Breakthrough Study Reveals Secrets of Dying Stars and FXTs

In a significant breakthrough for astrophysics, researchers at Northwestern University have identified the source of Fast X-ray Transients (FXTs), which have puzzled astronomers for nearly five decades. Published in *The Astrophysical Journal Letters* on July 10, 2025, this study reveals that FXTs are the result of failed gamma-ray bursts from massive stars at the end of their life cycles, fundamentally altering our understanding of stellar death.

For decades, scientists have observed brief, intense bursts of X-rays, lasting anywhere from a few seconds to several hours, without knowing their origin. Jillian Rastinejad, an astrophysicist at Northwestern University, leads a research team that has determined these FXTs stem from the collapsed cores of massive stars, which struggle to produce gamma-ray bursts (GRBs) under certain conditions. The research indicates that when a massive star, approximately 15 to 30 times the mass of the Sun, reaches the end of its life cycle, it collapses inward, creating jets that can release energy in the form of gamma-ray bursts if they escape the star's outer layers. However, in cases where these jets fail to break free, they become trapped, generating a shock that emits X-rays, which we observe as FXTs.

The pivotal moment for this research came on January 8, 2025, when NASA’s Einstein Probe telescope detected a powerful X-ray flash, designated EP 250108a. This flash was linked to a rare supernova, SN 2025kg, termed 'The Kangaroo.' This supernova is categorized as a fast-moving Type Ic-BL event, with material expanding at an astonishing speed of approximately 19,000 kilometers per second. In this instance, the jets produced by the dying star did not escape but were instead trapped, leading to the release of energy in the form of an FXT.

Rob Ailes-Ferris, an astrophysicist at the University of Leicester, highlighted the significance of this discovery, stating, "While FXTs resemble those linked to gamma-ray bursts, the critical distinction lies in the jets’ failure to emerge from the star. Previously, it was believed that gamma-ray bursts were a common outcome of stellar collapse, but this finding indicates that FXTs, resulting from trapped jets, may be more frequent than previously understood."

The implications of this study extend beyond the mere classification of these phenomena. Researchers are now investigating the underlying causes behind the failure of jets to escape. Factors such as internal forces, including magnetic fields and the star's composition, as well as external influences like surrounding material, are being considered as potential contributors. Understanding these dynamics could unlock further secrets of cosmic structures and behaviors.

In conclusion, this new understanding of FXTs not only enhances our knowledge of stellar evolution but also opens avenues for exploring other cosmic mysteries. The research by Rastinejad and her team emphasizes the complexity of stellar death and the diverse outcomes that can arise from the life of massive stars. With ongoing studies and technological advancements, the field of astrophysics is poised for significant revelations in the years to come.