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

TL;DR

The paper tackles the problem of assessing the rarity of an extremely bright, likely non-repeating FRB (FRB 20250316A) by applying a model-agnostic generalized extreme value (GEV) framework to the second CHIME/FRB catalog. It partitions the data into time blocks to form block maxima of peak flux and fluence, then fits a Bayesian GEV model to estimate return levels for extreme events. The peak flux analysis indicates a heavy-tailed Fréchet-type tail with a return period of about 593 years at 1-sigma, while the fluence analysis shows sensitivity to outliers, yielding a Fréchet tail with ~50-year return when all outliers are included and a Weibull tail with a finite upper bound when three outliers are removed. These results confirm FRB 20250316A as a profound outlier and suggest possible distinct sub-populations or channels in the FRB luminosity distribution; larger catalogs will be essential to refine tail shapes and understand the physical origin of such extreme bursts.

Abstract

We present a statistical analysis of the extreme brightness of the fast radio burst FRB 20250316A, a luminous, apparently non-repeating event detected by CHIME/FRB. Employing a model-agnostic framework based on the Generalized Extreme Value (GEV) distribution applied to the second CHIME/FRB catalog, we quantify its rarity within the current population. Bayesian fitting of GEV models to block maxima of peak flux and fluence data reveals FRB 20250316A to be a profound statistical outlier. For the peak flux, the analysis yields a return period of $\sim$ 600 years ($1σ$ credible level), with the underlying distribution being of the heavy-tailed, unbounded Fréchet type ($ξ> 0$). The fluence analysis indicates greater complexity: while the full sample suggests a Fréchet-type distribution with a $\sim50$-year return period in $1σ$ credible level, the removal of three other notable outliers points toward a light-tailed Weibull-type distribution ($ξ< 0$) with a finite upper bound far exceeded by FRB 20250316A. This dichotomy highlights the challenge in characterizing the tail of the FRB luminosity function with limited data. Although less extreme in recurrence time than the ``Brightest Of All Time'' gamma-ray burst GRB 221009A, FRB 20250316A constitutes a similarly exceptional event (a potential FRB ``BOAT'') within the short observational history of wide-field radio surveys. Our results underscore the existence of rare, highly luminous events that may probe the upper limits or distinct sub-populations of the FRB luminosity distribution.

Figures (4)

• Figure 1: The position of FRB 250316A in the peak flux vs. fluence diagram.
• Figure 2: GEV analysis of the peak flux. (a) Block maxima time series compared to FRB 20250316A (red horizontal dashed line). (b) Fitted GEV model and return level curve. The intersection with the FRB 20250316A flux defines return periods at $1\sigma$ and $2\sigma$ credible levels. (c) Posterior distributions of the GEV parameters $\mu$, $\sigma$, and $\xi$. (d) QQ plot for model diagnostics.
• Figure 3: GEV analysis of the fluence distribution,following the same panel layout as before.
• Figure 4: GEV analysis of the fluence distribution without three extreme outliers (FRBs 20200723B, 20220222B, 20210922C) removed, following the same panel layout as before.