

What did the collision leave behind?

The researchers believe they witnessed the birth of a magnetar. "Magnetars are rapidly spinning neutron stars with huge magnetic fields," explained Troja, who is part of the Einstein Probe European collaboration and co-corresponding author of the paper. "When they dump their magnetic power into the surroundings, they can make any explosion brighter and longer-lasting. When I saw the X-ray data from this new event, I realized something was up."

Short gamma-ray bursts have long served as the main signpost for neutron star mergers. "However, if the remnant of the collision is a magnetar, it could keep bursting for longer," Troja said. That, the researchers suggest, could explain why some merging neutron stars have been hiding among the unexplained fast X-ray transients.

What comes next?

The evidence is strong but still hinges on a single event, and the interpretation rests on the distance measurement, the missing supernova and the burst's properties taken together rather than on a direct detection of gravitational waves from this source.

More discoveries could firm up the picture. "Finding more of these X-ray flashes could help reveal how often neutron star mergers create magnetars," Passaleva said. "I am really excited for the next run of gravitational wave observations, when we could finally pair one of these X-ray flashes with a burst of gravitational waves from the same source."

The study, led by An Li and colleagues, appears in Science Bulletin (DOI: 10.1016/j.scib.2026.08.021) under the title "Minutes-long soft X-ray prompt emission from a compact object merger."