A neutron-star merger threw X-rays for nearly ten minutes

A burst of gamma rays lasting about half a second reached Earth on 4 July 2025, and soft X-rays kept pouring from the same patch of sky for nearly ten minutes afterward. That is the longest-lasting prompt X-ray flash ever observed from a neutron-star merger, according to a team led by An Li of Beijing Normal University and Chen-Wei Wang of the Chinese Academy of Sciences. The event carries two names, EP250704a and GRB 250704B, and its paper reached print in Science Bulletin in 2026.

Nothing about it matched the usual short gamma-ray burst, which is over within a couple of seconds and leaves little behind in X-rays. The authors treat the long X-ray tail as a fingerprint of what the merger left behind: a rapidly spinning, strongly magnetized neutron star, known as a magnetar, rather than a black hole.

Einstein Probe and SVOM: who recorded the flash

Two satellites caught the opening seconds. China-France mission SVOM registered the gamma-ray spike at 08:16:27 UTC, and an alert circulated through NASA’s GCN network reported a short burst with a T90 of about 0.7 seconds. Einstein Probe’s wide-field X-ray telescope triggered 25 seconds later, at 08:16:52 UTC, and assigned the EP250704a name. Insight-HXMT also saw the burst.

The ten-minute figure belongs to the X-ray instruments, not to any optical telescope. In the team’s preprint on the event, the soft X-ray emission, measured between 0.5 and 4 keV, lasted about 560 seconds, or a little over nine minutes, after a gamma-ray burst of 0.37 seconds. ScienceDaily rounds the same observation to “nearly 10 minutes” against a burst of about half a second, and the gap between 0.37 and 0.68 seconds reflects different instruments and different ways of measuring a burst’s length.

Redshift and a missing supernova from the VLT

The Very Large Telescope in Chile, ESO’s four-telescope flagship at Cerro Paranal, did not see the X-rays. It answered the questions that X-ray data cannot. Its X-shooter spectrograph measured a redshift of 0.661, which places the explosion more than six billion years back in time and rules out a nearby galaxy as the source. Its FORS2 instrument then took deep images looking for the glow of a supernova that would have signaled a collapsing massive star, and found none.

Niccolò Passaleva of the University of Rome Tor Vergata, one of the lead authors, described the scramble when the paper was summarized by ScienceDaily on 30 September 2026, which frames the event as a possible glimpse of magnetar birth. Returning home by train, he recalled, he was suddenly racing to commandeer one of the world’s largest telescopes from his laptop. The work sits within the QUEENB program, short for Quest for Elusive Neutron star and Black hole mergers, and Eleonora Troja of Tor Vergata is among the corresponding authors.

The magnetar reading, and a second paper

The time gap matters for how the result reads. The burst was observed on 4 July 2025, the preprints appeared in January 2026, and the peer-reviewed version carries the Science Bulletin citation volume 71, issue 18, so the finding is more than a year old as a measurement even though its journal publication is recent. Everything in the figures above dates from the 2025 observations, with no later re-measurement reported in the sources reviewed.

A short burst, a distance of billions of light-years and no supernova together point toward two neutron stars colliding, which is the basis for calling the event a merger. A collapsing massive star, the other usual source of long-lasting gamma-ray flashes, would have lit up the FORS2 images with a supernova, and none appeared. The word is the authors’ interpretation of those three clues, and the evidence in the sources reviewed is the electromagnetic signature alone: none of them reports a gravitational-wave signal.

A magnetar would explain the extended X-rays because a newborn, fast-spinning neutron star can keep injecting energy into the surrounding debris as it slows down. A separate analysis by Nissim Fraija and colleagues reached a compatible conclusion from multiwavelength data covering roughly two days: the long X-ray plateau, an optical and infrared plateau and a steep decline are consistent, in their words, with a millisecond magnetar undergoing accretion, and the remnant is most likely a long-lived magnetar.

The magnetar remains an inference from fitted light curves rather than a direct detection, and the two teams did not publish identical models. Radio follow-up from the Very Large Array and MeerKAT, logged in the same GCN archive between roughly four and six days after the burst, adds later-time measurements that any model of the remnant has to reproduce.

Li, Wang and their co-authors argue that bursts like this one may be more common than previously recognized, since the long soft X-ray phase is invisible to monitors that watch only gamma rays. EP250704a was caught with its tail intact because Einstein Probe was watching in soft X-rays, and a count of how many other short bursts carry a comparable minutes-long glow would show whether this one is an outlier.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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