10-minute X-ray burst suggests neutron star collision forged heavy elements

On July 4, 2025, astronomers watching the sky through the Einstein Probe detected something they had never quite seen before: a burst of gamma rays that flashed and vanished in half a second, followed by a sustained glow of X-rays that refused to fade for a full ten minutes. The event, catalogued as EP250704a, set off a chain of observations across multiple telescopes—the Very Large Telescope in Chile, the Very Large Array in New Mexico, and others—all aimed at understanding what had just happened in a galaxy so distant that its light had been traveling toward Earth for six billion years.

What astronomers believe they witnessed was a collision between two neutron stars, the densest objects in the universe short of black holes themselves. Neutron stars are born when massive stars exhaust their fuel and collapse catastrophically inward, compressing material equivalent to our entire sun into a sphere roughly the size of a city. The density is almost incomprehensible: a single teaspoon of neutron star matter, if somehow transported to Earth, would weigh as much as ten million tons. When two of these objects spiral into each other and merge, the violence unleashes forces extreme enough to forge elements that even the cores of living stars cannot create—gold, silver, platinum, and other heavy metals that end up scattered through space and eventually incorporated into planets and the beings that inhabit them.

Niccolò Passaleva, the astronomer who led the observations of EP250704a using the Very Large Telescope, described the event as unprecedented. This was, he noted, the longest-lasting prompt X-ray flash ever recorded from a neutron star merger. Astronomers had long expected these collisions to announce themselves through brief flashes of gamma rays that disappeared within two seconds, followed by a visible explosion called a kilonova. But EP250704a suggested something different was happening—a mechanism that could sustain the X-ray emission for minutes rather than seconds.

The team's leading hypothesis centers on a particular type of neutron star called a magnetar. All neutron stars form the same way, through the collapse of a massive star's core, but not all possess the same properties. When a stellar core collapses, it spins faster, much like an ice skater accelerating as they pull their arms inward. Some neutron stars spin more than 700 times per second. The collapse also compresses any existing magnetic field, pushing the field lines closer together and amplifying their strength. In rare cases, this process creates a neutron star with the most powerful magnetic field known to exist anywhere in the universe—a magnetar. Eleonora Troja, part of the Einstein Probe European collaboration, explained that if the remnant of the collision was indeed a magnetar, it could continue releasing bursts of energy for far longer than a typical neutron star merger. When a magnetar dumps its magnetic power into the surrounding space, it can make any explosion brighter and more prolonged.

The researchers were able to rule out other explanations for the X-ray burst. A supernova—the explosive death of a massive star—could not account for what they observed. The light's journey time of six billion years placed the event in a distant galaxy, far beyond our own. What made EP250704a particularly significant was that it may represent a new category of neutron star merger events, one that could explain hundreds of X-ray flashes detected by the Einstein Probe since its launch in 2024. Many of those flashes had resisted explanation, neither matching the signatures of supernovae nor fitting the expected behavior of standard neutron star collisions.

The discovery opens a new avenue for understanding how often magnetars form through these cosmic collisions. If astronomers can identify more events like EP250704a, they may be able to determine the frequency with which magnetars emerge from neutron star mergers and refine their understanding of how the universe's heaviest elements are distributed through space. For now, the ten-minute X-ray burst from six billion years ago stands as a window into the most extreme physics the cosmos has to offer—a reminder that the gold in human hands, the silver in human vaults, and the platinum in human technology all originated in moments of unimaginable violence between dead stars.