On March 15, 2024, the Einstein Probe, a cutting-edge space mission led by the Chinese Academy of Sciences (CAS), detected a rare and powerful burst of low-energy X-rays using its Wide-field X-ray Telescope (WXT). This burst, lasting for more than 17 minutes, exhibited fluctuations in brightness before it faded. Astronomers classify such bursts as fast X-ray transients (FXRTs). This particular event, designated EP240315a, has opened up new avenues for exploring the mysteries of the universe, offering a glimpse into phenomena previously uncharted.
For Yuan Liu, a researcher at the National Astronomical Observatories, CAS (NAO, CAS), and the first author of the study, this discovery held personal significance. Liu had designed the onboard software that triggered the detection algorithm for WXT, and seeing it successfully detect EP240315a was a proud moment. The study detailing the event was published in Nature Astronomy, marking a significant contribution to the field of astrophysics.
A Momentous Discovery from the Early Universe
The detection of EP240315a is not only scientifically significant due to its nature as a fast X-ray transient, but it is also remarkable because of its extraordinary distance from Earth. Approximately one hour after the initial X-ray detection, a telescope from the Asteroid Terrestrial-Impact Last Alert System (ATLAS) in South Africa spotted visible light from the same source. Subsequent follow-up observations were conducted using some of the most powerful telescopes in the world: the Gemini-North Telescope in Hawaii and the Very Large Telescope in Chile. The results from these observations were astounding.
The redshift measurements confirmed that EP240315a originated from 12.5 billion light-years away, meaning it began its cosmic journey to Earth when the universe was just about 10% of its current age. This finding makes EP240315a the first instance of detecting soft X-rays emitted from such an ancient cosmic explosion. Soft X-rays are highly energetic compared to visible or ultraviolet light but are considered “low-energy” in the context of high-energy astronomical phenomena.
As Xuefeng Wu, a researcher at the Purple Mountain Observatory, CAS, and one of the co-authors of the study, points out, the discovery highlights the Einstein Probe’s potential in uncovering transients from the early universe. This mission, Wu suggests, will play an essential role in international observations and collaborations going forward, offering new insights into the most mysterious and distant phenomena in space.
The Nature of Fast X-ray Transients (FXRTs)
A key feature of EP240315a is that it falls under the category of fast X-ray transients (FXRTs), a type of event that is difficult to study due to their brief and unpredictable nature. FXRTs are fleeting bursts of X-rays that can last from milliseconds to several minutes, making them challenging to detect and study in real-time. The ability of the Einstein Probe to quickly identify such events speaks to the advanced capabilities of the mission.
The Einstein Probe’s Wide-field X-ray Telescope (WXT) is designed to monitor a large portion of the sky, constantly scanning for unexpected X-ray transients. This mission’s sensitivity allows it to detect even the faintest and most elusive cosmic phenomena, such as EP240315a, that other telescopes might miss.
The discovery of this burst opens up the possibility of learning more about the processes behind these fast X-ray transients, potentially reshaping our understanding of these cosmic explosions and their connection to other high-energy astrophysical events, such as gamma-ray bursts (GRBs).
Collaboration and Follow-up Observations
The rapid detection of EP240315a led to an immediate collaboration with international researchers, including Roberto Ricci from the University of Rome Tor Vergata, Italy. Ricci and his team utilized the Australian Telescope Compact Array (ATCA) to observe the burst at radio wavelengths. This extended monitoring, which lasted for three months, helped establish that the energy output from EP240315a was consistent with that of a typical gamma-ray burst (GRB).
Gamma-ray bursts (GRBs) are among the most energetic and powerful explosions observed in the universe. They are typically associated with the death throes of massive stars or the merging of compact objects such as black holes or neutron stars. The combination of X-ray and radio observations provided a unique window into the burst’s properties, allowing astronomers to gain further insights into the nature of the explosion.
Interestingly, the X-ray emission from EP240315a was found to be coincident with GRB 240315C, a gamma-ray burst previously detected by instruments aboard NASA’s Neil Gehrels Swift Observatory and Wind spacecraft. This discovery reinforces the idea that a significant portion of FXRTs could be associated with gamma-ray bursts, offering a new perspective on how these two types of high-energy cosmic events may be linked.
The Mystery: A Long Delay between X-rays and Gamma Rays
Despite the clear connection between EP240315a and GRB 240315C, there is an unusual twist to the story. One of the defining characteristics of gamma-ray bursts is that their X-ray emissions usually precede the gamma rays by a few tens of seconds. However, in the case of EP240315a, the X-ray emission was observed more than six minutes (372 seconds) before the gamma-ray burst, which is an unprecedented delay. This long gap between the two types of emissions has never been observed in previous GRBs, posing an intriguing mystery for scientists.
As Hui Sun, a team member from the Einstein Probe Science Center, points out, such a significant delay challenges current understanding of the sequence of events in gamma-ray bursts. “Such a long delay has never been previously observed,” Sun says, suggesting that GRB models may need to be reconsidered to account for this unexpected behavior. It could indicate that the mechanisms driving gamma-ray bursts are far more complex than previously thought, or that there are additional processes at play in the explosion dynamics.
Reevaluating the Models of Gamma-Ray Bursts
The unusual delay between the X-ray and gamma-ray emissions could be a sign that scientists need to rethink their models for gamma-ray bursts. Weimin Yuan, the Principal Investigator for the Einstein Probe, emphasized that this discovery may require a fundamental shift in how astronomers approach the study of GRBs. The unexpectedly long duration of the X-ray emission, combined with the extended delay before the gamma rays appeared, suggests that new models of GRB explosions may be necessary to fully explain these phenomena.
The combination of X-ray and radio observations of EP240315a also highlights the importance of using multiple wavelengths to study cosmic explosions. This multi-wavelength approach allows scientists to examine different aspects of the event, providing a more comprehensive understanding of the explosion’s energy, duration, and potential progenitor.
The Role of Einstein Probe in Future Discoveries
The discovery of EP240315a represents just the beginning of what the Einstein Probe mission hopes to achieve. The superior sensitivity and field of view of its instruments position it to make significant contributions to the study of cosmic explosions. As Erik Kuulkers, ESA Einstein Probe Project Scientist, states, “As soon as we opened the eyes of Einstein Probe to the sky, it found interesting new phenomena. That’s pretty good and should mean that there are a lot more interesting discoveries to come.”
The Einstein Probe has the potential to become a vital tool in the study of high-energy astrophysics, uncovering rare and previously undetected events in the distant universe. Its ability to detect fast X-ray transients and gamma-ray bursts, combined with follow-up observations in radio and other wavelengths, will continue to offer valuable insights into the dynamics of cosmic explosions, the structure of the universe, and the nature of the early cosmos.
About Einstein Probe
The Einstein Probe (EP) mission, developed by the Chinese Academy of Sciences (CAS), in collaboration with the European Space Agency (ESA), Max-Planck-Institute for Extraterrestrial Physics (MPE), and the Centre National d’Études Spatiales (CNES), was launched on January 9, 2024, from the Xichang Satellite Launch Center in China. The mission carries two primary instruments: the Wide-field X-ray Telescope (WXT), which constantly monitors large portions of the sky for unexpected X-ray bursts, and the Follow-up X-ray Telescope (FXT), which zooms in on the sources of these X-rays for detailed analysis.
With the success of its first major detection, Einstein Probe promises to be a revolutionary tool in the field of high-energy astrophysics, offering new insights into some of the most extreme and distant phenomena in the universe. The continued success of the mission will undoubtedly lead to more groundbreaking discoveries that could redefine our understanding of the cosmos.
Reference: Y. Liu et al, Soft X-ray prompt emission from the high-redshift gamma-ray burst EP240315a, Nature Astronomy (2025). DOI: 10.1038/s41550-024-02449-8. www.nature.com/articles/s41550-024-02449-8