Longest-lasting X-ray flash that took 6 billion years to reach Earth, may have be a result of neutron star merger; study suggests

Astronomers studying a strange and rather unusual burst of long X-rays, whose light traveled for more than 6 billion years before being spotted last year, believe it was caused by two neutron stars colliding.

The event, named EP250704a, was detected on July 4, 2025. It began with a burst of gamma rays lasting about half a second, followed by an X-ray flash that lasted 10 minutes. Scientists are now trying to understand what made the flash last so long.

The study published in Science Bulletin on September 30, 2026, provides evidence for this connection. It suggests that merging neutron stars could be hiding among cosmic flashes whose origins are still a mystery.


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After China’s Einstein Probe space telescope, which has been orbiting LEO(Low Earth Orbit) for two years now, detected the burst, astronomers used several other observatories to investigate it, including the European Southern Observatory’s Very Large Telescope (VLT) and the Very Large Array.

The observations revealed an X-ray flash that stood out from previously observed neutron star mergers. “This is the longest-lasting prompt X-ray flash ever observed from a neutron star merger,” astronomer Niccolò Passaleva, who led the VLT observations, said in a statement to phys.org. “It is an opportunity to have a front-row seat to the most extreme forces of the universe and discover more of its secrets.”

In their research, they have ruled out that a supernova could have caused the X-ray burst and measured that the light from this collision that reached Earth was traveling for 6 billion years.

Neutron stars, which are among the densest objects known in the universe, form when massive stars collapse at the end of their lives. And when two neutron stars collide, the astronomical event known as a kilonova produces heavy elements like gold, silver and platinum.

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Short gamma rays of our universe are often the result of this astronomical event, which usually lasts less than two seconds.

The newly studied event suggests that some mergers may also produce X-ray emissions which could last for several minutes.

“When I saw the X-ray data from this new event, I realized something was up,” Eleonora Troja, a member of the Einstein Probe European collaboration, said to the outlet.

The team suspects that at least one of the merging neutron stars was a magnetar, a rapidly spinning neutron star with an exceptionally powerful magnetic field.

“If the remnant of the collision is a magnetar, it could keep bursting for longer,” she said. “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.”

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Since the Einstein Probe began operating in 2024, astronomers have identified hundreds of X-ray flashes from distant galaxies. Some have been connected to stellar explosions, others are unexplained.

So, more events like EP250704a could help scientists determine how frequently neutron star mergers produce magnetars and whether these collisions explain some of the mysterious X-ray flashes detected across the universe.

“Finding more of these X-ray flashes could help reveal how often neutron star mergers create magnetars.” concludes Passaleva.