GRB 170817A as a Refreshed Shock Afterglow viewed off-axis

Abstract

Energy injection into the external shock system that generates the afterglow to a gamma-ray burst (GRB) can result in a re-brightening of the emission. Here we investigate the off-axis view of a re-brightened refreshed shock afterglow. We find that the afterglow light-curve, when viewed from outside of the jet opening angle, could be characterised by a slow rise, or long-plateau, with a maximum flux determined by the total system energy. Using the broadband afterglow data for GRB170817A, 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 particular refreshed shock models: a single episode of energy injection; and a period of continuous energy injection. The best fit model parameters give a jet opening angle, for our first or second model of $\theta_j=5.2^{+1.1}_{-0.6}~$or$~6.3^{+1.7}_{-1.1}$ deg, an inclination to the line of sight $\iota=16.0^{+3.4}_{-1.1}~$or$~17.8^{+4.5}_{-2.9}$ deg, an initial isotropic equivalent kinetic energy $E_1 = (0.3^{+3.5}_{-0.3}~$or$~0.5^{+6.7}_{-0.2})\times10^{52}$erg and a total/final, refreshed shock energy $E_{\rm total}=(0.42^{+5.6}_{-0.4}~$or$~1.26^{+18.2}_{-0.7})\times10^{53}$erg. The first model fitting prefers an initial bulk Lorentz factor $\Gamma_{0,1}<60$, with a comparatively low central value of $\Gamma_{0,1}=19.5$, 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 $\Gamma_{0,1}=162.2^{+219.7}_{-122.1}$. Due to the low-Lorentz factor or the jet opening angles at $\theta_j\sim\iota/3$, both models are unable to reproduce the $\gamma$-ray emission observed in GRB170817A, which would therefore require an alternative explanation such as cocoon shock-breakout.