GRB 170817A: a short GRB seen off-axis

Abstract

The angular distribution of gamma-ray burst (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, θobs > θj, where θobs is the angle between our line of sight and the jet axis. As inferred by gravitational wave observations, as well as radio and X-ray afterglow modeling of GRB 170817A, it is likely that θobs ∼ 20° – 28°. In this work, we quantitatively consider two scenarios of angular energy distribution of GRB ejecta: a top-hat jet and a structured jet with a power law index s. For the top-hat jet model, we get a large θj (e.g., θj > 10°), a rather high local (i.e., z < 0.01) short GRB rate ∼8–15×103, Gpc−3, yr−1 (estimated to be 90∼1850 Gpc−3, yr−1 in Fong et al.) and an extremely high Epeak,0 (on-axis, rest-frame) > 7.5 × 104, keV (∼500, keV for a typical short GRB). For the structured jet model, we use θobs to give limits on s and θj for typical on-axis luminosity of a short GRB (e.g., 1049 erg s−1 ∼ 1051 erg s−1), and a low on-axis luminosity case (e.g., 1049 erg s−1) gives more reasonable values of s. The structured jet model is more feasible for GRB 170817A than the top-hat jet model due to the rather high local short GRB rate, and the extremely high on-axis Epeak,0 almost rules out the top-hat jet model. GRB 170817A is likely a low on-axis luminosity GRB (1049 erg s−1) with a structured jet.