Astronomers Catch Neutron Star Merger Glowing for Nine Minutes

On July 4, 2025, a neutron star merger halfway across the universe gave astronomers a show unlike anything they had seen before. The collision's gamma-ray flash lasted only about four-tenths of a second, but an X-ray telescope kept watching the same spot glow for nearly 10 minutes, with soft X-rays streaming for 562 seconds after the gamma-ray signal disappeared. The finding, published as a cover article in the journal Science Bulletin in October 2026, is being described as the longest-lasting prompt X-ray flash ever recorded from a neutron star merger, and it may finally link some of the sky's mysterious X-ray flashes to colliding dead stars.

Three satellites caught the event together. The China-led Einstein Probe, the French-Chinese Space Variable Objects Monitor and China's Insight-HXMT each played a role: the latter two recorded the half-second gamma-ray burst, catalogued as GRB 250704B, while the X-ray spacecraft captured the companion X-ray flash, EP250704a, from the very start and kept recording long after the gamma rays faded. Most X-ray telescopes can only swing toward a burst after a gamma-ray detector first finds it, according to the researchers, so the earliest X-ray light from any neutron star merger has typically been missed entirely.

What made this neutron star merger so unusual

Each neutron star merger normally announces itself with a short gamma-ray burst lasting under two seconds, and astronomers have generally assumed the central engine shuts down quickly after impact. This time the X-ray light told a different story. The spacecraft recorded three phases of activity: an initial spike matching the gamma-ray flash, a short tail, and then a much longer bump that stretched for hundreds of seconds. Through the entire extended phase, the gamma-ray instruments picked up almost nothing comparable, meaning the event would have looked utterly ordinary to anyone watching in gamma rays alone.

The flickering mattered as much as the duration. The X-rays changed brightness far too quickly, and their spectrum shifted over time, for the glow to be an ordinary afterglow, the fading light made when explosion debris slams into surrounding material. Instead, the rapid variability pointed to a central engine that kept feeding energy into the blast for minutes after the collision, according to the team behind the discovery, an unusual way for a neutron star merger to end.

A newborn magnetar may be powering the glow

The team's leading explanation is a newborn magnetar: a rapidly spinning neutron star with a magnetic field trillions of times stronger than Earth's, forged in the neutron star merger. "Magnetars are rapidly spinning neutron stars with huge magnetic fields. When they dump 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," said Eleonora Troja, an astrophysicist at the University of Rome Tor Vergata and a corresponding author of the paper, in a statement reported by Earth.com.

Measuring the distance took some detective work. Niccolò Passaleva, a graduate student in Troja's group, got the alert while riding a train and rushed to book time on the Very Large Telescope in Chile from his laptop. Its X-Shooter instrument split the fading light into a spectrum showing absorption features from magnesium and iron, placing the explosion at a redshift of 0.661, with its light traveling more than 6 billion years before reaching Earth. Deep follow-up images taken 12 and 20 days later showed no supernova at the site, ruling out the collapse of a massive star. "This is the longest lasting prompt X-ray flash ever observed from a neutron star merger," Passaleva said in a statement reported by IFLScience. "It is an opportunity to have a front-row seat to the most extreme forces of the Universe and discover more of its secrets."

The magnetar idea, however, is an inference rather than a direct detection. At that distance, gravitational waves from the collision sat beyond the reach of current ground-based detectors, and the kilonova glow from newly forged heavy elements would have been too faint to see. The researchers also could not fully rule out a newborn black hole at the center, though the way the X-rays faded fit that idea less well. And not every neutron star merger ends this way: one collision spotted through its gravitational waves in 2017 appears to have formed a black hole instead, according to IFLScience.

Why this neutron star merger matters for cosmic mystery-solving

The finding could rewrite what counts as a merger. Roughly 10 to 15 percent of bursts linked to these collisions have shown extended light before, but NASA's Swift satellite must swivel toward each burst and usually misses the start of the X-ray action. Judged on gamma rays alone, this event would have been a mere half-second spike. The team calculated that Swift's gamma-ray detector could have caught this X-ray glow only from far nearer collisions, and even then barely. That is why the researchers argue that long X-ray tails may be a normal feature of every neutron star merger, with most of them simply going unrecorded.

The result also touches a standing cosmic mystery. Since the X-ray mission launched in January 2024, astronomers have been puzzling over fast X-ray transients, brief flashes of X-rays with no obvious source. Yi-Han Iris Yin, a Ph.D. student at the University of Hong Kong who led the analysis of the prompt emission, said similar soft X-ray signals may have been present in earlier bursts but were too soft and faint for conventional gamma-ray instruments. "Einstein Probe is allowing us to uncover a part of compact star mergers that was hidden from previous gamma-ray observations," Yin said, in a statement reported by Phys.org. "The short gamma-ray flash may represent only the beginning of the high-energy activity."

Next up could be the ultimate confirmation. "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," Passaleva said. Catching both signals from one neutron star merger would reveal exactly what kind of crash makes such a flash, and settle how often these collisions give birth to magnetars rather than black holes. For more cosmic close encounters, see our science coverage and the story of the lowest-mass double neutron star pair found so far.