Einstein Probe detects puzzling cosmic explosion
Einstein Probe has opened a new window onto the distant X-ray Universe, promising us new views of the most faraway explosions in the cosmos. Less than three months after it launched, the spacecraft already discovered a puzzling blast of X-rays that could require us to change the way we explain the extraordinary explosions known as gamma-ray bursts.
In-depth
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.