An artist’s illustration of two neutron stars merging, creating a gammaray burst Copyright POOL/AFP/File Yuichi YAMAZAKI
A mysterious cosmic explosion has been traced to a massive stellar explosion. This cosmic event is showing fast Xray transients (FXTs), which are blasts of Xrays from distant galaxies. Astronomers have detected FXTs since the 1970s, but their origins have remained unknown.
The new discovery represents the closest FXT associated with a supernova ever detected. By studying the event, scientists found it was produced by a geyser of highenergy particles trapped inside the outer layer of a massive star.
This is the output from an international team of astrophysicists, led by Northwestern University and the University of Leicester in England. What is exciting astronomers is the fact the discovery has uncovered a possible origin of fast Xray transients (FXTs).
By using a combination of telescopes around the globe and in space, the team studied the closest FXT associated with the explosive death of a massive star, or supernova, ever observed. A geyser of highenergy particles, or jet, trapped inside a supernova produced the FXT, the scientists discovered.
When jets burst through a massive star’s onionlike layers, they generate gammaray bursts (GRBs), the most powerful and luminous explosions in the universe. When the jets are stifled, however, they emit lower levels of energy, which astronomers can detect only from Xray signals. The new observations now point to these “failed” jets as a source of the emission, explaining the historically elusive phenomena.
The research shows that FXTs can originate from the explosive death of a massive star. It also supports a causal link between GRBsupernovae and FXTsupernovae, in which GRBs are produced by successful jets, and FXTs are produced by trapped weak jets.
Although astronomers have detected FXTs for decades, the limited number of discoveries prevented detailed studies. But now, scientists have a new spacebased tool, called the Einstein Probe, which is dedicated to the search.
Launched in January 2024 by the Chinese Academy of Sciences in partnership with the European Space Agency and the Max Planck Institute for Extraterrestrial Physics, the Einstein Probe carries two scientific instruments, specially designed to observe Xray sources.
Shortly after its launch, the Einstein Probe captured the most nearby FXT, associated with a supernova, to date. Dubbed EP 250108a, the FXT was located 2.8 billion lightyears away from Earth, within the riverlike constellation Eridanus. Its proximity to Earth gave astronomers an unprecedented opportunity to observe the event’s evolution.
To track this evolving behaviour, a large international team captured the event’s signal across multiple wavelengths. The FLAMINGO2 spectrograph on the Gemini South telescope at the International Gemini Observatory provided nearinfrared data, and the Gemini MultiObject Spectrograph on the Gemini North telescope provided optical data. The Northwestern scientists also obtained optical spectroscopy from the W.M. Keck Observatory in Hawaii, infrared images from the MMT Observatory in Arizona and highly sensitive infrared data from the James Webb Space Telescope.
When astronomers pivoted the Gemini telescopes to the location of EP 250108a, they found the shining aftermath of a supernova. The supernova (dubbed SN 2025kg or affectionately known as “the kangaroo”) increased in brightness over several weeks before fading.
After examining its brightness and spectrum, the team confirmed “the kangaroo” was a broadlined Type Ic supernova. These powerful explosions are typically associated with highly energetic events like GRBs. But, in this case, evidence of a GRB was missing.
By analyzing its rapidly evolving signal, the scientists concluded EP 250108a is likely a “failed” GRB. Although EP 250108a is similar to a jetdriven explosion, its jets did not break through the outer layer of the dying star. Instead, the jets remained trapped inside.
To study the supernova itself, the team used the 4.1meter Southern Astrophysical Research Telescope at the National Science Foundation Cerro Tololo InterAmerican Observatory in Chile. With those observations, scientists estimate the progenitor star — whose death ignited EP 250108a and its associated supernova — contained a mass of about 15 to 30 times the mass of the sun.
Two companion studies, detailing various aspects of the event, have been accepted by The Astrophysical Journal Letters. The first is “The kangaroo’s first hop: the early fast cooling phase of EP250108a/SN 2025kg” and the second related paper is “EP 250108a/SN 2025kg: Observations of the most nearby BroadLine Type Ic Supernova following an Einstein Probe Fast Xray Transient.”
