Distributions of pseudo-redshifts and durations (observed and intrinsic) of Fermi GRBs

Ever since the insightful analysis of the durations of gamma-ray bursts (GRBs) by Kouveliotou et al. (Astrophys. J. Lett. 413:101, 1993), GRBs have most often been classified into two populations: “short bursts” (shorter than 2.0 seconds) and “long bursts” (longer than 2.0 seconds). However, recent works have suggested the existence of an intermediate population in the bursts observed by the Swift satellite. Moreover, some 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, and some long ones may originate from mergers, the usual progenitors of short GRBs.