Whenever we study space, we’re usually talking about long-lasting objects, like our own solar system or faraway galaxies that occasionally catch our attention when something extraordinary happens. But sometimes, the universe sends us quick, random bursts of energy that are usually too far away and too ephemeral for scientists to make any sense of—like fast X-ray transients (FXTs), whose elusive origins have long evaded astronomers.
Recently, however, astrophysicists had a lucky strike: spotting an FXT flashing unprecedentedly close to Earth and for a marginally longer time than usual. Not only that, but the X-ray burst, later named EP 250108a, seemed to be a faint spillover signal—likely the result of a cosmic jet—that barely escaped the powerful gravitational binds of a supernova.
Using multiple space telescopes around the world, an international team of astrophysicists from Northwestern University and the University of Leicester in England found compelling evidence that EP 250108a may have originated from the “failed” jets of a gamma-ray burst, likely triggered by the explosive death of a star around 2.8 billion light-years from Earth.
Their results—presented in two papers set for publication in The Astrophysical Journal Letters—offer some of the best evidence yet for at least one potential origin for fast X-ray transients (FXTs are distinct from fast radio bursts (FRBs), brief, extremely energetic bursts of radio waves with wavelengths much longer than those of X-rays).
When a star explodes in a fiery supernova, it swallows almost everything in its vicinity, eventually collapsing into a black hole. In this process of accretion, the star takes on an onion-like form, with different layers of gas, dust, and other cosmic material jostled around by gravitational forces. Some of this material escapes, usually in the form of jets that generate gamma-ray bursts, a class of the most powerful and luminous explosions in the universe.
But sometimes, the outer layers of an “onion-shaped” supernova exert a strong gravitational barrier on the gamma-ray bursts. In the case of this FXT, the tiny bits of energy that managed to leak through probably created EP 250108a, explained Jillian Rastijada PhD student at Northwestern University and lead author of the new paper, in a video call with Gizmodo.
“As the jet is being launched, that extra material from the star that didn’t collapse into the black hole [interacts] with the jet in such a way that sort of suppresses the jet from actually breaking out of the outer layers,” she said.
Rastinejad and colleagues first spotted EP 250108a in January using data from the Einstein Probea collaborative project between China and Europe tasked specifically with the observation of FXTs and other “fleeting” cosmic phenomena. Einstein Probe detects on average “maybe one [FXT] every three days or so,” Rastinejad recounted, but some of her collaborators followed it up with optical telescopes and found that this particular transient was unusually close to Earth.
“When something’s really nearby, it means that it’s going to be a lot brighter,” she explained. “So we can do a really detailed, beautiful, comprehensive, super exciting study of what else is going on at the location of the fast X-ray transient.”
“It’s always very exciting when there’s a transient object, just because it’s like there’s this sound of the record stopping, and you’ve got to stop what you’re doing and move over there,” said John O’Mearadeputy director and chief scientist at the W.M. Keck Observatory in Hawaii, during a video call with Gizmodo.
Keck Observatory was one of several huge space telescopes Rastinejad and colleagues pointed toward EP 250108a, allowing the team to capture a high-resolution view of the ephemeral object before it fades to oblivion.
Unlike most astronomical phenomena of interest to scientists—which lie on timescales that far exceed human lifespans—fast X-ray transients are part of the rare family of cosmic phenomena that evolve on a “human timescale,” Rastinejad said.
“If you took a picture of our Milky Way today, and you took a picture of it maybe a thousand years ago, it would look the same,” she explained. “But if you studied one type of massive star like we studied here, it changes a lot in what it looks like across the wavelengths on very human timescales.”
“The universe keeps trying to tell us very interesting things,” added O’Meara. But the universe “doesn’t care what telescope you build, but [EP 250108a] is a good example of proving that we’re ready to rise to the challenge of whatever the universe wants to throw at us—and I hope we get to keep doing that into the coming decades.”
In fact, Rastinejad, who just finished defending her PhD thesis, already has her eyes on another odd signal from the universe.
“Just a few days ago, [Einstein Probe] saw a fast X-ray transient that occurred in the same part of the sky at the same time as a signal from neutron star mergers,” she said excitedly. “Astronomy is like art. It doesn’t really affect our day-to-day lives. But it answers these questions that humans have always wondered about: where we come from and where we’re going.”
