A new kind of stellar explosion provides a novel way to probe the distant Universe, according to new results published today in Nature Astronomy.
Researchers from Lancaster University joined a global team, led by Radboud University, Netherlands, examining 'Fast X-ray transients (FXTs)' which are highly mysterious outbursts of X-ray radiation, typically lasting for just a few minutes.
"We've known that these unique explosions exist for some time, but it is only now, thanks to the new Einstein Probe mission, that we can pinpoint them in near real time", says astronomer Professor Peter Jonker, of Radboud University, Netherlands, discoverer of one of the first such events, and second author of today's paper.
The authors used a precise position provided first by an X-ray space telescope, the Einstein Probe, and refined on the ground by the ATLAS optical telescope to pinpoint the Fast X-ray Transient in space through observations with the Very Large Telescope in Chile and the Grantecan Telescope in Spain.
"These observations show that this explosion happened when the Universe was less than 10 per cent of its current age - the light has been travelling to us for 12 billion years," says Professor Andrew Levan, of Radboud University, and first author of the study.
"The combination of the distance and the brightness means this explosion gave off more energy in a few seconds than the Sun will over its entire life.
"These extreme energies, along with the detection of gamma-rays by the Fermi satellite, suggest that the Fast X-ray Transients are related, at least sometimes, to the extremely powerful gamma-ray bursts that have been studied for half a century."
"A real question is if all of the Fast X-ray Transients come from gamma-ray burst-like systems, or if there is much more diversity. Our paper shows that many of them might be gamma-ray bursts, but there are good reasons to think there is much more still to discover", added Professor Jonker.
When examining the observations from the Very Large Telescope, it became apparent that there is very little material, particularly hydrogen, surrounding this burst.
Hydrogen acts like a filter for ultraviolet light, blocking it from passing through the Universe.
However, in the history of the Universe, around the time of this Fast X-ray Transient, the Universe underwent its last global change in a process called reionisation, where the hydrogen between galaxies was hit with ultraviolet light and 'reionised'.
The challenge is that almost all the ultraviolet light created by stars in the Universe is blocked by hydrogen in the galaxy in which it sits.
"Our observations show that perhaps 10% of the ultraviolet light created in the host galaxy of the Fast X-ray Transient is escaping to ionise the Universe," said Andrea Saccardi, CNES Fellow at CEA Paris-Saclay, and third author of the paper.
"This is the most distant event where we can directly see light escaping from around stars. Galaxies like this are probably really important for reionisation."
These observations are of one of the first events detected by the Einstein Probe.
"In the year since this first object we have found and studied another 20 of these outbursts, they are living up to their promise as an exciting new way to explore both how stars end their lives, and also what the Universe was like in the distant past", added Professor Levan.
Research co-author and Lecturer in Astrophysics at Lancaster University Dr Samantha Oates was part of the research team.
She said: "This event is novel and interesting because only a handful of FXTs had been discovered until very recently, and their origin was a mystery, primarily because they had been found in archival observations.
"By the time they were discovered, it was too late to perform follow-up of these transient X-ray objects at other wavelengths to get other information about what might be causing them."
The Einstein Probe, launched in January 2024, is, says Dr Oates, revolutionising the detection and follow-up of FXTs, which are now regularly detected, and the community is alerted quickly, enabling rapid follow-up.
She added: "The Einstein Probe has opened up a new window on the universe, allowing us to probe the origin of these transient X-ray phenomena and widen our knowledge of the behaviour associated with the deaths of massive stars."
