Graduate student Niccolò Passaleva was on his way home when two neutron stars collided.
“I was traveling home by train,” Passaleva said in a press release, “and all of a sudden I was rushing against time to commandeer one of the largest telescopes in the world from my laptop.”
Passaleva is part of Professor Eleonora Troja’s research group, which is currently studying magnetars — spinning neutron stars with large magnetic fields. Some form after massive stars explode, but researchers believe others may form when two neutron stars collide. Astronomers have traditionally identified neutron star mergers through quick gamma-ray bursts that often dissipate in less than two seconds.
The team recently discovered that these rare neutron star collisions may also appear as X-ray flashes lasting several minutes, hence Passaleva rushing to commandeer a large telescope. Publishing their findings in Science Bulletin, the researchers believe that merging neutron stars could be lurking among mysterious cosmic flashes.
“This is the longest-lasting prompt X-ray flash ever observed from a neutron star merger,” said 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.”
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The Birth of a Magnetar
The bright glow of EP250704a is marked by white lines.
(Image Credit: Niccolò Passaleva, Eleonora Troja (ERC BHianca), ESO/VLT)
Bright X-ray flashes from distant galaxies are not uncommon and have been observed regularly since the 2024 launch of the Einstein Probe. Thanks to this satellite, researchers have identified hundreds of these bright flashes, also known as fast X-ray transients, and may have linked some to the death of stars, though researchers still need to learn more about them.
This is where Troja’s team, which is supported by a grant from the European Research Council (ERC) Consolidator, comes in.
When an alert from the Einstein Probe came through, the team quickly secured observations with some of the world’s most advanced telescopes, including the European Southern Observatory’s Very Large Telescope (VLT) and the Very Large Array. With the data collected, the team then studied the aftermath of the explosion.
Based on what they observed, the team suggests they may have captured the birth of a magnetar.
Analyzing Gamma-Ray Bursts
As massive stars die out, they leave behind neutron stars, essentially ultra-dense stellar corpses, according to the press release. As two neutron stars collide, they can send gravitational waves throughout space. The light emitted from this collision helps us understand what may be left behind, and it usually comes in the form of gamma-ray bursts.
“However, if the remnant of the collision is a magnetar, it could keep bursting for longer,” said Troja, who is part of the Einstein Probe European collaboration and co-corresponding author of the paper in the press release. “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 team named the event EP250704a/GRB 250704B, and it was first spotted on July 4, 2025, using the Space Variable Objects Monitor (SVOM), Insight Hard X-ray Modulation Telescope (HXMT), and the Einstein Probe.
The gamma-ray blast lasted about half a second, according to the study, but the Einstein Probe recorded bright X-ray radiation for closer to 10 minutes.
Signs of a Neutron Star Collision
To measure how far the burst occurred from Earth (also known as redshift), Passaleva and the team utilized the VLT’s X-Shooter instrument to separate the light into single components. This helped the team identify absorption patterns and the burst’s redshift. The measured redshift was z = 0.6610, indicating that the light traveled over six billion years to reach us and that the collision occurred before our Solar System formed.
Passaleva and the team then used images from the VLT’s FORS2 instrument to look for evidence of a bright supernova — the explosion of a star typically accompanied by a long-lasting X-ray flash. The search yielded no results, suggesting this burst likely came from a neutron star collision.
“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.”
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- This article references information from a study published in Science Bulletin: Minutes-long soft X-ray prompt emission from a compact object merger
