Researchers have found strong evidence linking one unusually long-lasting X-ray flash to a neutron star merger. The discovery raises the possibility that some mysterious, difficult-to-explain cosmic flashes are caused by collisions between neutron stars.

Since the launch of the Einstein spacecraft in January 2024, astronomers have detected hundreds of bright X-ray flashes from distant galaxies. These events, known as fast X-ray transients, can have several causes. Some are associated with the deaths of massive stars, while the origins of others remain unknown.

Determining the cause of a fast X-ray transient can be difficult because astronomers often do not know how far away the event occurred or how much energy it released.

A possible magnetar born from a neutron star merger

Researchers from Professor Eleonora Troja’s group, supported by a European Research Council Consolidator grant, obtained observations that helped identify the source of one such X-ray transient.

After receiving an alert from the Einstein spacecraft, the team quickly organized follow-up observations using several instruments, including the European Southern Observatory’s Very Large Telescope (VLT) and the Very Large Array. By studying the material left behind after the explosion, the researchers concluded that they may have witnessed the formation of a magnetar created by the collision, or merger, of two neutron stars.

Neutron stars are extremely dense stellar remnants left behind after massive stars reach the ends of their lives. When two neutron stars merge, they produce gravitational waves that travel through space. The light generated during these events helps astronomers determine what survived the impact.

Short gamma-ray bursts have traditionally been the main electromagnetic signal associated with neutron star mergers.

“But if the debris from the collision is a magnetar, it could continue to power an explosion for a longer period of time,” said Professor Troja, a member of the Einstein Spacecraft Europe Collaboration and co-corresponding author of the paper. “A magnetar is a rapidly rotating neutron star with a huge magnetic field. When a magnetar transfers its magnetic energy to its surroundings, it can make an explosion brighter and cause it to last longer. When we looked at the X-ray data from this new event, we knew something unusual was happening.”

A neutron star merger produced an X-ray flash lasting nearly 10 minutes

The event, designated EP250704a/GRB 250704B, was detected by the SVOM, Insight-HXMT and Einstein Probe satellites on July 4, 2025.

The gamma-ray burst lasted only about half a second. In contrast, the Einstein spacecraft recorded bright X-ray emissions for nearly 10 minutes.

“This is the longest-lasting instantaneous X-ray flash ever observed in a neutron star merger,” said Niccolo Passareva, a graduate student in Chile who led follow-up observations using the VLT. “This is your chance to have a front-row seat to the universe’s most extreme forces and discover more of their secrets.”

Researchers have spent several years searching for a strong link between fast X-ray transients and neutron star mergers. Previous candidates disappeared too quickly and failed to provide enough evidence.

This time, Pasareva was able to respond within minutes and begin observations while the event was still bright enough to study in detail.

“I was riding home on the train and suddenly I was racing against time to get the world’s largest telescope from my laptop,” Pasareva recalls.

Measuring an X-ray flash from more than 6 billion years ago

Pasareva and colleagues used the VLT’s X-Shooter instrument to break the event’s light into its individual components. They identified a distinct absorption pattern that allowed them to measure the event’s redshift and determine how far away its source was.

The research team measured a redshift of z = 0.6610. This indicates that the explosion occurred long before the Sun and its planets formed. Its light took more than 6 billion years to reach Earth.

The researchers then looked for another important clue: evidence of a supernova.

Using detailed observations from the VLT’s FORS2 instrument, they searched for a bright supernova. Supernovae are typically expected when the collapse of a massive star produces a long-lasting X-ray flash.

No supernova appeared.

Taken together, the measured distance, the absence of a supernova and the characteristics of the burst provided strong evidence that the event was caused by a neutron star merger.

A new way to detect neutron star mergers

Finding more events like this could help astronomers determine how often neutron star mergers produce magnetars.

“If we can find more of these X-ray flashes, it could help reveal how often neutron star mergers produce magnetars,” Passareva concluded. “I’m really looking forward to the next gravitational wave observation, when we can finally combine one of these X-ray flashes with a burst of gravitational waves from the same source.”

Additional information

This research was carried out by an international collaboration of astronomers.

First authors: An Li (Beijing Normal University), Chen-Wei Wang (Chinese Academy of Sciences), Niccolò Passaleva (University of Rome Tor Vergata) and Jie An (Chinese Academy of Sciences).

Corresponding authors: Binbin Zhang (Nanjing University), Eleonora Troja (Tor Vergata, University of Rome), Yi-Han Iris Ying (University of Hong Kong), Jing-Wei Hu (Chinese Academy of Sciences) and Hua-Li Li (Chinese Academy of Sciences).

The VLT observations used in this study were carried out as part of the large-scale program 114.27LW, led by Principal Investigator Eleonora Troja. The program is titled “QUEENB: The Quest for the Elusive Neutron Star-Black Hole Merger.”