Fast X-ray transients and dying stars

Read the original Volkskrant press release (in Dutch): here
Link to the Nature Astronomy paper: here

In 2013, Peter Jonker of Radboud University Nijmegen discovered the first of these "fast X-ray transients" in thirteen years of data from an American satellite. Several dozen more were later found. Everything pointed to them originating from great distances, but their true nature remained unknown.

That changed in early 2024 with the launch of a sensitive Chinese-European X-ray satellite, the so-called Einstein Probe (see image below). It monitors a much larger portion of the universe and transmits its measurements directly to astronomers on Earth.

"The Einstein Probe is crazy about discovering new X-ray flashes," says Jonker. "In a year and a half, it's already found more than a hundred." By quickly looking at the explosion site with ground-based telescopes, a possible burst of visible light can also be observed, and the origin of the X-ray flash can be determined.

Faint Dot of Light
On March 15, 2024, this was achieved for the first time. That day, the Einstein Probe discovered an X-ray flash in the constellation Hydra ('flash' is actually a misnomer; the outburst lasted almost half an hour). Almost simultaneously, two other satellites registered an explosion of gamma rays. And an hour later, the Atlas telescope network spotted a faint dot of light in the same area of ​​the sky, which had not been seen before.

Jonker and his Nijmegen colleague Andrew Levan quickly deployed large telescopes on La Palma and in northern Chile. Only a few hours after the Atlas discovery was announced, they studied the slowly fading dot of light. It turned out to be so far away that the light from the explosion took over 12.5 billion years to reach Earth. The results were recently published in the journal Nature Astronomy.

The measurements of EP240315a, as the X-ray burst is called, leave no doubt: it is associated with a gamma-ray burst in the early universe, says flash expert Joshua Bloom of the University of California, Berkeley, who was not involved in the research. These types of long gamma-ray bursts are known to originate from the catastrophic explosions of extremely massive stars.

This explanation also applies to a number of X-ray bursts later discovered by the Einstein Probe, the distance of which could not always be determined. But according to Jonker, X-ray bursts have also been discovered that appear to originate in a completely different way, for example, when a dwarf star is swallowed by a black hole.

Mysteries
Bloom sees a fascinating future for X-ray burst research. "As more are discovered and studied, we'll start to distinguish all sorts of subclasses," he says, "just like we did with gamma-ray bursts in the past."

The EP240315a burst also poses a few puzzles for astronomers, says Jonker. For example, it's not entirely clear why the X-rays from the exploding star were emitted almost seven minutes before the gamma-rays. "That's a bit odd."