Einstein Probe detects soft X-ray phase after neutron star merger

The Einstein Probe detected soft X-ray emission lasting nearly ten minutes after a short gamma-ray burst faded, indicating extended central engine activity. The event, named EP250704a in X-rays and GRB 250704B in gamma rays, occurred on July 4, 2025. The initial gamma-ray flash lasted about 0.4 seconds, but soft X-rays continued for about 562 seconds afterward. This prolonged X-ray emission carried substantial energy but had a soft spectrum below the detection threshold of conventional gamma-ray instruments.

Einstein Probe detected the event from its onset and observed several episodes of soft X-ray emission after the gamma-ray signal disappeared. The rapid variability and changing spectrum of these X-rays indicate ongoing energy release from the central engine. The source remained active long after the gamma-ray burst.

Origin and interpretation of the emission

The prolonged X-ray activity is linked to the merger remnant, likely a rapidly spinning, highly magnetized neutron star known as a magnetar, not the blast wave. Researchers favor this model to explain the extended emission. The discovery shows that short gamma-ray bursts may conceal longer, richer activity at soft X-ray energies. It provides a new electromagnetic counterpart to gravitational-wave signals from neutron star mergers.

Such mergers generate gravitational waves and energetic explosions. The findings support the idea that fast X-ray transients are electromagnetic counterparts to gravitational-wave sources from compact object mergers. This may provide another way to study gravitational-wave sources. Prolonged soft X-ray emission could be more common in compact-object mergers than previously recognised.

Multiwavelength confirmation and host galaxy

Follow-up observations across X-ray, optical, and radio bands identified the host galaxy, measured distance, ruled out a supernova, and confirmed the emission originated from a compact object merger. EP-WXT detected X-rays during the event, while SVOM-GRM and Insight-HXMT detected gamma rays simultaneously.

Follow-up observations measured a redshift of 0.661 for the event. No accompanying supernova was found. SVOM is a China-France joint mission led by CNSA, CNES, and CAS. Insight-HXMT is China's first space X-ray telescope, jointly funded by CNSA and CAS.

Research team and publication

The study was led by an international team including researchers from The University of Hong Kong, Nanjing University, University of Rome 'Tor Vergata', and the Chinese Academy of Sciences. It was published as a cover article in Science Bulletin on August 10, 2026. The paper's title is 'Minutes-long soft X-ray prompt emission from a compact object merger'.

Yi-Han Iris Yin, a PhD student at The University of Hong Kong, is a co-corresponding author and led the analysis of the high-energy prompt emission. Professor Bin-Bin Zhang of Nanjing University worked with the international team and is a corresponding author. An Li, a PhD student at Beijing Normal University, led the international team as the Transient Advocate for Einstein Probe. Professor Bing Zhang, Founding Director of the Hong Kong Institute of Astronomy and Astrophysics, co-authored the study and contributed to the physical interpretation.

Einstein Probe is a space mission led by the Chinese Academy of Sciences with international partners including ESA, MPE, and CNES.

Implications for future observations

The finding suggests that similar soft X-ray phases may have been missed in past short gamma-ray bursts due to instrumental limitations. Einstein Probe monitors a wide area of the sky at soft X-ray energies, avoiding delays from gamma-ray alerts. Most other X-ray telescopes can only observe after a gamma-ray detector locates the burst, potentially missing this early X-ray emission. The research contributes to multi-messenger astronomy by linking gravitational waves and electromagnetic radiation.

The paper was published in Science Bulletin, 2026; 71 (18): 4657, with DOI 10.1016/j.scib.2026.08.021.