Einstein Probe detects puzzling cosmic explosion

On 15 March 2024, Einstein Probe’s Wide-field X-ray Telescope (WXT) detected a burst of low-energy X-rays. Astronomers call such X-rays ‘soft’ even though they are still far more energetic than visible or ultraviolet light. The burst lasted for more than 17 minutes and fluctuated in brightness before fading away again. Such an event is known as a fast X-ray transient (FXRT) and this particular transient was given the designation EP240315a.

For Yuan Liu, National Astronomical Observatories, Chinese Academy of Sciences (NAO, CAS) and first author on the newly published paper detailing the study, it was a special moment because he had designed the onboard software trigger for WXT. “It was really good to see the algorithm working fine for this event,” he says.

Ancient explosion

About 1 hour after the X-rays were seen, a telescope situated in South Africa as part of the Asteroid Terrestrial-Impact Last Alert System (ATLAS) detected visible light from the same location. Follow-up observations from the Gemini-North telescope on Hawaii and the Very Large Telescope in Chile returned redshift measurements that confirmed that the burst had come from around 12.5 billion light-years away, beginning its cosmic journey to us when the Universe was just 10 percent its current age.

This meant EP240315a was the first time astronomers had detected soft X-rays for such a long duration from such an ancient explosion.

“The detection of EP240315a demonstrates Einstein Probe’s great potential for discovering transients from the early Universe. The mission will play an important role in international observations and collaborations,” says Xuefeng Wu, a researcher at the Purple Mountain Observatory, CAS, and one of the paper’s authors.

A mystery to be solved

The rapid detection of EP240315a also allowed the team to collaborate with Roberto Ricci, University of Rome Tor Vergata, Italy. They began watching the burst at radio wavelengths using the Australian Telescope Compact Array (ATCA). Monitoring it for three months, they established that the energy output was consistent with a typical gamma-ray burst (GRB).

GRBs are extremely powerful events that release extraordinary amounts of energy. Typically, long GRBs come from the explosion of massive stars.

In later analyses, the X-rays were indeed found to be coincident with a gamma-ray burst known as GRB 240315C. This burst had been seen by the Burst Alert Telescope (BAT) on NASA’s Neil Gehrels Swift Observatory and the Russian Federations’ 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. “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.”

Yet there is a mystery to be solved. Although GRBs are associated with X-rays, EP240315a is different.

Rethink our ideas of gamma-ray bursts

Usually, the X-rays are observed to 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,” says Hui Sun, a team member from the Einstein Probe Science Center at the NAO, CAS.

Combine this with the unexpectedly long duration of the X-rays and it could be telling us that we do not understand how GRBs 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, NAO, CAS Einstein Probe Principal Investigator.

Time and more data will help. Although past missions have been able to detect soft X-rays, Einstein Probe’s superior sensitivity and field of view really opens this 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.

“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,” says Erik Kuulkers, ESA Einstein Probe Project Scientist.

About Einstein Probe

Einstein Probe (EP) is a mission of the Chinese Academy of Science (CAS) working in partnership with the European Space Agency (ESA), the Max-Planck-Institute for extraterrestrial Physics (MPE), Germany, and the Centre National d’Études Spatiales (CNES), France. It was launched from the Xichang Satellite Launch Centre in China on 9 January 2024, and carries two instruments. The Wide-field X-ray Telescope (WXT) constantly monitors a large portion of the sky for unexpected X-rays, and the Follow-up X-ray Telescope (FXT) that homes in on the X-ray sources found by WXT for a more detailed look.

Notes for editors

‘Soft X-ray prompt emission from the high-redshift gamma-1ray burst EP240315a’ by Liu et al. is published today in Nature Astronomy. https://www.nature.com/articles/s41550-024-02449-8

‘Long-term radio monitoring of the fast X-ray transient EP240315a: evidence for a relativistic jet’ by Ricci et al. is published today in The Astrophysical Journal Letters. https://iopscience.iop.org/article/10.3847/2041-8213/ad8b3

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