2020ApJ...899..105L

Energy injection into the external shock system that generates the afterglow to a gamma-ray burst (GRB) can result in a rebrightening of the emission. Here we investigate the off-axis view of a rebrightened refreshed shock afterglow. We find that the afterglow light curve, when viewed from outside of the jet opening angle, could be characterized by a slow rise, or long plateau, with a maximum flux determined by the total system energy. Using the broadband afterglow data for GRB 170817A, associated with the gravitational-wave-detected binary neutron star merger GW170817, we show that a refreshed shock model with a simple top-hat jet can reproduce the observed afterglow features. We consider two refreshed shock models: a single episode of energy injection, and an episode of continuous energy injection. The best-fit model parameters give a jet opening angle for our first and second models, respectively, of θ_j = 5.°2_–0.6^+1.1 and 6.°3_–1.1^+1.7, an inclination to the line of sight ι = 16.°0_–1.1^+3.4 and 17.°8_–2.9^+4.5, an initial on-axis isotropic equivalent kinetic energy E₁=(0.3_–0.3^+3.5 and 0.5_–0.2^+6.7)×10⁵² erg, and a total/final, on-axis isotropic equivalent refreshed shock energy E_total=(0.42_–0.4^+5.6 and 1.26_–0.7^+18.2)×10⁵³ erg. The first model fitting prefers an initial bulk Lorentz factor Γ_0,1 < 60, with a comparatively low central value of Γ_0,1 = 19.5, indicating that, in this case, the on-axis jet could have been a "failed GRB." Alternatively, our second model is consistent with a bright GRB for an on-axis observer, with Γ_0,1=162.2_–122.1^+219.7. Due to the low Lorentz factor and/or the jet opening angles at θ_j ∼ ι/3, both models are unable to reproduce the γ-ray emission observed in GRB 170817A, which would therefore require an alternative explanation such as cocoon shock breakout.