Discrete and Continuous Ejection Models of the Radio Source Associated with GW170817

Abstract

The gravity wave source, GW170817, and associated gamma ray burst (GRB), GRB 170817A, produced radio emission that was detected in multiple epochs of Very Long Baseline Interferometry (VLBI) and with broadband radio photometry. Three unique pieces of observational evidence were determined: a discrete radio emitting region that moves with an apparent velocity of $\approx 4$c, the discrete region includes all of the radio flux, and there is likely a synchrotron self absorption (SSA) spectral turnover on day $\sim 110$ and day $\sim 160$ after ejection. This unprecedented wealth of data for a GRB provides a unique opportunity to understand the radio emitting plasma that was ejected by the putative merger event. The velocity can constrain the kinematics and the SSA turnover has been used to constrain the size to much smaller than can be done with an unresolved VLBI image, allowing one to estimate the associated plasmoid size directly from the data and improve estimates of the energetics. Models of the radio emission for both a turbulent, protonic, discrete ballistic ejection and a high dissipation region within an otherwise invisible Poynting flux dominated positron-electron jet are considered. On days $\sim 110$ and $\sim 160$ post-merger, for the range of models presented, the jet power is $2\times 10^{39} - 8\times 10^{40} \rm{ergs/s}$ and the ballistic plasmoid kinetic energy is $3\times 10^{45} - 1.5\times 10^{47} \rm{ergs}$. Even though only valid after day 110, this independent analysis augments traditional GRB light curve studies, providing additional constraints on the merger event.