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.
“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.”
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 Rastinejad, a 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 Probe, a 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.”
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