  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 reshapes 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 multi-wavelength 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
$\theta_{j}\approx0.11$ rad, and the viewing angle $\theta_{v}\approx0.23$ rad.
The best-fitting value of $\theta_{v}$ is close to the lower limit of the prior
which is chosen based on the gravitational-wave and electromagnetic
observations. In addition, we also derive the initial Lorentz factor
$\Gamma_{0}\approx47$ and the isotropic kinetic energy $E_{\rm
K,iso}\approx2\times10^{52}\rm\ erg$. A consistence 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 $B_{p}\approx1.6\times10^{13}\rm\ G$ is
obtained in our fitting, indicating that a relatively low thermalization
efficiency $\eta\lesssim10^{-3}$ 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.
