Short GRBs Viewed from Far Off Axis

Understand

The recent radio observations (Mooley et al, 2018) of a superluminal radio afterglow following GRB 170817A are interpreted in terms of a jet impacting a baryonic cloak, which is presumably the material caught at the front of the jet as the latter emerges from a denser ejected material.

• Assuming that we the observers are located {\bf at a viewing angle of $\sim 0.2$ radians from the emitting material (perhaps slightly more from jet axis)}, we suggest that the Lorentz factor of the jet is $\lesssim 20$ at the time of the prompt emission, and that, as suggested previously, it is accelerated to much higher values before finally decelerating during the afterglow phase.
• A less extreme example of a short GRB being observed off axis may have been GRB 150101b (Fong, et al., 2016).
• A feature of GRBs viewed from large offset angles is a large afterglow isotropic equivalent energy as compared to prompt emission, as predicted (Eichler, 2017), and this is born out by the observations of these two GRB.