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

Understand

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 $\Gamma - E_{\rm iso}$ relations to determine the Lorentz factor $\Gamma$ and the viewing angle from the edge of the jet $\theta'_{\rm obs}$ of GRB 170817A.
• Our result shows, $\Gamma = 13.4 ^{+9.8}_{-5.5}$ and $\theta'_{\rm obs} = 4.3^{+1.8}_{-1.5} \,^{\circ}$.