High-energy explosions occur again and again in the cosmos, producing short-term bright gamma and X-ray bursts. But astronomers do not know all the causes of these phenomena. The so-called fast X-ray transients (almost X-ray transients) have been causing a lot of guesswork for a good two years now. These are intense bursts of soft X-rays that can last up to ten minutes.
These X-ray bursts were only discovered by the Chinese-European space telescope “Einstein Probe”, which was launched in January 2024. This specializes in capturing short-lived events in the soft X-ray range – and has already detected hundreds of such X-ray flashes. Some of these could be traced back to supernovae of massive stars, but others remained unexplained.
First a gamma burst, then an X-ray explosion
Now a seemingly completely different celestial event has revealed the secret behind these slow X-ray flashes. On July 4, 2025, several gamma-ray satellites detected the brief but intense flash of a gamma-ray burst – the gamma-ray burst lasting less than half a second. “This event appeared to be a typical short gamma ray burst,” explains first author An Li from the Normal University in Beijing.
But contrary to what was expected, the gamma ray burst called GRB 250704B had an aftermath: When Li looked at the data recorded at the same time from the Einstein sample, he discovered a signal there too: The source of the gamma ray flash repeatedly released bursts of soft X-ray radiation in the energy range between 0.5 and 4 kiloelectron volts for almost ten minutes – as is typical for fast X-ray t-transients.
On July 4, 2025, the gamma-ray satellites SVOM and HXMT detected a short gamma-ray burst (blue). The Einstein sample recorded soft X-rays (red) coming from the same source. — © Yi-Han Iris Yin/University of Hong Kong
Collision instead of collapse
Could both phenomena be related? To clarify this, Li and his team asked colleagues at other telescopes to follow what was happening in wavelengths from the UV range through visible light and infrared to the radio range. “These coordinated observations were crucial,” explains co-author Eleonora Troja from the University of Rome. The data revealed that the energetic burst occurred in a galaxy around six billion light-years away.
In search of a cause, astronomers then looked for evidence of a supernova in this galaxy – but they found none. “Our optical limits exclude a wide range of supernovae, making a stellar core collapse unlikely as the origin,” report Li and his colleagues. Instead, they identified evidence of a neutron star collision in the spectrum of the radiation burst. Such a merger between two ultra-dense star relics releases enormous amounts of energy – including in the form of gravitational waves and gamma rays.
Neutron star collisions release enormous amounts of energy – and in some cases also cause fast X-ray transients. — © NSF/LIGO/ Sonoma State University, A. Simonnet, CC by 4.0
Magnetar as the author?
Now it has been shown for the first time that such neutron star collisions can also cause X-ray transients – longer-lasting bursts of soft X-rays that go beyond a mere afterglow. “Such a connection between short gamma ray bursts and long-lasting X-ray transients has already been suspected, but has never been proven before,” explain the astronomers. Now this has been achieved for the first time.
The observations also provide initial evidence as to why only some neutron star collisions result in such soft X-ray bursts: “A plausible explanation would be that the merger created a rapidly rotating, strongly magnetized neutron star – a so-called magnetar,” says co-author Yi-Han Iris Yin from the University of Hong Kong. The strong magnetic fields of such a rapidly rotating neutron star release additional energy and radiation.
“Expands our view of neutron star collisions”
The radiation burst on July 4, 2025 not only revealed the causes of the persistent X-ray flashes, it also provides new insights into the processes involved in the merger of neutron stars. “This discovery expands our view of neutron star collisions beyond the brief gamma-ray burst – and reveals a previously hidden phase of soft X-rays,” says Yin. So far it has only been partially clarified what happens during these mergers and what radiation is released.
At the same time, the current event also reveals why the connection between short gamma ray bursts and X-ray transients was not recognized earlier. “Although this long-lasting X-ray emission carried considerable energy, its spectrum was so soft that it remains below the detection threshold of common gamma ray detectors,” explains co-author Bin-Bin Zhang from Nanjing University. “As a result, only the short gamma-ray burst was detected, but not the aftermath in the X-ray range.” Only the Einstein sample, which specializes in soft X-rays, made this visible.
Source: An Li (Beijing Normal University, China) et al., Science Bulletin, 2026; doi: 10.1016/j.scib.2026.08.021