GRB 170817A: a short GRB seen off-axis

Understand

The angular distribution of GRB jets is not yet clear.

• The observed luminosity of GRB 170817A is the lowest among all known short GRBs, which is best explained by the fact that our line of sight is outside of the jet opening angle, $\theta_{obs}$ $>$ $\theta_j$, where $\theta_{obs}$ is the angle between our line of sight and jet axis.
• Inferred by gravitational wave observations, as well as radio and X-ray afterglow modeling of GRB 170817A, it is likely that $\theta_{obs}$ $\sim$ 20$^{\circ}$ -- 28$^{\circ}$.
• In this work, we quantitatively consider two scenarios of angular energy distribution of GRB ejecta: top-hat jet, and structured jet with a power law index $s$.