http://arxiv.org/abs/1710.06407
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, and was also accompanied by an electromagnetic counterpart – the short-duration GRB 170817A. It occurred in the nearby ($D\approx40\;$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 late time X-ray (at $\geq 9\;$days) and radio (at $\geq 17\;$days) emission. There was a delay of $\Delta t \approx 1.74\;$s between the GW merger chirp signal and the prompt-GRB emission onset, and an upper limit of $\theta_{\rm obs}<28^\circ$ 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 $\Delta t$ sets an upper limit on the prompt-GRB emission radius, $R_\gamma\lesssim 2c\Delta t/(\theta_{\rm obs}-\theta_0)^2$, for a jet with sharp edges at an angle $\theta_0<\theta_{\rm obs}$. GRB 170817A’s relatively low isotropic equivalent $\gamma$-ray energy-output and peak $\nu F_\nu$ photon energy suggest either a viewing angle slightly outside the jet’s sharp edge, $\theta_{\rm obs}-\theta_0\sim(0.05-0.1)(\Gamma/100)^{-1}$, or that the jet does not have sharp edges and the prompt emission was dominated by less energetic material along our line of sight, at $\theta_{\rm obs}\gtrsim 2\theta_0$. 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 $\sim1\;$s appears to be somewhat favored over the direct formation of a black hole.
J. Granot, D. Guetta and R. Gill
Wed, 18 Oct 2017
46/62
Comments: 5 pages, 3 figure; minor changes