Since their initial discovery, powerful bursts of X-rays from distant galaxies, termed fast X-ray transients (FXTs), have puzzled astronomers. These elusive cosmic events occur at vast distances from Earth and typically last only seconds to hours. The launch of the Einstein Probe (EP) in 2024, dedicated to observing transient X-ray events, is revolutionizing the understanding of these mysterious phenomena.
In January 2025, the Einstein Probe detected the nearest FXT observed so far, named EP 250108a, located 2.8 billion light-years away. This proximity offered an unprecedented opportunity for detailed observation of the event’s evolving behavior. An international team of astronomers quickly mobilized to capture its signal across multiple wavelengths.
Unraveling the Mysteries of EP 250108a
Following the initial detection of EP 250108a, the team utilized the FLAMINGOS-2 spectrograph on the Gemini South telescope and the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North telescope to gather near-infrared and optical data, respectively. Thanks to Gemini’s rapid response capabilities, astronomers quickly pinpointed the location of EP 250108a, revealing the explosive death of a massive star, known as a supernova.
Through analysis of EP 250108a’s rapidly evolving signal over the first six days post-detection, researchers concluded that this FXT likely represents a ‘failed’ variation of a gamma-ray burst (GRB). GRBs are the universe’s most powerful explosions, often preceding supernovae. During these events, high-energy particle jets burst through a star’s outer layers as it collapses, detectable by their gamma-ray emissions.
“This FXT supernova is nearly a twin of past supernovae that followed GRBs,” said Rob Eyles-Ferris, a postdoctoral researcher at the University of Leicester and lead author of a companion paper to appear in The Astrophysical Journal Letters. “Our observations of the early stages of EP 250108a’s evolution show that the explosions of massive stars can produce both phenomena.”
Decoding the FXT-Supernova Connection
While early observations provided insights into the mechanisms driving the FXT, extended monitoring was necessary to understand the progenitor star’s characteristics. The team continued observing EP 250108a beyond the first six days, noting that as the trapped jet’s emission faded, the optical signal from its associated supernova, SN 2025kg, dominated the spectra.
“The X-ray data alone cannot tell us what phenomena created the FXT,” explained Jillian Rastinejad, a PhD student at Northwestern University and lead author of the second companion paper. “Our optical monitoring campaign of EP 250108a was key to identifying the aftermath of the FXT and assembling the clues to its origin.”
At EP 250108a’s location, the team observed a rise in optical brightness lasting several weeks before fading, accompanied by spectra with broad absorption lines. These features indicate the FXT’s association with a Type Ic broad-lined supernova. Near-infrared observations from the Southern Astrophysical Research (SOAR) Telescope further constrained the supernova’s peak brightness, offering more clues about the progenitor star, estimated to have a mass 15–30 times that of the Sun.
“Our analysis shows definitively that FXTs can originate from the explosive death of a massive star,” said Rastinejad. “It also supports a causal link between GRB-supernovae and FXT-supernovae, where GRBs are produced by successful jets and FXTs by trapped or weak jets.”
Implications for Stellar Evolution Research
The team’s companion papers offer the most detailed dataset to date of a supernova accompanying an EP FXT. Their analysis suggests that ‘failed’ jets associated with FXTs are more common in massive star explosions than the ‘successful’ jets associated with GRBs. Since EP’s launch, FXTs have been detected several times monthly, whereas GRB detections have historically been rarer, occurring roughly once per year.
“This discovery heralds a broader understanding of the diversity in massive stars’ deaths and a need for deeper investigations into the whole landscape of stellar evolution,” remarked Eyles-Ferris.
Astronomers’ understanding of stars is set to expand significantly with the upcoming NSF–DOE Vera C. Rubin Observatory. Funded by the NSF and the U.S. Department of Energy’s Office of Science, its decade-long Legacy Survey of Space and Time (LSST) will provide immense amounts of detailed time-domain data on stellar explosions, revealing the internal workings of FXTs and other exotic stellar events.
“The International Gemini Observatory combines rapid response capabilities with world-leading sensitivity to faint, distant sources,” said Martin Still, NSF program director for the International Gemini Observatory. “This optimizes Gemini to be a premier follow-up machine for explosive event alerts from gravitational wave and particle detectors, space-borne surveys, and the upcoming Legacy Survey of Space and Time by the NSF-DOE Vera C. Rubin Observatory.”
Looking Ahead
This research was presented in the companion papers: “The kangaroo’s first hop: the early fast cooling phase of EP250108a/SN 2025kg” and “EP 250108a/SN 2025kg: Observations of the most nearby Broad-Line Type Ic Supernova following an Einstein Probe Fast X-ray Transient,” both to appear in The Astrophysical Journal Letters. The teams include R. A. J. Eyles-Ferris (University of Leicester), P. G. Jonker (Radboud University), A. J. Levan (Radboud University), et al.; J. C. Rastinejad (Northwestern University), A. J. Levan (Radboud University), P. G. Jonker (Radboud University), et al.
NSF NOIRLab, the U.S. National Science Foundation center for ground-based optical-infrared astronomy, operates the International Gemini Observatory, NSF Kitt Peak National Observatory, NSF Cerro Tololo Inter-American Observatory, the Community Science and Data Center, and NSF–DOE Vera C. Rubin Observatory, managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF. The scientific community acknowledges the cultural significance of I’oligam Du’ag to the Tohono O’odham Nation and Maunakea to the Kanaka Maoli (Native Hawaiians).
