On the limit between short and long GRBs

Abstract

Two classes of GRBs have been identified thus far without doubt and are prescribed to different physical scenarios -- NS-NS or NS-BH mergers, and collapse of massive stars, for short and long GRBs, respectively. The existence of two distinct populations was inferred through a bimodal distribution of the observed durations $T_{90}$, and the commonly applied $2\,{\rm s}$ limit between short and long GRBs was obtained by fitting a parabola between the two peaks in binned data from BATSE 1B. Herein, by means of a maximum likelihood (ML) method a mixture of two Gaussians is fitted to the datasets from BATSE, $Swift$, $BeppoSAX$, and $Fermi$ in search for a local minimum that might serve as a new, more proper, limit for the two GRB classes. It is found that $Swift$ and $BeppoSAX$ distributions are unimodal, hence no local minimum is present, $Fermi$ is consistent with the conventional limit, whereas BATSE gives the limit significantly longer (equal to $3.38\pm 0.27\,{\rm s}$) than $2\,{\rm s}$. These new values change the fractions of short and long GRBs in the samples examined, and imply that the observed $T_{90}$ durations are detector dependent, hence no universal limiting value may be applied to all satellites due to their different instrument specifications. Because of this, and due to the strong overlap of the two-Gaussian components, the straightforward association of short GRBs to mergers and long ones to collapsars is ambiguous.