Einstein probe detects mysterious burst of X-rays from the early universe

On March 15, 2024 space observatory detected bursts of low-energy X-rays from the depths of the ancient universe, varying in brightness for more than 17 minutes before disappearing. About an hour later, ground-based telescopes picked up visible light from the same source, tracing it back to when the universe was only about a billion years old.

The the newly launched Einstein probe is already spotting distant explosions with the potential to overturn 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 (FXRTs), and the newly observed burst is designated EP240315a.

After monitoring the burst at wavelengths over a three-month period, the team of astronomers behind the discovery 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 approximate age of the universe at 13.8 billion years, the explosion occurred when the universe was roughly 1.38 billion years old.

“These results show that a significant fraction of FXRT can be associated with (gamma-ray bursts) and that sensitive X-ray monitors such as the Einstein Probe can detect 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 with details of the discovery, said in a statement. “Combining the power of X-rays and radio observations gives us a new way to study these ancient explosions, even without detecting their gamma rays.”

EP240315a marks the first time soft X-rays have been detected from an ancient explosion that lasted this long. Follow-up observations using the Gemini-North telescope in Hawaii and the Very Large Telescope in Chile measured visible light from the same location, confirming that the burst came from about 12.5 billion light-years away.

Gamma-ray bursts are brief flashes of high-energy light and the most powerful explosions in the universe, usually caused by the collapse of massive stars or the merger of neutron stars. These bursts are also known to emit copious amounts of X-rays. The newly detected 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 the Wind spacecraft of NASA.

Although gamma-ray bursts are associated with X-rays, the newly discovered fast X-ray transition is an anomaly. 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. “Never before has such a long delay been observed,” Hui Sun, a team member from the Einstein Probe Science Center at the National Astronomical Observatories, Chinese Academy of Sciences and a co-author of the new study, said in a statement.

The mystery behind the long duration of time between X-rays and gamma rays, in addition to the long duration of the X-rays themselves, is reason to doubt whether gamma rays explode the way scientists believe.

The Einstein probe, an X-ray telescope operated by the Chinese Academy of Sciences and created in collaboration with the European Space Agency and the Max Planck Institute for Extraterrestrial Physics, was launched on January 9, 2024. Its wide-field X-ray telescope takes in X-ray light in square tubes in a grid, meaning it can observe 3,600 square degrees (just under a tenth of a celestial sphere) in a single shot.

“As soon as we opened the Einstein probe’s eyes to the sky, it discovered interesting new phenomena,” Eric Kulkers, ESA’s Einstein Probe project scientist, said in a statement. “That’s pretty good and should mean there are many more interesting discoveries to come.”