Analysis of Fermi GRB durations in the observed and intrinsic reference frames

Gamma-ray bursts (GRBs) are usually classified into two populations, according to their durations, denoted by T_{90}: long bursts (T_{90} > 2.0 s) and short bursts (T_{90} < 2.0 s). Recent work has suggested the existence of an intermediate population in the bursts observed by the Swift satellite (Bromberg et al. 2013, Zitouni et al. 2015, Horváth & Tóth 2016). Moreover, a number of researchers have questioned the universality of the 2.0-second dividing line between short and long bursts: some bursts may be short but actually result from collapsars, the physical mechanism behind normally long bursts.In this work, we focus on GRBs detected by the Fermi satellite and study the distribution of their durations measured in the observer's reference frame and, to the extent possible, in the bursts' reference frames. However, bursts with measured redshifts are limited, and this makes a statistical study difficult. We thus proceed in two ways. First, following Zhang & Wang (2017), we determine pseudo-redshifts from the correlation relation between the luminosity L_p and the energy E_p both calculated at the peak of the flux. Interestingly, we find that the uncertainties over the quantities used in the determination of pseudo-redshifts affect the precision of the results significantly, although they remain within acceptable limits. We thus present the advantages and disadvantages of using these correlation relations as cosmological candles. Secondly, we compare the results obtained with the pseudo-redshifts with those obtained from a sample of 43 GRBs with actual redshifts given by Heussaff (2013). A chi-square statistical test shows that the distribution of the durations of Fermi bursts, both observed and intrinsic, are composed of two peaks, not three, as we had found for the BATSE bursts. We propose an explanation for the presence of an intermediate class of bursts in the Swift data, based on a selection effect related to the Swift/BAT energy band.