A team of astronomers has identified what they describe as the most convincing example yet of an "orphan afterglow" — the fading radio echo of a gamma-ray burst whose original explosion was never observed from Earth. The discovery, detailed in a paper accepted for publication in The Astrophysical Journal and posted to arXiv on February 24, opens a new window into some of the most powerful and elusive events in the universe.
The radio source, designated ASKAP J005512-255834, was found using the Australian SKA Pathfinder (ASKAP), a 36-antenna radio telescope in Western Australia. It sits in a small, actively star-forming galaxy roughly 1.7 billion light-years from Earth.
Ruling Out Alternatives
The signal was visible almost exclusively at radio wavelengths, with no counterpart in visible light or X-rays — a hallmark of an orphan afterglow. Follow-up observations spanning frequencies from 0.3 to 9 GHz, using instruments including the Australia Telescope Compact Array, the upgraded Giant Metrewave Radio Telescope, and MeerKAT, revealed an evolving spectrum consistent with synchrotron emission.
The team systematically ruled out other explanations, including pulsars, supernovae, and active galactic nuclei. Only two scenarios fit the observed behavior: the late-time afterglow of a long gamma-ray burst viewed off-axis, or a star being torn apart by an intermediate-mass black hole — a rare and still-hypothetical class of black holes.
Opening a Hidden Population
Either explanation would represent an exceptionally rare detection. The paper notes that if ASKAP J005512-255834 is confirmed as an orphan afterglow, it would be only the second such discovery made through radio observations. The team expressed hope that ASKAP and future radio survey instruments could uncover many more of these hidden events, offering a fuller census of gamma-ray bursts across the cosmos.
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