For decades, astronomers have primarily detected short GRBs through brief flashes of gamma rays. Their early soft X-ray emission has been much harder to observe because most narrow-field X-ray telescopes rely on gamma-ray alerts to locate a burst after it occurs.

The Einstein spacecraft’s wide-field soft X-ray monitoring has now revealed this previously hidden phase, allowing astronomers to observe the onset of a short GRB and the activity that followed.

A Half-Second Gamma-Ray Flash Followed by 10 Minutes of X-Rays

The event, designated EP250704a/GRB 250704B, surprised researchers on July 4, 2025. An Li, a doctoral student at Beijing Normal University and temporary Einstein Probe advocate, was at work when the signal appeared.

“The phenomenon initially appeared like a regular short GRB, producing a bright flash lasting less than half a second, and was detected simultaneously in gamma rays by SVOM-GRM and Insight-HXMT and in X-rays by EP-WXT,” Li said.

After receiving the in-flight alert from Einstein Probe, Li immediately began an initial analysis. The source then produced an unexpected afterglow.

“However, instead of disappearing, the source continued to emit soft X-rays for nearly 10 minutes.”

The burst generated widespread X-ray activity at energies that would have been difficult for conventional gamma-ray instruments to detect.

“While this long-lasting emission had considerable 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 Professor Bingbing Zhang of Nanjing University, co-corresponding author of the paper that initiated a detailed study of the event.

“As a result, previous missions would have recorded only short gamma-ray flashes and missed the long-term activity revealed by EP. Our observations show that what appears to be a typical short gamma-ray burst may actually be hiding longer, richer episodes of activity at soft X-ray energies.”

Multi-Wavelength Observations Trace the Explosion

To determine the cause of the unusual signal, researchers organized an international follow-up campaign that collected observations across X-ray, optical and radio wavelengths.

“Coordinated observations across multiple wavelengths were essential,” said co-corresponding author Professor Eleonora Troja of the University of Rome Tor Vergata, whose group obtained important redshift information through spectroscopy.

“They not only allowed us to identify and study the host galaxy of the burst and measure its distance, but also ruled out an accompanying supernova and provided strong evidence linking this unusual X-ray emission to a compact object merger.”

The combined observations show that the anomalous X-ray activity was associated with a compact-object merger rather than the explosion of another type of star.

Evidence for a Long-Lived Central Engine

Closer analysis revealed that the long-lasting X-ray emission came directly from debris left behind by the merger, rather than from the outward-moving blast wave.

Yi-Han Iris Yin, a PhD student in the Department of Physics at the University of Hong Kong and the Hong Kong Institute of Astronomy and Astrophysics, led the analysis of the high-energy radiation.

As co-corresponding author, Yin found that EP250704a’s rapidly changing brightness, evolving spectrum, and subsequent X-ray and optical afterglow all point to continued activity from the central engine after the initial short GRB had faded.

“One plausible explanation is that the merger produced a rapidly rotating, highly magnetized neutron star known as a magnetar, which magnified the X-ray emission and continued the energy injection,” Yin said.

A magnetar could naturally explain why the system remained active long after the short gamma-ray flash ended.

A New Tool for Multi-Messenger Astronomy

The discovery could have important implications for studying neutron star mergers and the sources of gravitational waves.

Since the first joint detection of electromagnetic radiation and gravitational waves from merging neutron stars in 2017, astronomers have searched for additional electromagnetic signals that can reveal what happens during and after these extreme collisions.

“The newly discovered soft X-ray component provides a new probe and shows that fast X-ray transients are also the electromagnetic counterpart of gravitational wave sources and may arise from the merger of compact objects,” Professor Troja said.

The researchers suggest that long-lasting soft X-ray activity may not be uncommon. Similar signals could accompany other short GRBs but may have gone unnoticed because earlier missions were unable to capture prompt emissions below the gamma-ray range.

Revealing the Hidden Stages of Neutron Star Mergers

“This discovery extends our view of neutron star mergers beyond brief gamma-ray flashes,” Professor Yin said. “By revealing previously hidden soft X-ray phases, the Einstein spacecraft opens a new window for studying neutron star merger debris and may ultimately help constrain the neutron star’s equation of state.”

By detecting activity that traditional gamma-ray observations can miss, Einstein Probe could provide astronomers with a new way to study the immediate effects of neutron star mergers and their connection to gravitational-wave events.

“This discovery also demonstrates the Einstein spacecraft’s unique ability to discover new types of transient phenomena and strengthen its role in the era of multi-messenger astronomy, where gravitational waves and electromagnetic radiation are studied together to understand some of the most extreme phenomena in the universe,” Professor Zhang added.