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

Gavin P. Lamb; Andrew J. Levan; Nial R. Tanvir

doi:10.3847/1538-4357/aba75a

paper

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

arXiv:2005.12426 · doi:10.3847/1538-4357/aba75a

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 $θ_{j} = 5. 2_{- 0.6}^{+ 1.1}$ or $6. 3_{- 1.1}^{+ 1.7}$ deg, an inclination to the line of sight $ι = 16. 0_{- 1.1}^{+ 3.4}$ or $17. 8_{- 2.9}^{+ 4.5}$ deg, an initial isotropic equivalent kinetic energy $E_{1} = (0. 3_{- 0.3}^{+ 3.5}$ or $0. 5_{- 0.2}^{+ 6.7}) \times 1 0^{52}$ erg and a total/final, refreshed shock energy $E_{total} = (0.4 2_{- 0.4}^{+ 5.6}$ or $1.2 6_{- 0.7}^{+ 18.2}) \times 1 0^{53}$ 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 $Γ_{0, 1} = 162. 2_{- 122.1}^{+ 219.7}$ . Due to the low-Lorentz factor or the jet opening angles at $θ_{j} \sim ι /3$ , both models are unable to reproduce the $γ$ -ray emission observed in GRB170817A, which would therefore require an alternative explanation such as cocoon shock-breakout.

arXiv comment14 pages, 6 figures - Version accepted for publication in ApJ. Analysis now includes two refreshed shock models and expanded discussion

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

Abstract

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