Imagine a star so tough it survives the initial stages of its own death. Astronomers have detected a powerful, mysterious blast of X-rays that seems to come from just such a celestial object: a massive star that underwent a supernova explosion but still sent out a powerful signal, much like a cosmic action hero refusing to quit. This discovery is helping scientists better understand how the largest stars in the universe meet their fiery end, leaving behind exotic objects like black holes and neutron stars.
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This new X-ray signal, known as a fast X-ray transient (FXT), might be evidence of a “failed” version of an even more extreme event called a gamma-ray burst (GRB). Studying this specific FXT, the closest one ever observed, could revolutionize our understanding of massive stellar evolution and the diverse ways stars explode.
Decoding Mysterious Cosmic X-Ray Bursts
For years, fast X-ray transients (FXTs) have puzzled astronomers. These intense, brief flashes of X-rays appear suddenly in distant galaxies, lasting only seconds to hours before fading away. Their fleeting nature and far-off locations – often billions of light-years away – have made them difficult to study in detail and understand their origins.
Scientists hoped that new tools designed to scan the X-ray sky could help solve this mystery. The Einstein Probe, a satellite launched in January 2024, was specifically built for this purpose.
In early 2025, just about a year after its launch, the Einstein Probe hit the jackpot: it detected the closest FXT ever seen, designated EP 250108a. This burst originated from a galaxy located a mere 2.8 billion light-years away – relatively close in cosmic terms, offering an unprecedented opportunity for follow-up studies.
The relative proximity of EP 250108a allowed astronomers to quickly mobilize a network of powerful telescopes, including the Gemini North and South observatories, to investigate its source and watch how the event unfolded over time.
Tracing the X-Ray Blast Back to a Stellar Corpse
By tracing the signal back to its point of origin, the research team linked EP 250108a to the aftermath of a massive star’s destruction. This stellar explosion is known as the supernova SN 2025kg.
Sequence of images showing the fading light of supernova SN 2025kg
Observing EP 250108a’s evolution for the first six days, the astronomers noticed something intriguing: it behaved much like what they would expect from a “failed GRB.”
Gamma-ray bursts (GRBs) are the most powerful explosions known in the universe. They are often associated with certain types of supernovas, occurring as a massive star collapses. GRBs are thought to be powered by incredibly fast jets of particles launched from the core of the dying star. These jets punch through the star’s outer layers at nearly the speed of light, emitting intense gamma-rays that we can detect across vast cosmic distances.
The Case of the Trapped Jet
The FXT EP 250108a strongly resembled a jet-driven explosion, but critically, it lacked the powerful gamma-ray signature of a typical GRB. This led the scientists to a compelling hypothesis: perhaps this was a GRB jet that didn’t quite make it out.
Instead of successfully bursting through the star’s outer envelope, the jets were likely trapped inside. As these powerful jets struggled against the dense stellar material, they lost energy and generated the intense X-rays detected by the Einstein Probe, rather than escaping as gamma-rays.
As team member Rob Eyles-Ferris from the University of Leicester explained, this discovery broadens our understanding of how massive stars die and highlights the need for deeper investigation into stellar evolution.
“This FXT supernova is nearly a twin of past supernovae that followed GRBs,” Eyles-Ferris noted. “Our observations of the early stages of EP 250108a’s evolution show that the explosions of massive stars can produce both phenomena.”
Beyond X-Rays: Optical Light Reveals More Clues
To fully piece together the story of this cosmic event, the astronomers needed to look beyond the initial X-ray burst. After the first six days, the X-ray emission faded, and the explosion’s light began to be dominated by lower-energy optical light, which is visible light like we see with our eyes.
Distant galaxies visible in a telescope image
Jillian Rastinejad, a researcher at Northwestern University and another team member, emphasized the importance of this multi-wavelength approach. “The X-ray data alone cannot tell us what phenomena created the FXT,” she said. “Our optical monitoring campaign of EP 250108a was key to identifying the aftermath of the FXT and assembling the clues to its origin.”
The way the optical brightness of EP 250108a increased over several weeks provided crucial information. Its characteristics pointed to a specific type of stellar explosion: a Type Ic broad-lined supernova.
The Anatomy of a Dying Star
Type Ic supernovas are a category of core-collapse supernovas. These happen when massive stars, much larger than our Sun, exhaust the nuclear fuel in their core. No longer able to support themselves against their own immense gravity, their core collapses, triggering a catastrophic explosion.
What makes Type Ic supernovas distinct is that the star is believed to have lost its outer layers of hydrogen and helium before exploding. This “stripped” state influences the kind of light we see from the supernova.
Using the Southern Astrophysical Research (SOAR) Telescope in Chile, the team further analyzed EP 250108a. This allowed them to estimate the mass of the original star that exploded. Their analysis suggests the star was truly massive, between 15 and 30 times the mass of the Sun.
The SOAR Telescope under the Milky Way
“Our analysis shows definitively that FXTs can originate from the explosive death of a massive star,” Rastinejad concluded. “It also supports a causal link between GRB-supernovas and FXT-supernovas, in which GRBs are produced by successful jets and FXTs are produced by trapped or weak jets.”
More Failed Explosions Than Successful Ones?
The Einstein Probe is detecting several FXTs each month. In contrast, GRBs are much rarer, detected only about once a year. This difference in detection rates suggests that the “failed jet” scenarios that might create FXTs could be more common than the “successful jet” events that produce GRBs.
Understanding this difference is vital for building a complete picture of stellar death and the formation of exotic objects like black holes and neutron stars.
Astronomers are eager for future observations that can shed more light on these events. The upcoming Legacy Survey of Space and Time (LSST) by the Vera C. Rubin Observatory, set to begin soon, will survey the entire night sky over a decade, potentially discovering many more FXTs and associated supernovas, helping to solve these cosmic mysteries.
The detailed findings about EP 250108a and its associated supernova SN 2025kg are presented in two scientific papers, providing astronomers with the most comprehensive data yet on a fast X-ray transient linked to a stellar explosion. This research opens a new window into the diverse and dramatic ways massive stars end their lives.
