Extreme-value Distribution Analysis of the Second CHIME/FRB Catalog: Assessing the Rarity of the One-off FRB 20250316A

We present a statistical analysis of the extremely bright, apparently nonrepeating fast radio burst FRB 20250316A, detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME), to assess its rarity. Using a model-agnostic framework based on the generalized extreme-value (GEV) distribution and the second CHIME/FRB catalog, we perform Bayesian fits to the block-maxima of its peak flux and fluence. Our analysis confirms FRB 20250316A as a pronounced statistical outlier in both quantities. For the peak flux, the best-fit GEV model follows an unbounded, heavy-tailed Fréchet-type distribution, yielding return periods of approximately 802 yr at the 68% confidence level (CL), 81 yr at the 95% CL, and 30 yr at the 99% CL. The fluence distribution exhibits greater complexity: while the full sample is consistent with a Fréchet-type distribution (return period of approximately 55, 15, and 8 yr at the 68%, 95%, and 99% CLs, respectively), removing three other conspicuous outliers reveals a light-tailed Weibull-type distribution with a finite upper bound that is far exceeded by the fluence of FRB 20250316A. Although its inferred recurrence time is shorter than that of the “Brightest Of All Time” (BOAT) gamma-ray burst GRB 221009A, FRB 20250316A represents a similarly exceptional event (a potential FRB "BOAT") within the relatively short observational baseline of wide-field radio surveys. This work affirms the existence of rare, extremely luminous events at the extreme upper end of the FRB luminosity distribution, which may delineate a distinct physical channel or the extreme tail of a complex luminosity function.