Astronomers Link X-Ray Flashes to Neutron Star Collisions

For decades, astronomers have relied on a single calling card to spot the collision of two neutron stars: a burst of gamma rays that flares and vanishes in less than two seconds. But a discovery announced this fall suggests that some of these violent cosmic encounters may linger far longer than anyone expected—visible not as a quick flash but as a sustained X-ray glow that can persist for minutes. The finding, published in Science Bulletin, opens a new way to hunt for these events and hints that many more neutron star mergers may be hiding in plain sight among the unexplained X-ray flashes astronomers have been cataloging since the Einstein Probe satellite launched in early 2024.

Since that satellite began operations, it has detected hundreds of bright X-ray transients—sudden flashes from distant galaxies that appear and fade. Some have been traced to the deaths of massive stars. Others remain mysteries. Without knowing how far away these flashes are or how much energy they release, astronomers struggle to identify their source. A team led by Professor Eleonora Troja at the University of Rome Tor Vergata, supported by a European Research Council grant, set out to solve this puzzle for one particular event. When the Einstein Probe alerted them to an X-ray transient on July 4, 2025, they mobilized quickly, arranging follow-up observations with the European Southern Observatory's Very Large Telescope in Chile and the Very Large Array to study what came after the initial burst. What they found was evidence of something extraordinary: the birth of a magnetar—a rapidly spinning neutron star with an enormous magnetic field—created by the merger of two neutron stars.

Neutron stars are the ultra-dense remnants left when massive stars exhaust their fuel and collapse. When two of them collide, they send ripples of gravitational waves through space and release a flood of light. For years, the short gamma-ray bursts that accompany these mergers have been the primary way astronomers identify them. But Troja realized that if the collision produces a magnetar, the story changes. As a magnetar releases its magnetic energy into the surrounding material, it can make the explosion brighter and stretch it out over a much longer timescale. "When I saw the X-ray data from this new event, I realized something was up," Troja said in a statement.

The event, designated EP250704a/GRB 250704B, was spotted by three satellites: SVOM, Insight-HXMT, and Einstein Probe. The initial gamma-ray burst lasted roughly half a second—typical for a neutron star merger. But the Einstein Probe continued recording bright X-ray radiation for nearly ten minutes. This duration set a record: the longest-lasting prompt X-ray flash ever observed from a neutron star merger. "It is an opportunity to have a front-row seat to the most extreme forces of the Universe," said Niccolò Passaleva, a graduate student who led the follow-up observations, "and discover more of its secrets."

Passaleva's team had been searching for a connection between fast X-ray transients and neutron star mergers for several years, but earlier candidates had faded before yielding definitive answers. This time, Passaleva acted within minutes of the alert. Traveling home by train, he found himself racing against the clock to secure time on one of the world's largest telescopes from his laptop. Using the VLT's X-Shooter instrument, he and his colleagues separated the light from the explosion into its component wavelengths and identified absorption patterns that revealed how far away the burst had occurred. The redshift measurement—z=0.6610—showed that the explosion had taken place more than six billion years ago, long before the Sun and Earth formed. The light from that ancient collision had traveled across the expanding universe for over six billion years before reaching Earth.

To confirm the neutron star merger hypothesis, the team searched for a supernova—the bright explosion of a massive star that would be expected if the X-ray flash came from a stellar collapse rather than a merger. Using the VLT's FORS2 instrument, they found none. Combined with the distance measurement and the properties of the burst itself, the absence of a supernova provided strong evidence that they were indeed witnessing the aftermath of two neutron stars colliding. "Finding more of these X-ray flashes could help reveal how often neutron star mergers create magnetars," Passaleva said. The next frontier, he added, would come when gravitational wave detectors—which sense the ripples in spacetime produced by merging neutron stars—begin their next observing run. If astronomers can pair an X-ray flash with a gravitational wave signal from the same event, they will have a far more complete picture of what happens when the universe's densest objects collide.