2021ApJ...918...52L

A relativistic electron-positron (e⁺e^–) pair wind from a rapidly rotating, strongly magnetized neutron star (NS) would interact with a gamma-ray burst (GRB) external shock and reshape afterglow emission signatures. Assuming that the merger remnant of GW170817 is a long-lived NS, we show that a relativistic e⁺e^– pair wind model with a simple top-hat jet viewed off-axis can reproduce multiwavelength afterglow lightcurves and superluminal motion of GRB 170817A. The Markov Chain Monte Carlo (MCMC) method is adopted to obtain the best-fitting parameters, which give the jet half-opening angle θ_j ≃ 0.12 rad, and the viewing angle θ_v ≃ 0.23 rad. The best-fitting value of θ_v is close to the lower limit of the prior that is chosen based on the gravitational-wave and electromagnetic observations. In addition, we also derive the initial Lorentz factor Γ₀ ≃ 49 and the isotropic kinetic energy E_K,iso ≃ 1x 10⁵² erg. Consistency between the corrected on-axis values for GRB 170817A and typical values observed for short GRBs indicates that our model can also reproduce the prompt emission of GRB 170817A. An NS with a magnetic field strength B_p ≃ 1.6 x 10¹³ G is obtained in our fitting, indicating that a relatively low thermalization efficiency η <= 10^–3 is needed to satisfy observational constraints on the kilonova. Furthermore, our model is able to reproduce a late-time shallow decay in the X-ray lightcurve, and predicts that the X-ray and radio flux will continue to decline in the coming years.