http://arxiv.org/abs/1909.03049
\cite{Kienlin+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 $\epsilon_{\rm p}$-$E_{\rm\gamma,iso}$ (spectral peak energy – isotropic-equivalent energy) plane and using compactness arguments to estimate the minimal Lorentz factor, $\Gamma$, we find that all the bursts in this sample are consistent with being regular sGRBs if they are located at $z\simeq0.3-3$. They are also consistent with being similar to GRB 170817A if they are located at $z\lesssim0.1$. Even in the latter case, the events must involve at least mildly relativistic ($\Gamma \gtrsim 2$) motion within the sources. We, further, show that only two of the bursts, GRBs 120915A and 180511A, are consistent with the cocoon shock-breakout model, which is a leading scenario for the $\gamma$-ray emission observed in GRB 170817A. This fraction is consistent with the event rate expected from the rate of binary neutron star mergers. Finally, we calculate the event rate of the sample for either off-axis emission from a jet core or from a jet-wing (surrounding the core). We find that the off-axis emission model 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.
T. Matsumoto and T. Piran
Mon, 9 Sep 19
59/67
Comments: 8 pages, 4 figures, 1 table, submitted to MNRAS, comments welcome !!!