Revisiting the Parameter Space of Binary Neutron Star Merger Event GW170817

Abstract

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 ($\theta_{obs}/\theta_j$) between 4 - 6. We use a parameterized model and broadband synchrotron data up to $\sim 800$ 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, $\eta_0$, and the bulk Lorentz factor, $\gamma_B(\sim 1/\theta_j)$ with shapes varying smoothly from a quasi-spherical outflow for low values of $\gamma_B$ to a highly collimated jet for high values. We run simulations with $\gamma_B$ in the range $1-20$ and $\eta_0$ in the range $2-15$. To calculate light curves we use a synchrotron radiation model characterized by $F_{peak}$, $\nu_m$, and $\nu_c$ and calculate millions of spectra at different times and $\theta_{obs}$ values using the \texttt{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 $E_j\sim10^{50}$ ergs and with an observer angle of $\theta_{obs}=0.65^{+0.13}_{-0.14}$ radians and ratio to jet opening angle of ($\theta_{obs}/\theta_j$) = 5.4$^{+0.53}_{-0.38}$.