On March 15, 2024, a space-based observatory detected bursts of low-energy X-rays from deep in the ancient universe, fluctuating in brightness for over 17 minutes before fading away. About an hour later, ground-based telescopes picked up visible light from the same source, tracing it back to when the universe was only around one billion years old.
The newly launched Einstein Probe is already spotting distant explosions with the potential to upend what we know about the universe’s earliest years. Using its wide-field X-ray telescope, the spacecraft detected so-called soft, or low-energy, X-rays, that lasted for an unusually long period of time. These events are known as fast X-ray transients (FXRT), and the newly spotted blast has been designated EP240315a.
After monitoring the burst at radio wavelengths for a period of three months, the team of astronomers behind the detection confirmed that the energy output was consistent with a gamma-ray burst dating back to when the universe was only 10 percent of its current age. So given the universe’s estimated age of 13.8 billion years, the burst occurred when the universe was approximately 1.38 billion years old.
“These results show that a substantial fraction of FXRTs may be associated with [gamma-ray bursts] and that sensitive X-ray monitors, such as Einstein Probe can pinpoint them in the distant Universe,” Roberto Ricci, a researcher at the University of Rome Tor Vergata, Italy, and one of the authors behind the new paper detailing the discovery, said in a statement. “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.”
EP240315a marks the first time soft X-rays were detected from an ancient explosion that lasted for such a long duration. Follow-up observations using the Gemini-North telescope in Hawaii and the Very Large Telescope in Chile measured visible light from the same location that confirmed the burst had come from around 12.5 billion light-years away.
Gamma-ray bursts are brief flashes of high-energy light and the most powerful explosions in the universe, typically triggered by the collapse of massive stars or the merger of neutron stars. Those bursts are also known to emit copious amounts of X-rays. The recently discovered soft X-rays were traced back to GRB 240315C, a gamma-ray burst first detected by the Burst Alert Telescope (BAT) on NASA’s Neil Gehrels Swift Observatory, with additional data provided by the Konus instrument aboard NASA’s Wind spacecraft.
Although gamma-ray bursts are associated with X-rays, the newly discovered fast X-ray transient is an anomaly. The X-rays typically precede the gamma rays by a few tens of seconds, but EP240315a was seen more than six minutes (372 seconds) before GRB 240315C. “Such a long delay has never been previously observed,” Hui Sun, a team member from the Einstein Probe Science Center at the National Astronomical Observatories, Chinese Academy of Sciences, and co-author of the new study, said in a statement.
The mystery behind the long duration of time between the X-rays and the gamma-ray burst, in addition to the long duration of the X-rays themselves, are reason to question whether gamma-ray bursts explode the way scientists believe them to.
The Einstein Probe, an X-ray telescope managed by the Chinese Academy of Sciences, and built in collaboration with European Space Agency and the Max Planck Institute for Extraterrestrial Physics, launched on January 9, 2024. Its Wide-field X-ray Telescope takes in X-ray light in square tubes in a grid, which means it can observe 3,600 square degrees (just under one-tenth of the celestial sphere) in a single shot.
“As soon as we opened the eyes of Einstein Probe to the sky, it found interesting new phenomena,” Erik Kuulkers, ESA’s Einstein Probe project scientist, said in a statement. “That’s pretty good and should mean that there are a lot more interesting discoveries to come.”
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