Astronomers have traced a mysterious and powerful blast of X-rays to a massive star that, defying a complete stellar death, has been dubbed the "Die Hard" star. This cosmic event, detected in early 2025 by the Einstein Probe and named EP 250108a, was followed by a supernova designated SN 2025kg, affectionately nicknamed "The Kangaroo."
For decades, astronomers have been puzzled by fast X-ray transients (FXTs) – blasts of X-rays from distant galaxies that can last from seconds to hours. The new observations of EP 250108a and SN 2025kg have provided crucial insights, revealing that these FXTs can originate from the explosive death of massive stars.
The prevailing theory suggests that the X-ray blast is the result of a "failed" gamma-ray burst (GRB). GRBs are the most energetic explosions in the universe, typically occurring when a massive star collapses and launches powerful jets of high-energy particles that successfully punch through its outer layers. In the case of the "Die Hard" star, the jets erupted from its core but remained trapped within the star's outer shell. These trapped jets interacted with the star's material, losing energy and emitting the observed X-rays.
The star responsible for this phenomenon is estimated to have been between 15 and 30 times the mass of the Sun. The subsequent supernova, SN 2025kg, was identified as a rare and energetic Type Ic-BL supernova, which are typically associated with GRBs. This discovery supports a causal link between GRB-supernovae (where jets succeed) and FXT-supernovae (where jets are trapped).
The finding suggests that "failed jets" might be more common than "successful ones," as the Einstein Probe has detected several FXTs per month, while GRBs are detected far less frequently. This opens up a broader understanding of the diverse ways in which massive stars meet their ends and highlights the need for further investigations into stellar evolution.