A neutron star crash hid 10 minutes of X-rays behind a half-second gamma flash

Neutron star collisions usually announce themselves in less than two seconds: a spike of gamma rays, then a fading afterglow. One collision six billion light-years away followed that script for less than half a second. Then, instead of going quiet, it kept pouring out X-rays for nearly ten minutes.

China’s Einstein Probe satellite caught the whole sequence, and it is the longest-lasting prompt X-ray flash ever recorded from a neutron star merger. The international team behind the study, the cover article of the journal Science Bulletin, argues that the extra minutes came from whatever the crash left behind. Their most plausible culprit is a magnetar, a newborn neutron star spinning fast and carrying a magnetic field strong enough to keep pumping energy into the explosion. If they are right, gamma-ray observatories have been catching only the opening beat of these collisions.

That matters well beyond one event. Neutron star mergers are where the universe forges heavy elements such as gold and platinum, and they shake spacetime hard enough for gravitational-wave detectors on Earth to feel it. What survives the crash, a black hole or a super-magnetized neutron star, tells physicists how much matter can be squeezed together before gravity wins.

How a half-second burst turned into ten minutes

The event carries two names, EP250704a in X-rays and GRB 250704B in gamma rays, because three spacecraft saw it at once. The Chinese-French SVOM mission and China’s Insight-HXMT observatory recorded the gamma-ray flash, while Einstein Probe’s Wide-field X-ray Telescope watched the same spot in soft, lower-energy X-rays.

Most X-ray telescopes have a narrow view and only swing toward a burst after a gamma-ray detector raises the alarm, so the earliest X-rays are usually gone by the time they look. Einstein Probe uses lobster-eye optics that stare at a large patch of sky continuously, which meant it was already watching when the collision went off.

“The event initially appeared to be an ordinary short GRB, producing a bright flash lasting less than half a second,” said An Li, a PhD student at Beijing Normal University who was on duty as the mission’s transient advocate. “However, instead of fading away, the source continued emitting episodes of soft X-rays for nearly ten minutes.”

The race to pin down the source fell to Niccolò Passaleva, a graduate student in the group of Eleonora Troja at the University of Rome Tor Vergata. “I was traveling home by train, and all of a sudden I was rushing against time to commandeer one of the largest telescopes in the world from my laptop,” he said. Within minutes, the European Southern Observatory’s Very Large Telescope in Chile was pointed at the fading glow.

Its X-Shooter spectrograph split the light into its component colors and found absorption lines that gave a redshift of 0.661, a measure of how much the expanding universe has stretched the light on its way here. That places the explosion more than six billion light-years away: it happened before the Sun and its planets existed.

Why it was not a dying star

Long X-ray flashes are not new on their own. Massive stars that collapse at the end of their lives also produce long-lasting bursts, and those deaths come with a supernova. So the team used the VLT’s FORS2 camera to look for one in deep images of the host galaxy. They found nothing.

The distance, the missing supernova and the burst’s own properties together point to a merger of compact objects, the umbrella term for neutron stars and black holes. The X-rays also behaved in a way the textbook picture cannot explain. Instead of a hard spike from matter falling onto the remnant followed by a smoothly fading afterglow from the expanding blast, the emission flickered and changed its spectrum for minutes. According to the team, that pattern means the central engine itself kept firing long after the gamma rays stopped.

“One plausible explanation is that the merger produced a rapidly rotating, highly magnetized neutron star, known as a magnetar, that powered the extended X-ray emission and continued energy injection,” said Yi-Han Iris Yin, a PhD student at the University of Hong Kong who led the analysis of the high-energy emission.

0.4 seconds of gamma rays, 560 seconds of X-rays

The numbers explain why astronomers kept missing this phase. The gamma-ray flash lasted about 0.4 seconds. The soft X-rays lasted roughly 560 seconds, about 1,400 times longer. Earlier work had inferred that around 30 percent of short gamma-ray bursts might hide an engine-driven X-ray phase of about 100 seconds, judging from how quickly their afterglows faded, but nobody had seen one directly. This one ran more than five times longer than those estimates.

“Although this long-lasting emission carried substantial energy, its spectrum was so soft that, for a burst at this typical cosmological distance, it would have remained below the detection threshold of conventional gamma-ray instruments, such as Swift’s Burst Alert Telescope,” said Bin-Bin Zhang of Nanjing University, a co-corresponding author. In other words, earlier missions would have logged a perfectly ordinary half-second burst.

A magnetar would fit that behavior. A neutron star packs more mass than the Sun into a ball about 20 kilometers across, roughly the size of a city, and a magnetar’s magnetic field is around a thousand times stronger than an ordinary neutron star’s. “Magnetars are rapidly spinning neutron stars with huge magnetic fields. When they damp their magnetic power into the surroundings, they can make any explosion brighter and longer-lasting,” said Troja, one of the paper’s corresponding authors. “When I saw the X-ray data from this new event, I realized something was up.”

What it doesn’t settle

The magnetar is an interpretation, not an observation. The researchers call it the most plausible explanation, and the paper’s own title is careful, speaking of a “compact object merger” rather than naming the pair. A neutron star colliding with a black hole also falls under that term.

There is no gravitational-wave signal to settle the question. At six billion light-years, the collision sat far beyond the reach of today’s detectors, which can hear neutron star mergers only out to a few hundred million light-years. The case for a merger rests on the missing supernova and on the burst’s properties, which are strong evidence but indirect.

It is also a single event. Earlier candidates faded too fast to analyze, so nobody knows yet how many short bursts hide an X-ray phase, or how many leave a magnetar rather than a black hole. The team suspects the phenomenon is common; only a larger sample can show it.

Common questions about neutron star mergers and magnetars

What is a magnetar?

A magnetar is a neutron star with an extreme magnetic field, around a thousand times stronger than that of an ordinary neutron star. Young magnetars spin rapidly, and as they lose magnetic energy they can feed power into their surroundings, making an explosion brighter and longer-lasting.

What is a short gamma-ray burst?

A short gamma-ray burst is a flash of high-energy radiation lasting less than about two seconds. Most are thought to come from collisions between two neutron stars or between a neutron star and a black hole. Longer bursts usually come from collapsing massive stars and arrive with a supernova.

How far away was this neutron star collision?

The light traveled for more than six billion years to reach Earth. The team measured a redshift of 0.661 with the Very Large Telescope’s X-Shooter spectrograph, which means the explosion happened before the Sun and the planets formed.

What is the Einstein Probe?

Einstein Probe is a China-led X-ray space telescope built with partners including the European Space Agency and Germany’s Max Planck Institute for Extraterrestrial Physics. Its wide-field telescope scans large areas of the sky in soft X-rays and has detected hundreds of brief X-ray flashes from distant galaxies.

What comes next

The event was detected on July 4, 2025, and the paper appeared in Science Bulletin in 2026. Einstein Probe, in orbit since January 2024, keeps scanning the sky, and every new fast X-ray flash with a measured distance adds to the sample the team needs.

The bigger target is a match with gravitational waves. Since 2017, when a neutron star merger was seen in both gravitational waves and light for the first time, astronomers have looked for more signals that can travel alongside those ripples. “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. A closer merger caught in both would show exactly what the collision left behind.

Reference: Li, Wang, Passaleva, An et al., “Minutes-long soft X-ray prompt emission from a compact object merger,” Science Bulletin, 2026. DOI: 10.1016/j.scib.2026.08.021

Tags: astronomy, magnetar, Very Large Telescope, Einstein Probe, Eleonora Troja, gamma-ray bursts