On July 4, 2025, the Einstein Probe observatory captured an unusually long X-ray flash designated EP250704a/GRB 250704B, which continued emitting soft X-rays for nearly ten minutes after a standard short gamma-ray burst. The discovery reveals a previously hidden phase of neutron star collisions, pointing to prolonged activity from a central merger remnant.
Catching the Hidden X-Ray Phase With the Einstein Probe
Short gamma-ray bursts usually flare for less than half a second before disappearing. Narrow-field X-ray telescopes historically struggled to catch these initial moments because they required a separate gamma-ray alert to locate the blast before repointing. The Einstein Probe avoids this delay by continuously monitoring a wide area of the sky at soft X-ray energies.
When the observatory caught EP250704a on July 4, 2025, onboard alerts triggered an immediate response. An Li, a PhD student at Beijing Normal University and the Transient Advocate for Einstein Probe, began preliminary analysis while on shift.
“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. “However, instead of fading away, the source continued emitting episodes of soft X-rays for nearly ten minutes.”
Measuring a Six-Billion-Year-Old Cosmic Journey
Following the initial detection, an international collaboration initiated a multiwavelength follow-up campaign spanning X-ray, optical, and radio observations. Eleonora Troja, a professor at Tor Vergata and co-author of the paper, worked alongside Passaleva to capture the fading glow. Passaleva monitored the event from a laptop while traveling on a train, racing against time to secure observation slots on the Very Large Telescope (VLT) in Chile.
The VLT’s X-Shooter instrument split the incoming light to reveal absorption lines of iron and magnesium. These lines yielded a measured redshift of z = 0.6610, indicating that the light traveled through space for more than six billion years. Observations with the FORS2 instrument confirmed the absence of an accompanying supernova, helping researchers rule out the collapse of a massive star.
“The coordinated observations across multiwavelengths were essential,” said Eleonora Troja. “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.”
Implications for Magnetar Remnants and Future Collisions
Bin-Bin Zhang of Nanjing University noted that conventional instruments like Swift’s Burst Alert Telescope would have missed the prolonged emission because its spectrum fell largely at softer X-ray energies below their detection thresholds. The timing and rapid variation of the signal instead pointed directly toward sustained activity from the central engine.
“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,” explained 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.
Since the first joint detection of electromagnetic radiation and gravitational waves from a neutron star merger in 2017, astronomers have sought definitive signatures to track post-collision behavior. This newly documented soft X-ray phase establishes an additional observational bridge linking fast electromagnetic transients directly to gravitational wave sources.
