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Since their first detection, powerful bursts of X-rays from distant galaxies, known as fast X-ray transients (FXTs), have mystified astronomers. FXTs have historically been elusive events, occurring at vast distances away from Earth and only lasting seconds to hours. Einstein Probe (EP), launched in 2024, is dedicated to observing transient events in the X-ray and is changing the game for astronomers looking to understand the origin of these exotic events.

In January 2025 EP alerted astronomers to the nearest FXT known at the time, named EP 250108a. Its proximity to Earth (2.8 billion light-years away) provided an unprecedented opportunity for detailed observations of the event’s evolving behavior.

After the initial detection of EP 250108a, a large, international team of astronomers jumped into action to capture its signal in multiple wavelengths. The FLAMINGOS-2 spectrograph on the Gemini South telescope, one half of the International Gemini Observatory, provided near-infrared data, while the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North telescope provided optical. Gemini’s rapid response capabilities allowed the team to quickly point to the location of EP 250108a where they found the shining aftermath of 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 following initial detection, the team found that this FXT is likely a ‘failed’ variation of a gamma-ray burst (GRB). GRBs are the most powerful explosions in the Universe and have been observed preceding supernovae. During these events, violent geysers of high-energy particles burst through a star’s outer layers as it collapses in on itself. These jets flow at nearly the speed of light and are detectable by their gamma-ray emission.

EP 250108a appears similar to a jet-driven explosion, but one in which the jets do not break through the outer layers of the dying star and instead remain trapped inside. As the stifled jets interact with the star’s outer layers, they decelerate and their kinetic energy is converted to the X-rays detected by Einstein Probe.

“This FXT supernova is nearly a twin of past supernovae that followed GRBs,” says Rob Eyles-Ferris, a postdoctoral researcher at the University of Leicester and lead author of one of two companion papers presenting these results, 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.”

While these early-stage observations provide insight into the mechanisms driving the FXT, longer-term monitoring of the event is necessary to piece together the characteristics of the progenitor star. So the team continued observing EP 250108a beyond the first six days, watching as the emission from the trapped jet faded and 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,” says Jillian Rastinejad, 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 the location of EP 250108a, the team observed a rise in optical brightness that lasted a few weeks before fading, along with spectra containing broad absorption lines. These characteristics indicate that the FXT is associated with a Type Ic broad-lined supernova.

Near-infrared observations from the 4.1-meter Southern Astrophysical Research (SOAR) Telescope at NSF Cerro Tololo Inter-American Observatory (CTIO) in Chile further helped to constrain the supernova’s peak brightness, offering more clues as to what the progenitor star looked like. The team estimates that the star whose death ignited EP 250108a and its associated supernova had a mass of about 15–30 times that of the Sun.

“Our analysis shows definitively that FXTs can originate from the explosive death of a massive star,” says Rastinejad. “It also supports a causal link between GRB-supernovae and FXT-supernovae, in which GRBs are produced by successful jets and FXTs are produced by trapped or weak jets.”

Together, the team’s companion papers present the most detailed dataset to date of a supernova accompanying an EP FXT. Their combined analysis indicates that ‘failed’ jets associated with FXTs are more common in massive star explosions than ‘successful’ jets associated with GRBs. Since the launch of EP, FXTs have been detected several times each month. Meanwhile, GRB detections have historically been sparse, 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,” says Eyles-Ferris.

Astronomers’ understanding of stars will be significantly expanded upon by the upcoming NSF–DOE Vera C. Rubin Observatory, funded by the NSF and the U.S. Department of Energy’s Office of Science (DOE/SC). Its decade-long Legacy Survey of Space and Time (LSST) will provide astronomers with immense amounts of detailed time-domain data on stellar explosions, revealing the internal workings of FXTs and many other exotic stellar events.

“The International Gemini Observatory combines rapid response capabilities with world-leading sensitivity to faint, distant sources,” says 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.”

More information

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 are composed of: 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 (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), NSF Kitt Peak National Observatory (KPNO), NSF Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and NSF–DOE Vera C. Rubin Observatory (in cooperation with DOE’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. 

The scientific community is honored to have the opportunity to conduct astronomical research on I’oligam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence of I’oligam Du’ag to the Tohono O’odham Nation, and Maunakea to the Kanaka Maoli (Native Hawaiians) community.

The Southern Astrophysical Research (SOAR) Telescope is a joint project of the Ministério da Ciência, Tecnologia e Inovações do Brasil (MCTIC/LNA), NSF NOIRLab, the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).

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