Short gamma-ray bursts, or short GRBs, are believed to occur when two compact objects, such as neutron stars, spiral toward one another before colliding and merging. These extraordinarily violent events produce gravitational waves and rank among the most powerful explosions in the Universe.
For decades, astronomers have mainly detected these events through their gamma-ray flashes. Their earliest soft X-ray emission has been far more difficult to observe because most narrow-field X-ray telescopes depend on gamma-ray alerts to locate a burst before turning toward it. Einstein Probe’s wide-field soft X-ray monitoring has now exposed this previously unseen stage, giving astronomers a direct look at the opening moments of a short GRB.
A Half-Second Flash With a Much Longer Aftermath
The event, designated EP250704a/GRB 250704B, caught researchers by surprise on July 4, 2025. An Li, a PhD student at Beijing Normal University and the Transient Advocate for EP, was on duty when the signal appeared.
“The event initially appeared to be an ordinary short GRB, producing a bright flash lasting less than half a second that was detected simultaneously in gamma rays by SVOM-GRM and Insight-HXMT and X-rays by EP-WXT,” said Li.
After receiving the onboard alerts from EP, Li quickly began the initial analysis. What happened next was unexpected.
“However, instead of fading away, the source continued emitting episodes of soft X-rays for nearly ten minutes.”
The extended X-ray activity was energetic, but it occurred at wavelengths that conventional gamma-ray instruments would have struggled to detect.
“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. As a result, previous missions would have recorded only the brief gamma-ray flash, missing the prolonged activity revealed by EP,” said Professor Bin-Bin Zhang of Nanjing University, a co-corresponding author of the paper who initiated the in-depth study of the event. “Our observations show that what appears to be a typical short gamma-ray burst can actually conceal a much longer and richer episode of activity at soft X-ray energies.”
Tracking the Explosion Across the Spectrum
To determine what had produced the unusual signal, researchers organized an extensive international follow-up campaign using observations ranging from X-ray and optical wavelengths to radio.
“The coordinated observations across multiwavelengths were essential,” said Professor Eleonora Troja of the University of Rome “Tor Vergata,” a co-corresponding author of the paper, whose group obtained the key information of the redshift from spectroscopic analysis. “They not only allowed us to identify and study the burst’s host galaxy and measure its distance, but also enabled us to rule out an accompanying supernova and provide strong evidence linking this extraordinary X-ray emission to a compact object merger.”
The combined observations helped establish that the unusual X-ray activity was connected to a compact object merger rather than another type of stellar explosion.
Evidence for a Long-Lived Central Engine
A closer examination showed that the prolonged X-ray emission was being powered directly by the remnant left behind by the merger, rather than by the outward-moving blast wave.
Yi-Han Iris Yin, a PhD student in the Department of Physics and the Hong Kong Institute of Astronomy and Astrophysics at The University of Hong Kong, led the analysis of the high-energy emission. As a co-corresponding author, she found that EP250704a’s rapidly changing brightness, evolving spectrum, and later X-ray and optical afterglows all pointed to continued activity from the central engine after the initial short GRB had already faded.
“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 Yin.
Such a magnetar could provide a natural explanation for why the system remained active long after the brief gamma-ray flash had ended.
A New Tool for Multi-Messenger Astronomy
The discovery could have important consequences for the study of neutron star mergers and the sources of gravitational waves.
Since electromagnetic radiation and gravitational waves were jointly detected from merging neutron stars for the first time in 2017, astronomers have been searching for additional electromagnetic signals that can reveal what occurs during and after these extreme collisions.
“The newly discovered soft X-ray component provides a new probe, indicating that the fast X-ray transients are also electromagnetic counterparts to gravitational-wave sources and may originate from compact object mergers,” said Professor Troja.
The researchers also suggest that this type of extended soft X-ray activity may not be unusual. Similar signals could accompany other short GRBs but may have gone unnoticed because earlier missions were not able to capture prompt emission at energies below the gamma-ray range.
A Hidden Phase of Neutron Star Mergers
“This discovery extends our view of neutron star mergers beyond the brief gamma-ray flash,” said Yin. “By revealing this previously hidden soft X-ray phase, Einstein Probe opens a new window for studying neutron star merger remnants and may ultimately help constrain the neutron star equation-of-state.”
By revealing activity that traditional gamma-ray observations can miss, Einstein Probe may give astronomers a new way to study both the immediate aftermath of neutron star mergers and their connection to gravitational-wave events.
“The findings also demonstrate Einstein Probe’s unique capability to uncover new classes of transient phenomena and strengthen its role in the era of multi-messenger astronomy, in which gravitational waves and electromagnetic radiation are studied together to understand some of the most extreme events in the Universe,” added Professor Zhang.
