On March 15, 2024, the spacecraft's Wide-field X-ray Telescope (WXT) captured a burst of low-energy X-rays - termed "soft" X-rays despite their high energy compared to visible light. This phenomenon, classified as a fast X-ray transient (FXRT) and named EP240315a, persisted for over 17 minutes with fluctuating brightness before fading.
For Yuan Liu of the National Astronomical Observatories, Chinese Academy of Sciences (NAO, CAS), and lead author of the study, this detection was particularly meaningful. "It was really good to see the algorithm working fine for this event," he noted, referencing the onboard software trigger he designed for WXT.
A relic from the early Universe
Approximately one hour after the X-ray detection, South Africa's Asteroid Terrestrial-Impact Last Alert System (ATLAS) telescope recorded visible light from the same location. Follow-up observations by the Gemini-North telescope in Hawaii and the Very Large Telescope in Chile revealed redshift measurements, pinpointing the event at a distance of 12.5 billion light-years. This indicated that EP240315a originated when the Universe was just 10% of its current age, marking it as the longest-duration soft X-ray emission ever observed from such a distant explosion.
"The detection of EP240315a demonstrates Einstein Probe's great potential for discovering transients from the early Universe," said Xuefeng Wu, a researcher at the Purple Mountain Observatory, CAS, and co-author of the study. "The mission will play an important role in international observations and collaborations."
Deciphering a cosmic mystery
The swift identification of EP240315a enabled collaboration with Roberto Ricci of the University of Rome Tor Vergata, Italy. Using the Australian Telescope Compact Array (ATCA), researchers monitored the event at radio wavelengths for three months, confirming its energy output matched that of a typical GRB. GRBs, known for their immense energy release, often result from the collapse of massive stars.
Further analysis linked the X-ray transient to GRB 240315C, detected by NASA's Neil Gehrels Swift Observatory and the Russian Federation's 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," Ricci commented. "Combining the power of X-ray and radio observations hands us a new way to explore these ancient explosions even without detecting their gamma rays."
However, EP240315a's unique characteristics challenge existing GRB models. While X-rays generally precede gamma rays by seconds, the X-rays from EP240315a appeared over six minutes before GRB 240315C. "Such a long delay has never been previously observed," said Hui Sun, a team member from the Einstein Probe Science Center, NAO, CAS.
Reevaluating gamma-ray burst theories
The unusual duration and timing of EP240315a's X-rays suggest gaps in our understanding of GRBs. "This tells us something really new and maybe we have to rethink the models we have for gamma-ray bursts," said Weimin Yuan, Principal Investigator for Einstein Probe at NAO, CAS.
Future data may shed light on these mysteries. With its advanced sensitivity and wide field of view, Einstein Probe promises to expand our knowledge of cosmic explosions. "This is just the starting point and really demonstrates the potential of Einstein Probe to detect cosmic explosions from the early Universe," Weimin added.
"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," said Erik Kuulkers, ESA Einstein Probe Project Scientist.
Research Report:Soft X-ray prompt emission from the high-redshift gamma-ray burst EP240315a
Research Report:Long-term radio monitoring of the fast X-ray transient EP240315a: evidence for a relativistic jet
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