On short GRBs similar to GRB 170817A detected by Fermi-GBM

Von Kienlin et al. (2019) selected 11 short gamma-ray bursts (sGRBs) whose characteristics are similar to GRB 170817A. These bursts, like GRB 170817A, have a hard spike followed by a soft thermal tail. However, as their redshifts are unknown it is not clear if their luminosities are as low as that of GRB 170817A. Comparing the positions in the ∈p-Eγ ,iso (spectral peak energy - isotropic-equivalent energy) plane and using compactness arguments to estimate the minimal Lorentz factor, γ, we find that all the bursts in this sample are consistent with being regular sGRBs if they are located at z ≈ 0.3-3. They are also consistent with being similar to GRB 170817A if they are located at z ≈0.1. Even in the latter case, the events must involve at least mildly relativistic (γ ≥ 2) motion within the sources. We, further, find that at most one or two bursts in the sample are consistent with the cocoon shock-breakout model. Finally, we calculate the event rate of off-axis emission either from a jet core or from a jet wing (surrounding the core). We find that the off-axis emission model as an origin of the sample is rejected as it predicts too small event rate. The wing model can be consistent with the observed rate but the model parameters cannot be constrained by the current observations.