2023ApJ...945..135M

Since the gravitational wave event GW170817 and gamma-ray burst GW170817A, there have been numerous studies constraining the burst properties through analysis of the afterglow light curves. Most agree that the burst was viewed off-axis with a ratio of the observer angle to the jet angle (θ_obs/θ_j) between 4 and 6. We use a parameterized model and broadband synchrotron data up to ∼800 days post-merger to constrain parameters of the burst. To reproduce the hydrodynamics of a gamma-ray burst outflow, we use a two-parameter “boosted fireball” model. The structure of a boosted fireball is determined by the specific internal energy, η₀, and the bulk Lorentz factor, γ_B( ∼ 1/θ_j), with shapes varying smoothly from a quasi-spherical outflow for low values of γ_B to a highly collimated jet for high values. We run simulations with γ_B in the range 1-20 and η₀ in the range 2-15. To calculate light curves, we use a synchrotron radiation model characterized by F_peak, ν_m, and ν_c, and calculate millions of spectra at different times and θ_obs values using the boxfit radiation code. We can tabulate the spectral parameter values from our spectra and rapidly generate arbitrary light curves for comparison to data in MCMC analysis. We find that our model prefers a gamma-ray burst with jet energy E_j ∼ 10⁵⁰ erg and with an observer angle of ${\theta }_{\mathrm{obs}}={0.65}_{-0.14}^{+0.13}$ radians and ratio to the jet opening angle of (θ_obs/θ_j) = ${5.4}_{-0.38}^{+0.53}$.