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The most sensitive search to date for a persistent radio source associated with an extremely bright, non-repeating fast radio burst (FRB) has yielded no results, significantly narrowing the range of possible explanations for these mysterious cosmic phenomena.

A joint research team from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences (CAS) and the University of Science and Technology of China used the Very Large Array to search for potential persistent radio emission from FRB 20250316A, one of the brightest non-repeating bursts ever detected.

The results, in The Astrophysical Journal Letters, provide a stringent upper limit yet on persistent radio emission from a non-repeating FRB.

Fast radio bursts are among the most energetic phenomena in the universe, releasing as much energy in milliseconds as the sun produces in days. Since their discovery more than a decade ago, astronomers have identified two distinct classes: repeating bursts that flash multiple times from the same location, and non-repeating bursts that appear only once.

A key question is whether these different types arise from fundamentally different physical processes. One major clue lies in whether FRB sources are associated with persistent radio emission between bursts. Some repeating FRBs exhibit such persistent sources, which could indicate ongoing activity in their local environment. However, for non-repeating FRBs, this question has largely remained unanswered due to the extreme sensitivity required for detection.

FRB 20250316A provided an ideal test case. The burst was both exceptionally bright and relatively nearby, enabling the search for even the faintest persistent emission. Using observations taken within one month of the burst, the researchers achieved unprecedented sensitivity, thus setting an upper limit of 2.8 micro Jansky (1 sigma) at 15 GHz, consistent with independent observations by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Collaboration at 9.9 GHz about two months after burst.

"This non-detection is actually an interesting result," said Prof. An Tao, first author of the study. "We can now rule out several theoretical models that predict bright, persistent radio sources around non-repeating FRBs. The environment appears much 'cleaner' than some theories suggested."

Specifically, the findings eliminate magnetar–nebula models, a leading explanation for some repeating FRBs, as a viable option for this non-repeating burst. Instead, they favor scenarios such as neutron star mergers or giant flares from more evolved, isolated magnetars, which occur in low-density environments unlikely to produce long-lasting radio emission.

This work represents a crucial step towards understanding whether repeating and non-repeating FRBs arise from distinct physical mechanisms, a question that has important implications for our understanding of extreme physics in the universe. As larger samples of FRBs are discovered and studied with increasing precision, astronomers are moving closer to solving one of modern astrophysics' most intriguing puzzles.