GW170817, General Relativistic Magnetohydrodynamic Simulations, and the Neutron Star Maximum Mass

Coincident detections of gravitational waves with electromagnetic signals can be used to constrain the nature of the progenitor of GW170817/GRB170817A. Combining the observational data with our recent fully general relativistic magnetohydrodynamic numerical simulations we conclude that the progenitor represents the merger of a magnetized binary neutron star that undergoes delayed collapse to a black hole immersed in a magnetized disk of tidal debris. This conclusion leads to a bound on the maximum mass of a cold, spherical neutron star (the TOV limit): ${M_{\rm max}^{\rm sph}}\leq2.74/\beta$, where $\beta$ is the ratio of the maximum mass of a uniformly rotating neutron star (the supramassive limit) over the maximum mass of a nonrotating star. Causality arguments allow $\beta$ to be as high as $1.27$, while most realistic candidate equations of state predict $\beta$ to be closer to $1.2$, yielding ${M_{\rm max}^{\rm sph}}$ in the range $2.16-2.28 M_\odot$. Assumptions and caveats, which can be removed by further numerical simulations and analysis, are also discussed.