GRB 170817A as a Refreshed Shock Afterglow Viewed Off-axis

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 ${E}_{1}=({0.3}_{-0.3}^{+3.5}\,$ and $\,{0.5}_{-0.2}^{+6.7})\times {10}^{52}$ erg, and a total/final, on-axis isotropic equivalent refreshed shock energy ${E}_{\mathrm{total}}=({0.42}_{-0.4}^{+5.6}\,$ and $\,{1.26}_{-0.7}^{+18.2})\times {10}^{53}$ 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 ${{\rm{\Gamma }}}_{\mathrm{0,1}}={162.2}_{-122.1}^{+219.7}$ . 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.