Analysis of Fermi gamma-ray burst duration distribution

TL;DR

This study analyzes Fermi gamma-ray burst durations and finds that, despite fitting with three log-normal distributions, the data is intrinsically bimodal, indicating only two distinct GRB classes are supported.

Contribution

It provides a detailed statistical analysis of Fermi GRB durations, challenging the existence of a third class suggested by previous datasets.

Findings

No trimodal distribution found in Fermi data

Duration distribution is well described by a bimodal mixture

Log-normal fit may not be the most adequate model

Abstract

Two classes of GRBs, short and long, have been determined without any doubts, and are usually prescribed to different physical scenarios. A third class, intermediate in $T_{90}$ durations, has been reported to be present in the datasets of BATSE, Swift, RHESSI and possibly BeppoSAX. The latest release of $>1500$ GRBs observed by Fermi gives an opportunity to further investigate the duration distribution. The aim of this paper is to investigate whether a third class is present in the $\log T_{90}$ distribution, or is it described by a bimodal distribution. A standard $\chi^2$ fitting of a mixture of Gaussians is applied to 25 histograms with different binnings. Different binnings give various values of the fitting parameters, as well as the shape of the fitted curve. Among five statistically significant fits none is trimodal. Locations of the Gaussian components are in agreement with previous works. However, a trimodal distribution, understood in the sense of having three separated peaks, is not found for any binning. It is concluded that the duration distribution in Fermi data is well described by a mixture of three log-normal distributions, but it is intrinsically bimodal, hence no third class is present in the $T_{90}$ data of Fermi. It is suggested that the log-normal fit may not be an adequate model.