2019ApJ...871..200F

Binary neutron star mergers are believed to eject significant masses with a diverse range of velocities. Once these ejected materials begin to be decelerated by a homogeneous medium, relativistic electrons are mainly cooled down by synchrotron radiation, generating a multiwavelength long-lived afterglow. Analytic and numerical methods illustrate that the outermost matter, the merger shock-breakout material, can be parametrized by power-law velocity distributions α(β_cΓ)^-α_s. Considering that the shock-breakout material is moving on-axis toward the observer and the relativistic jet off-axis, we compute the light curves during the relativistic and the lateral expansion phase. As a particular case, we successfully describe the X-ray, optical, and radio light curves alongside the spectral energy distribution from the recently discovered gravitational-wave transient GW170817, when the merger shock-breakout material moves with mildly relativistic velocities and achieves the near-Newtonian phase and the jet moves with relativistic velocities. Future electromagnetic counterpart observations of this binary system could be able to evaluate different properties of these light curves.