Constraining the Outflow Structure of the Binary Neutron Star Merger Event GW170817/GRB170817A with a Markov Chain Monte Carlo Analysis

Abstract

The multiwavelength nonthermal emission from the binary neutron star merger event GW170817/GRB170817A has raised a heated debate concerning the post-merger outflow structure. Both a relativistic structured jet viewed off-axis and a mildly relativistic quasi-spherical outflow can explain the observational data up to ∼260 days. We utilize a physically motivated analytic two-parameter model called the “boosted fireball,” for the outflow structure after it has expanded far from the merger site. This model consists of a family of outflows with structures varying smoothly between a highly collimated ultra-relativistic jet and an isotropic outflow. We simulate the dynamical evolution of 240 “boosted fireball” outflows using the moving-mesh relativistic hydrodynamics code JET following their evolution through the full afterglow phase. We compute ∼2,000,000 synchrotron spectra from the hydrodynamic simulations. Using scaling relations for the hydrodynamic and radiation equations, we develop a synthetic light-curve generator with efficient sampling speed. This allows the observational data to be fit using Markov Chain Monte Carlo analysis in an eight-dimensional parameter space of hydrodynamic, radiation, and observational parameters. Our results favor the relativistic structured jet, with an opening angle θ 0  ≈ 5° and Lorentz factor Γ ≈ 175, viewed from off-axis angle degrees. Due to parameter degeneracies, we find broad distributions for the explosion energy E 0 , the circumburst density n 0 , and the electron and magnetic energy fractions ϵ e and ϵ B . High n 0 and low ϵ B can also produce a good fit, indicating that very low n 0 may not be required for GW170817/GRB170817A.