Short GRBs are thought to be produced when two compact stars, such as neutron stars, orbit each other, collide, and merge. These cataclysmic events generate gravitational waves and are among the most energetic explosions in the Universe. For decades, astronomers have observed these events primarily through their gamma-ray emission, while their prompt soft X-ray phase remained hidden because most narrow-field X-ray telescopes relied on gamma-ray triggers for localization before repointing to the source. EP’s wide-field soft X-ray monitoring has now revealed this missing phase, providing a direct observation of the earliest stage of the short GRB explosion.
The observations of EP250704a/GRB 250704B came as a remarkable surprise to An Li, a PhD student at Beijing Normal University and the Transient Advocate for EP who was on shift on July 4, 2025. “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. He swiftly responded to the onboard alerts from EP and conducted preliminay analysis. “However, instead of fading away, the source continued emitting episodes of soft X-rays for nearly ten minutes.”
“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.”
To determine the nature of the explosion, the team carried out an extensive international follow-up campaign spanning X-ray, optical, to radio wavelengths. “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.”
Evidence for a Long-lived Central Engine
Digging deeper into the observations, the team found that the long-lasting X-ray emission was powered directly by the merger remnant, rather than by the expanding 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 rapid variability, spectral evolution, and the subsequent behaviors of the X-ray and optical afterglows all point to sustained 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—that powered the extended X-ray emission and continued energy injection.” said Yin.
Opening a New Window for Multi-messenger Astronomy
The discovery has broad implications for the study of neutron star mergers and gravitational-wave sources.
Since the first joint detection of electromagnetic signals and gravitational waves from merging neutron stars in 2017, astronomers have been searching for electromagnetic counterparts of gravitational wave sources that can reveal what happens during and after these violent cosmic 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.
Importantly, the researchers argue that this phenomenon may not be rare. Similar soft X-ray emission could accompany more short GRBs but may have escaped detection because previous missions lacked the capability to capture the prompt emission below gamma-ray energies.
“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.”
“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.
Science Bulletin