Analysis of Fermi gamma-ray burst duration distribution

Astronomical Observatory, Jagiellonian University, Orla 171, 30-244 Cracow, Poland
e-mail: mariusz.tarnopolski@uj.edu.pl

Received: 26 April 2015
Accepted: 24 June 2015

Abstract

Context. Two classes of gamma-ray bursts (GRBs), short and long, have been determined without any doubts, and are usually prescribed to different physical scenarios. A third class, intermediate in T₉₀ durations has been reported 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.

Aims. The aim of this paper is to investigate whether a third class is present in the log T₉₀ distribution, or whether it is described by a bimodal distribution.

Methods. A standard χ² fitting of a mixture of Gaussians was applied to 25 histograms with different binnings.

Results. 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.

Conclusions. Locations of the Gaussian components are in agreement with previous works. However, a trimodal distribution, understood in the sense of having three distinct peaks, is not found for any binning. It is concluded that the duration distribution in the 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 T₉₀ data of Fermi. It is suggested that the log-normal fit may not be an adequate model.