Short gamma-ray bursts, or short GRBs, are believed to occur when two compact objects, such as neutron stars, spiral toward one another before colliding and merging. These extraordinarily violent events produce gravitational waves and rank among the most powerful explosions in the Universe. For decades, astronomers have mainly detected these events through their gamma-ray flashes.
Their earliest soft X-ray emission has been far more difficult to observe because most narrow-field X-ray telescopes depend on gamma-ray alerts to locate a burst before turning toward it. Einstein Probe's wide-field soft X-ray monitoring has now exposed this previously unseen stage, giving astronomers a direct look at the opening moments of a short GRB. A Half-Second Flash With a Much Longer Aftermath The event, designated EP250704a/GRB 250704B, caught researchers by surprise on July 4, 2025.
An Li, a PhD student at Beijing Normal University and the Transient Advocate for EP, was on duty when the signal appeared. "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. After receiving the onboard alerts from EP, Li quickly began the initial analysis.
What happened next was unexpected. "However, instead of fading away, the source continued emitting episodes of soft X-rays for nearly ten minutes." The extended X-ray activity was energetic, but it occurred at wavelengths that conventional gamma-ray instruments would have struggled to detect. "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." Tracking the Explosion Across the Spectrum To determine what had produced the unusual signal, researchers organized an extensive international follow-up campaign using observations ranging from X-ray and optical wavelengths to radio. "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." The combined observations helped establish that the unusual X-ray activity was connected to a compact object merger rather than another type of stellar explosion. Evidence for a Long-Lived Central Engine A closer examination showed that the prolonged X-ray emission was being powered directly by the remnant left behind by the merger, rather than by the outward-moving 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.
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