Lessons from the Short GRB 170817A: The First Gravitational-wave Detection of a Binary Neutron Star Merger

The first, long-awaited, detection of a gravitational-wave (GW) signal from the merger of a binary neutron star (NS-NS) system was finally achieved (GW170817) and was also accompanied by an electromagnetic counterpart-the short-duration gamma-ray burst (GRB) 170817A. It occurred in the nearby (D ≈ 40 Mpc) elliptical galaxy NGC 4993 and showed optical, IR, and UV emission from half a day up to weeks after the event, as well as latetime X-ray (at≥8.9 days) and radio (at≥16.4 days) emission. There was a delay of Δt ≈1.74 s between the GW merger chirp signal and the prompt GRB emission onset, and an upper limit of θ_obs < 28° was set on the viewing angle w.r.t the jet's symmetry axis from the GW signal. In this letter we examine some of the implications of these groundbreaking observations. The delay Δt sets an upper limit on the prompt GRB emission radius, R_γ ≲ 2cΔt (θ_obs - θ₀)², for a jet with sharp edges at an angle θ₀ < θ_obs. GRB 170817A's relatively low isotropic equivalent γ -ray energy output may suggest a viewing angle slightly outside the jet's sharp edge, θ_obs - θ₀ ∼ (0.05 - 0.1)(Γ/100)^-1, but its peak νF_ν photon energy and afterglow emission suggest instead that the jet does not have sharp edges and the prompt emission was dominated by less energetic material along our line of sight, at θ_obs ≳ 2θ₀. Finally, we consider the type of remnant that is produced by the NS-NS merger and find that a relatively long-lived (>2 s) massive NS is strongly disfavored, while a hyper-massive NS of lifetime ∼1 s appears to be somewhat favored over the direct formation of a black hole.