GW170817 Afterglow Reveals that Short Gamma-Ray Bursts are Neutron Star Mergers

We systematically investigate the outflow structure of GW170817 in comparison with a sample of 27 cosmological short GRBs by modelling their afterglow light curves. We find that cosmological short GRBs share the same outflow structures with GW170817, relativistic structured jets. The jet opening angle of GW170817 is $6.3^{+1.1}_{-0.6}{}^{\circ}$, which is consistent with that of cosmological short GRBs ($\theta_0 = 6.9^\circ \pm 2.3^{\circ}$). Our analysis indicates that GW170817 is viewed off-axis ($\theta_{\rm obs} = 30^{+7}_{-4}{}^{\circ}$), while cosmological short GRBs are viewed on-axis ($\theta_{\rm obs} \lesssim \theta_0$). The exceptional properties of the GW170817 afterglow can be explained by the difference in observation angle alone. We demonstrate that the light curves of the GW170817 afterglow, if viewed on-axis, are consistent with those of cosmological short GRBs. Other properties of GW170817, such as Lorentz factor $\Gamma \approx 150$, spectral index $p \approx 2.15$, isotropic equivalent energy $E_{\rm iso} \approx 8 \times 10^{52}$ erg and interstellar medium density $n_0 \approx 10^{-2}$ proton cm$^{-3}$, fit well within the ranges of those of cosmological short GRBs. The similarity between the GW170817 outflow structure and those of cosmological short GRBs indicates that cosmological short GRBs are likely neutron star mergers.