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

TL;DR

This paper estimates the Lorentz factor and viewing angle of GRB 170817A using empirical relations, revealing it as an intrinsically weak short GRB with potential shared radiation mechanisms with blazars.

Contribution

It applies $E_{p,i}-E_{iso}$ and $\Gamma - E_{iso}$ relations to determine key jet parameters of GRB 170817A, providing new insights into its nature.

Findings

Lorentz factor $\Gamma$ = 13.4 (+9.8/-5.5)

Viewing angle $ heta'_{obs}$ = 4.3° (+1.8/-1.5)

GRB shares universal Doppler-luminosity relation with blazars

Abstract

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 $\theta'_{\rm obs} = 4.3^{+1.8}_{-1.5} \,^{\circ}$. This corresponds to the on-axis $E_{\rm p,i} = 415^{+361}_{-167}$ keV and $E_{\rm iso} = (2.4^{+1.6}_{-1.9}) \times 10^{47}$ 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.