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 ${\theta }_j=5.2_{-0.6}^{+1.1}$or$6.3_{-1.1}^{+1.7}$ deg, an inclination to the line of sight $\iota =16.0_{-1.1}^{+3.4}$or$17.8_{-2.9}^{+4.5}$ deg, an initial isotropic equivalent kinetic energy $E_1=(0.3_{-0.3}^{+3.5}$or$0.5_{-0.2}^{+6.7})\times 10^{52}$erg and a total/final, refreshed shock energy $E_{\mathrm{total}}=(0.42_{-0.4}^{+5.6}$or$1.26_{-0.7}^{+18.2})\times 10^{53}$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 ${\Gamma }_{0,1}=162.2_{-122.1}^{+219.7}$. 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.

14 pages, 6 figures - Version accepted for publication in ApJ. Analysis now includes two refreshed shock models and expanded discussion

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• GRB 170817A Afterglow from a Relativistic Electron-Positron Pair Wind Observed Off-axis
• Broadband Emission from a Kilonova Ejecta-Pulsar Wind Nebula System: Late-Time X-ray Afterglow Rebrightening of GRB 170817A
• Probing particle acceleration at trans-relativistic shocks with off-axis gamma-ray burst afterglows
• Reverse Shock Emission in an Off-axis Top-hat Jet Model for Gamma-Ray Bursts