Einstein Probe is a small orbiting telescope that operates in the X-ray band and has an extremely wide field of view. It recently picked up high-energy particles from a burst that occurred on the other side of the Universe.

X-ray burst
On 15 March 2024, Einstein Probe’s Wide Field X-ray Telescope (WXT) detected a burst of low-energy X-ray emission. Astronomers call such X-rays “soft,” although they are still much more energetic than visible or ultraviolet light. The burst lasted more than 17 minutes and fluctuated in brightness before fading out again. Such an event is known as a fast X-ray transient (FXRT), and this particular transient has been designated EP240315a.
For Yuan Liu of the National Astronomical Observatory of the Chinese Academy of Sciences (NAO, CAS) and first author of the recently published paper detailing the study, this was a special moment as he developed the software trigger for WXT. “It was really good to see the algorithm working fine for this event,” he says. The study is published in the journal Nature Astronomy.
Ancient Explosion
About an hour after the X-rays were detected, a telescope located in South Africa as part of the Asteroid Terrestrial-Impact Last Alert System (ATLAS) detected visible light from the same location. Further observations with the Gemini-North telescope in Hawaii and the Very Large Telescope in Chile measured the redshift, which confirmed that the burst occurred about 12.5 billion light years away, beginning its cosmic journey to us when the Universe was only 10 per cent younger than its current age.
This means that EP240315a is the first time astronomers have detected soft X-ray emission of such a long duration from such an ancient explosion.
A mystery to solve
The rapid discovery of EP240315a also allowed the team to collaborate with Roberto Ricci of the University of Rome Tor Vergata, Italy. They began observing the burst in the radio wavelength range with the Australian Telescope Compact Array (ATCA). After observing it for three months, they found that the energy output corresponded to a typical gamma-ray burst (GRB).
Gamma-ray bursts are extremely powerful flashes that release huge amounts of energy. Long GRBs usually result from the explosion of massive stars.
Later analyses showed that the X-ray emission really coincided with the gamma ray burst known as GRB 240315C. This burst was detected by the Burst Alert Telescope (BAT) at NASA’s Neil Gehrels Swift Observatory and by the Russian Konus instrument on NASA’s Wind spacecraft.
“These results show that a substantial fraction of FXRTs may be associated with GRBs and that sensitive X-ray monitors, such as Einstein Probe can pinpoint them in the distant universe,” says Roberto. But there’s another mystery to solve. Although GRBs are associated with X-rays, EP240315a is different.
Rethinking our ideas about gamma-ray bursts
X-rays usually precede gamma rays by a few tens of seconds, but EP240315a was seen more than six minutes (372 seconds) before GRB 240315C. Combine this with the unexpectedly long duration of X-rays, and it may indicate that we don’t understand how supernovae galaxies explode as well as we thought.
“This tells us something really new and maybe we have to rethink the models we have for gamma-ray bursts,” says Weimin Yuan, principal investigator of the CAS Einstein Probe, NAO.
Time and more data will help. While previous missions have managed to detect soft X-rays, the Einstein Probe’s superior sensitivity and field of view really opens that window. “This is just the starting point and really demonstrates the potential of Einstein Probe to detect cosmic explosions from the early universe,” says Weimin.
About Einstein Probe
Einstein Probe (EP) is a mission of the Chinese Academy of Sciences (CAS) working in cooperation with the European Space Agency (ESA), the Max Planck Institute for Extraterrestrial Physics (MPE), Germany, and the Centre National d’Etudes Spatiales (CNES), France.
It was launched from Xichang Space Center in China on January 9, 2024, and it carries two instruments on board. The Wide Field X-ray Telescope (WXT) continuously monitors a large part of the sky for unexpected X-ray emission, while the X-ray Telescope (FXT) focuses on X-ray sources found by the WXT for more detailed study.
According to phys.org