Astronomers Detect Long-Duration X-Ray Flashes From Neutron Star Merger, Opening New Detection Method

A New Signature for Cosmic Collisions

For decades, astronomers have identified neutron star collisions primarily through gamma-ray bursts that flash and fade in seconds. Researchers have now discovered that these catastrophic events may also produce extended X-ray emissions that linger for minutes—a finding that could fundamentally expand how scientists hunt for these violent cosmic encounters. The discovery, published in Science Bulletin, emerged from observations of a neutron star merger detected on July 4, 2025, designated EP250704a/GRB 250704B. The gamma-ray burst lasted roughly half a second, yet the Einstein Probe satellite captured bright X-ray emissions persisting for nearly ten minutes, marking the longest prompt X-ray flash ever recorded from a neutron star collision.

The Event and Its Detection

The breakthrough came through rapid-response observations following an alert from the Einstein Probe satellite, which launched in January 2024 and has detected hundreds of bright X-ray flashes from distant galaxies. Graduate student Niccolò Passaleva led the follow-up campaign, mobilizing major observatories including the European Southern Observatory's Very Large Telescope in Chile and the Very Large Array within minutes of the initial alert. Passaleva coordinated observations from a train while traveling home, using his laptop to command access to one of the world's most powerful telescopes. The multi-instrument approach proved decisive: by analyzing the event's light spectrum using the VLT's X-Shooter instrument, researchers identified absorption patterns revealing the explosion occurred more than 6 billion light-years away, with its light traveling through space for over six billion years before reaching Earth.

The team measured a redshift of z=0.6610, establishing the cosmic distance definitively. Critically, deep observations using the VLT's FORS2 instrument found no associated supernova—a phenomenon that would normally accompany prolonged X-ray flashes from dying massive stars. The combination of measured distance, absence of supernova signatures, and burst characteristics provided strong evidence pointing to a neutron star merger rather than alternative explanations.

The Magnetar Connection

The extended X-ray emission likely originates from a magnetar, an extraordinarily dense neutron star with an intense magnetic field spinning rapidly. When a neutron star merger occurs, the collision may produce such a magnetar, which then releases magnetic energy into surrounding material, dramatically amplifying and prolonging the visible explosion. According to research team leader Eleonora Troja, the magnetic dampening process can make any resulting explosion significantly brighter and more enduring than simple neutron star collision models would predict. This mechanism explains why the X-ray signature persisted so dramatically longer than the brief gamma-ray flash—the magnetar's energy injection sustained the visible emission.

Implications for Future Searches

The discovery opens a new detection pathway for neutron star mergers that have historically been difficult to identify amid the universe's cacophony of transient events. Fast X-ray transients detected by satellites like the Einstein Probe have puzzled astronomers for years, with some tied to stellar deaths while others remaining orphaned without clear sources. If additional similar events emerge, astronomers could establish how frequently neutron star collisions produce magnetars—a crucial unknown in understanding merger physics.

The research team spent several years searching for a definitive connection between fast X-ray transients and neutron star mergers. Previous candidates faded too rapidly to provide sufficient evidence, but this event's extended brightness allowed detailed characterization. Passaleva emphasized the scientific opportunity: detecting the longest-lasting prompt X-ray flash from a neutron star merger provides a front-row perspective on the universe's most extreme forces. Looking ahead, the community anticipates pairing these X-ray detections with gravitational wave signals—ripples in spacetime emanating from the same merger events—when the next generation of gravitational wave observations commences. Such simultaneous detections would provide unprecedented insight into merger dynamics and the resulting compact object formation.