The classification and formation rate of $\mathbf{Swift/BAT}$ gamma-ray bursts

TL;DR

This study re-examines the classification of Swift/BAT gamma-ray bursts, revealing three subclasses—short, intermediate, and long—based on statistical modeling of their duration and hardness ratio distributions, and analyzing their luminosity functions and formation rates.

Contribution

It introduces a three-component Gaussian mixture model for GRB classification and analyzes the distinct luminosity functions and formation rates of each subclass.

Findings

Three subclasses of GRBs identified: short, intermediate, long.

Distinct luminosity functions for each subclass.

Different formation rates support the intermediate subclass existence.

Abstract

Gamma-ray bursts (GRBs) are usually classified into long/short categories according to their durations, but controversy still exists in this aspect. Here we re-examine the long/short classification of GRBs and further compare the cosmological distribution and evolution of each potential subclass. A large number of $Swift/BAT$ GRBs are analyzed in this study. The Gaussian mixture model is used to fit the duration distribution as well as the joint distribution of duration and hardness ratio, and the Akaike and Bayesian information criteria are adopted to assess the goodness of fit. It is found that three Gaussian components can better fit both the univariate and bivariate distributions, indicating that there are three subclasses in the $Swift/BAT$ GRBs, namely short, intermediate, and long subclasses. The non-parametric Efron-Petrosian and Lynden-Bell's $c^{-}$ methods are used to derive the luminosity function and formation rate from the truncated data of bursts with known redshift in each subclass. It is found that the luminosity distributions and birth rates of the three subclasses are different, further supporting the existence of the intermediate subclass in the $Swift/BAT$ GRBs.