Einstein Probe Captures a Hidden Soft X-Ray Phase in Event EP250704a
Astronomers monitoring the cosmos on July 4, 2025, recorded a transformative event designated EP250704a, associated with the gamma-ray burst GRB 250704B. Initially appearing as an ordinary short gamma-ray burst with a bright flash lasting less than half a second, the source surprised researchers by emitting prolonged episodes of soft X-rays for nearly ten minutes. This discovery exposes a previously hidden phase of neutron star collisions that decades of narrow-field telescopes had systematically missed.
Short gamma-ray bursts rank among the most powerful explosions in the universe, typically occurring when two compact objects like neutron stars spiral inward and merge. These violent collisions generate gravitational waves alongside energetic electromagnetic radiation. Historically, astronomers detected these events primarily through their initial gamma-ray flashes. Narrow-field X-ray telescopes relied on gamma-ray alerts to locate a burst before turning toward the source, a delay that allowed the earliest and softest X-ray emissions to slip past undetected.
Wide-Field Monitoring Defeats Traditional Observatory Blind Spots
The Einstein Probe changed this dynamic by maintaining wide-field soft X-ray monitoring across the sky. When event EP250704a unfolded, the observatory caught the prompt emission without waiting for a secondary cue. An Li, a PhD student at Beijing Normal University and Transient Advocate for the Einstein Probe, was on duty during the alert. “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 X-rays by EP-WXT,” Li stated. Instead of fading instantly, the source sustained soft X-ray emissions for close to ten minutes.
This extended emission carried substantial energy. However, its spectrum was exceptionally soft. For a cosmic event situated at a typical cosmological distance, conventional gamma-ray instruments like the Swift Burst Alert Telescope would have remained below the detection threshold. Professor Bin-Bin Zhang of Nanjing University, a co-corresponding author of the study published in Science Bulletin, emphasized that earlier missions recorded only the brief gamma-ray flash while missing the prolonged activity entirely. “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,” Zhang explained.
Multiwavelength Follow-Up Campaigns Rule Out Supernovae
To verify the physical origin of the signal, an international team organized a coordinated follow-up campaign spanning X-ray, optical, and radio wavelengths. Professor Eleonora Troja of the University of Rome Tor Vergata led the spectroscopic analysis group that acquired crucial redshift data. “The coordinated observations across multiwavelengths were essential,” Troja noted. These observations allowed researchers to identify the host galaxy, measure its distance, and rule out an accompanying supernova. This exclusion provided strong evidence linking the X-ray emission directly to a compact object merger.
Further analysis of the high-energy emission was led by Yi-Han Iris Yin, a PhD student at the Department of Physics and the Hong Kong Institute of Astronomy and Astrophysics at The University of Hong Kong. Yin observed that the rapidly changing brightness, evolving spectrum, and subsequent afterglows pointed to continuous activity from a central engine long after the primary short gamma-ray burst had vanished. “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,” Yin stated.
Implications for Multi-Messenger Astronomy and Magnetar Engines
This discovery provides a new electromagnetic counterpart for gravitational-wave astronomy, expanding our observational toolkit following the joint detection of gravitational waves and electromagnetic radiation from merging neutron stars in 2017. Researchers suggest that this extended soft X-ray activity may not be unusual for short GRBs, but previous technological limitations obscured it. By exposing this hidden phase, the Einstein Probe grants astronomers a direct probe into the remnants of stellar mergers.
As international teams continue processing multi-messenger data, the Einstein Probe solidifies its role in uncovering transient phenomena that bridge gravitational-wave physics with high-energy astrophysics.
