Determine the Lorentz factor and the viewing angle of GRB 170817A

GRB 170817A is a weak short gamma-ray burst (GRB) accompanied by the gravitational wave (GW) event GW170817. It is believed, that an off beaming relativistic jet, produces this weak GRB. Here we use the $E_{\rm p,i}- E_{\rm iso}$and$\Gamma - E_{\rm iso}$relationstodeterminetheLorentzfactor$\Gamma$andtheviewinganglefromtheedgeofthejet$\theta’{\rm obs}$ of GRB 170817A. Our result shows, $\mathrm{\Gamma }=13.4_{-5.5}^{+9.8}$ and $\theta’{\rm obs} = 4.3^{+1.8}{-1.5} ,^{\circ}$\mathrm{.}Thiscorrespondstotheon-axis$E{\rm p,i} = 415^{+361}{-167}$keVand$E{\rm iso} = (2.4^{+1.6}_{-1.9}) \times 10^{47}$ ergs of the GRB, which is an intrinsically weak short GRB. Interestingly, the Doppler factor and the luminosity follow a universal relation from GRBs and blazars, which indicates they may share similar radiation mechanism.