Off-Axis Emission of Short GRB Jets from Double Neutron Star Mergers and GRB 170817A

Understand

The short-duration ($\lesssim2\;$s) GRB 170817A in the nearby ($D=40\;$Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave (GW) detection of a binary neutron-star (NS-NS) merger.

• It was followed by optical, IR, and UV emission from half a day up to weeks after the event, as well as late time X-ray and radio emission.
• The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactive-decay powered kilonova-like emission.
• Comparison to kilonova models favors the formation of a short-lived ($\sim1\;$s) hypermassive NS, which is also supported by the $\Delta t\approx1.74\;$s delay between the GW chirp signal and the prompt GRB onset.