Observations led by a graduate student and his colleagues helped connect an X-ray flash lasting nearly ten minutes to a merger of neutron stars.
For decades, astronomers have identified the collisions of two neutron stars through bursts of gamma rays that can disappear in less than two seconds. But some of these violent encounters may also reveal themselves through X-ray flashes lasting several minutes. A new study published in Science Bulletin provides compelling evidence for this connection, suggesting that merging neutron stars could be hiding among cosmic flashes whose origins have remained uncertain.
Since the launch of the Einstein Probe satellite in January 2024, astronomers have been discovering hundreds of bright flashes of X-rays from distant galaxies. Known as fast X-ray transients, some of these flashes have been linked to the deaths of massive stars, but others remain unexplained. Without knowing their distance and the energy they release, astronomers struggle to determine what produced them.
Researchers in Professor Eleonora Troja’s group, supported by a European Research Council (ERC) Consolidator grant, obtained observations crucial to identifying the source of one such X-ray transient. Following an alert from the Einstein Probe satellite, the team quickly arranged observations with other telescopes, including the European Southern Observatory’s Very Large Telescope (VLT) and the Very Large Array, to study the explosion’s aftermath. They believe to have witnessed the birth of a magnetar produced by the collision (or merger) of two neutron stars.
An X-ray flash with a record-setting duration
Neutron stars are the ultra-dense stellar corpses left behind when massive stars reach the end of their lives. When two neutron stars collide, they send gravitational waves rippling through space. The light from these events offers a glimpse of what the collision leaves behind.
Gamma-ray bursts of short duration have long been the main signpost for these events. “However, if the remnant of the collision is a magnetar, it could keep bursting for longer” said Prof. Troja, who is part of the Einstein Probe European collaboration and co-corresponding author of the paper. “Magnetars are rapidly spinning neutron stars with huge magnetic fields. When they damp 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.”
The event, known as EP250704a/GRB 250704B, was discovered on 4 July 2025 by the SVOM, Insight-HXMT and Einstein Probe satellites. Its blast of gamma-rays lasted about half a second, while the Einstein Probe satellite recorded bright X-ray radiation for nearly ten minutes. “This is the longest lasting prompt X-ray flash ever observed from a neutron star merger,” said graduate student Niccolò Passaleva, who led the follow-up observations using the VLT in Chile. “It is an opportunity to have a front-row seat to the most extreme forces of the Universe and discover more of its secrets.”
A new window on stellar collisions
The team had been searching for a connection between fast X-ray transients and neutron star mergers for several years, but earlier candidates faded away before providing a decisive answer. This time, Passaleva reacted within minutes and began observing while the light from the explosion was still bright enough to study in detail. “I was traveling home by train,” recalls Passaleva, “and all of a sudden I was rushing against time to commandeer one of the largest telescopes in the world from my laptop.”
Using the VLT’s X-Shooter instrument, Passaleva and his colleagues separated the light into its components and identified clear absorption patterns, whose position revealed the burst’s distance from Earth, known as redshift. The measured redshift of z=0.6610 revealed that the explosion had occurred long before our Sun and its planets formed, and the burst’s light had travelled for more than six billion years before reaching us.
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-lasting X-ray flash caused by the collapse of a massive star. None appeared. The distance measurement, the absence of a supernova and the properties of the burst together provided strong evidence for a neutron star merger.
“Finding more of these X-ray flashes could help reveal how often neutron star mergers create magnetars,” concludes Passaleva, “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.”