2018ApJ...852L...1Z

The weak short gamma-ray burst (GRB) 170817A was accompanied by the GW170817 gravitational-wave event and is believed to have been produced by an off-beam relativistic jet. Here, we use the E_p,i–E_iso and Γ–E_iso relations to determine its Lorentz factor Γ and the viewing angle from the edge of the jet θ_obs^′^ of GRB 170817A. Our results indicate that Γ=13.4_–5.5^+9.8 and θ_obs^′^=4○ / . 3_–1.5^+1.8, corresponding to an on-axis E_p,i=415_–167⁺³⁶¹ keV and E_iso=(2.4_–1.9^+1.6)×10⁴⁷ erg. Therefore, the GRB was an intrinsically weak short GRB. We also find that the afterglow emission was in good agreement with the follow-up multiband observations and that the radio emissions at around 20 days may have come from the off-axis jet. Interestingly, the Doppler factor and luminosity follow a universal relation for GRBs and blazars, thus suggesting that they may share a similar radiation mechanism.