Short GRBs Viewed from Far Off Axis

Abstract

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. It is also shown that the prompt emission of GRB 170817A, if seen way off-axis ($\theta \gg 1/\Gamma$), could not be made by internal shocks in the baryonic material that powers the afterglow.

accepted for publication in ApJ Letters

References in corpus (6)

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Cited by in corpus (8)

• The optical afterglow of GW170817 at one year post-merger
• The electromagnetic counterparts of compact binary mergers
• Physics of radiation mediated shocks and its applications to GRBs, supernovae, and neutron star mergers
• Prospects of joint detections of neutron star mergers and short-GRBs with Gaussian structured jets
• A backscattering dominated prompt emission model for the prompt phase of Gamma ray bursts
• A Summary of Multimessenger Science with Neutron Star Mergers
• Predicting spectral parameters in the backscattering dominated model for the prompt phase of GRBs
• Effect of Fluid Composition on a Jet Breaking Out of a Cocoon in Gamma-ray Bursts: A Relativistic de Laval Nozzle Treatment