Extreme value distribution for gamma-ray-burst prompt data -- How unexpected was the BOAT event?

TL;DR

This paper applies extreme value theory to gamma-ray burst data to assess the rarity of the exceptional GRB221009A event, finding it highly unlikely under standard models and suggesting it is an outlier.

Contribution

The study introduces a model-independent approach using generalized extreme value distribution to analyze the extremities of GRB data, highlighting the rarity of the BOAT event.

Findings

GRB data's extreme values are consistent with a single population.

GRB221009A's fluence and peak flux are statistically exceptional.

The probability of observing such an event is about once per millennium.

Abstract

Gamma-Ray Bursts (GRBs) are known to be unpredictable in time and position. A few (observationally) exceptional events have been observed, as GRB221009A that stands out for its fluence and peak flux, being orders of magnitude higher than what measured so far. Analyzing the observed fluence, peak flux or duration distributions typically requires one to assume some scenarios, and the consistency of the observed data with the predictions turns out to be an important model diagnostic. However, it is also of interest to model these distributions using general statistical properties that do not rely on specific model assumptions, allowing one to derive inferences only based on the consistency of the observed distributions with the hypothesis of one single population of events that generate them. We obtained fluences, peak fluxes and durations from the catalogues of GRBs observed by the CGRO-BATSE and Fermi-GBM instruments. We selected the extreme values in slots of equal duration and modelled their distributions by the generalized extreme value (GEV) formalism. The GEV distribution is a limit distribution naturally arising when the number of observations is large and is essentially independent of the phenomena producing the observed data. The distributions of extreme values for fluences, peak fluxes and durations are consistent with being extracted from a single population of events but the fluence and peak flux recorded for GRB221009A constitutes a striking exception. The probability to observe such an event, assuming it is a cosmological GRB, is low, with a median value of about one event per millennium for the fluence and about one event per century for the peak flux.