The reason this phase had remained hidden for so long lies in the architecture of previous X-ray missions. Most narrow-field X-ray telescopes could not find transient events on their own; they relied on gamma-ray detections to provide an initial localization before repointing their instruments toward the source. By the time they arrived, the earliest and softest X-ray emission had already passed. Einstein Probe, with its wide-field soft X-ray monitoring capability, was watching the sky at the right moment and in the right energy band, providing a direct observation of the earliest stage of the short gamma-ray burst explosion. The finding, published in Science Bulletin, suggests that the standard picture of these mergers, in which the action is essentially over within a couple of seconds, is incomplete.

An Li, a PhD student at Beijing Normal University who serves as a Transient Advocate for Einstein Probe, was on shift when the event unfolded. According to Li, the event initially appeared to be an ordinary short gamma-ray burst, producing a bright flash lasting less than half a second that was detected simultaneously in gamma rays by SVOM-GRM and Insight-HXMT and in X-rays by EP-WXT. He swiftly responded to the onboard alerts and conducted a preliminary analysis. Rather than disappearing, the source kept producing episodes of soft X-ray emission for nearly ten minutes, an behavior that immediately set the event apart from the hundreds of short bursts recorded by earlier missions.

The energy budget of this extended emission was substantial, but its spectral character made it extraordinarily difficult to detect with conventional instrumentation. Professor Bin-Bin Zhang of Nanjing University, a co-corresponding author of the paper who initiated the in-depth study, explained that the spectrum was so soft that, for a burst at a typical cosmological distance, it would have fallen below the detection threshold of conventional gamma-ray instruments such as Swift’s Burst Alert Telescope. Previous missions would therefore have recorded only the brief gamma-ray flash and missed the prolonged activity that Einstein Probe revealed. As Zhang put it, the 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.

Determining the physical nature of the explosion required more than a single instrument. The team mounted an extensive international follow-up campaign spanning X-ray, optical, and radio wavelengths, coordinating facilities across the globe and in orbit. Professor Eleonora Troja of the University of Rome Tor Vergata, a co-corresponding author whose group obtained the key redshift information from spectroscopic analysis, emphasized that the coordinated multiwavelength observations were essential. They allowed the team to identify and study the burst’s host galaxy, measure its distance, and, critically, rule out an accompanying supernova. That exclusion provided strong evidence linking the extraordinary X-ray emission to a compact object merger rather than the death of a massive star, which produces long gamma-ray bursts and supernovae.

The deeper analysis delivered perhaps the most consequential result of the study: the long-lasting X-ray emission appeared to be powered directly by the merger remnant itself, not by the expanding blast wave slamming into surrounding material. 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 the event’s rapid variability, its spectral evolution, and the subsequent behavior of both the X-ray and optical afterglows all pointed toward sustained activity from a central engine operating long after the initial short gamma-ray burst had faded. In standard afterglow models, emission is dominated by the external shock; here, the data told a different story.

One plausible explanation advanced by the team is that the merger produced a rapidly rotating, highly magnetized neutron star, known as a magnetar, which powered the extended X-ray emission through continued injection of energy. A stable magnetar remnant of this kind would rotate hundreds of times per second, winding up extreme magnetic fields that can tap the enormous rotational energy of the newborn object and channel it into electromagnetic radiation. Such a remnant has major implications for fundamental physics, because whether a merger produces a long-lived neutron star or collapses promptly into a black hole depends sensitively on the true maximum mass of neutron stars, which in turn depends on the poorly constrained equation of state of matter at nuclear densities.

The discovery also resonates strongly with the field of multi-messenger astronomy. Since the first joint detection of electromagnetic signals and gravitational waves from merging neutron stars in 2017, astronomers have been searching for electromagnetic counterparts to gravitational-wave sources that can reveal what happens during and after these violent cosmic collisions. Troja noted that the newly discovered soft X-ray component provides a new probe, indicating that fast X-ray transients are also electromagnetic counterparts to gravitational-wave sources and may originate from compact object mergers. If confirmed by future joint detections, fast X-ray transients could join kilonovae as standard signposts of mergers, helping telescopes and gravitational-wave detectors triangulate events across the sky.

Importantly, the researchers argue that this phenomenon may not be rare at all. Similar soft X-ray emission could accompany many more short gamma-ray bursts, but it may have escaped detection simply because previous missions lacked the capability to capture prompt emission below gamma-ray energies. In that sense, EP250704a may be the first clearly observed member of a population that has been hiding in plain sight, its soft X-ray glow too faint and too long-lived for gamma-ray monitors, and too prompt for repointing X-ray telescopes. Wide-field, sensitive soft X-ray monitoring changes the observational calculus entirely, potentially multiplying the number of merger events whose central engines can be studied directly.

The broader stakes of the finding extend to some of the deepest questions in astrophysics. Yin observed that the discovery extends our view of neutron star mergers beyond the brief gamma-ray flash, and that 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. Zhang added that the findings 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. As more wide-field X-ray monitors come online and gravitational-wave detectors grow more sensitive, events like EP250704a are likely to become a routine part of the astronomer’s toolkit, transforming short gamma-ray bursts from split-second flashes into extended laboratories for probing matter under the most extreme conditions nature allows.

Subject of Research: Soft X-ray emission from a neutron star merger associated with a short gamma-ray burst

Article Title: A hidden X-ray chapter of cosmic collisions comes to light

Article References: A hidden X-ray chapter of cosmic collisions comes to light. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Einstein Probe, short gamma-ray burst, neutron star merger, magnetar, soft X-ray transient, gravitational waves, multi-messenger astronomy, central engine, kilonova, SVOM, Insight-HXMT, equation of state