Einstein Probe detects longest-lasting x-ray flash from merger

Astronomers have recorded the longest-lasting prompt X-ray flash ever detected from a neutron star merger, capturing an explosion six billion light-years away that continued emitting X-rays for nearly ten minutes after a half-second burst of gamma rays. According to a study published in Science Bulletin, the event was simultaneously detected by three spacecraft, including China’s Einstein Probe satellite, providing new data on the central engines that drive these cosmic collisions.

The event received two designations: EP250704a for its X-ray signature and GRB 250704B for its gamma-ray emission. Most X-ray telescopes rely on narrow fields of view that require a cue from a gamma-ray detector before turning toward a transient event, which usually misses the earliest X-ray stages. The Einstein Probe utilized its lobster-eye optics to continuously monitor a wide swath of the sky, allowing it to capture the transition as it happened.

An Li, a doctoral student at Beijing Normal University and duty transient manager for the mission, stated that the event initially appeared to be a standard short gamma-ray burst lasting less than half a second. Instead of fading away, the source continued emitting episodes of soft X-rays for nearly ten minutes. The gamma-ray burst lasted approximately 0.4 seconds, while the soft X-ray emission endured for about 560 seconds, stretching roughly 1,400 times longer than the initial flash.

Niccolò Passaleva, a doctoral student working with Eleonora Troja at the University of Rome Tor Vergata, initiated observations of the fading glow while commuting on a train. Within minutes, his remote commands directed the European Southern Observatory’s Very Large Telescope in Chile toward the coordinates of the blast.

The facility’s X-Shooter spectrograph split the incoming light into component colors, revealing absorption lines that established a redshift of 0.661. This measurement placed the cosmic collision more than six billion light-years away, meaning the event occurred before the formation of the Solar System. To rule out a collapsing massive star, the team used the VLT’s FORS2 camera to search deep images of the host galaxy for an accompanying supernova, finding no evidence of one.

The observed X-ray behavior defied standard textbook predictions for compact object mergers, which typically feature a hard spike of falling matter followed by a smoothly fading afterglow. In contrast, the emission flickered and shifted in spectrum across several minutes, indicating that the central engine remained active long after the gamma-ray burst ceased.

Yi-Han Iris Yin, a doctoral student at the University of Hong Kong who led the high-energy analysis, explained that the most plausible explanation involves the creation of a rapidly rotating, highly magnetized neutron star known as a magnetar. Previous studies suggested that roughly 30 percent of short gamma-ray bursts might conceal a similar motor-driven X-ray phase lasting around 100 seconds, but EP250704a lasted more than five times longer than those theoretical estimates.

How far away was the EP250704a neutron star merger?

The cosmic collision occurred more than six billion light-years away, determined by spectral measurements showing a redshift of 0.661 taken by the Very Large Telescope in Chile.

Which spacecraft detected the gamma-ray burst and X-ray flash?

Three spacecraft recorded the event simultaneously: China’s Einstein Probe observed the soft X-rays with its wide-field lobster-eye optics, while the Sino-French SVOM mission and China’s Insight-HXMT observatory registered the initial gamma-ray burst.

Why did this merger last longer than previous observations?

While the initial gamma-ray burst lasted only 0.4 seconds, the resulting magnetar continued pumping energy into the remnant, driving an extended soft X-ray emission that lasted for approximately 560 seconds.

Neutron star mergers are known sites where the universe forges heavy elements such as gold and platinum while generating gravitational waves detectable on Earth.

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