GW170817, general relativistic magnetohydrodynamic simulations, and the neutron star maximum mass

Recent numerical simulations in general relativistic magnetohydrodynamics (GRMHD) provide useful constraints for the interpretation of the GW170817 discovery. Combining the observed data with these simulations leads to a bound on the maximum mass of a cold, spherical neutron star (the TOV limit): $M_{\max }^{\mathrm{sph}}\lesssim 2.74/\beta$, 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 $M_{\max }^{\mathrm{sph}}$ in the range $2.16–2.28M_{\odot }$. A minimal set of assumptions based on these simulations distinguishes this analysis from previous ones, but leads a to similar estimate. There are caveats, however, and they are enumerated and discussed. The caveats can be removed by further simulations and analysis to firm up the basic argument.

Figure 1

Snapshots of the rest-mass density, normalized to its initial maximum value (log scale), for a supramassive remnant that persists, [first column, case (1b)], a HMNS remnant that undergoes delayed collapse [second column, case (2a)], a HMNS remnant that undergoes prompt collapse [third column, case (2b)]. First row shows NSs at the time of B-field insertion; second row shows the final quasistationary remnants. Third row shows the $B^2/(8\pi {\rho }_0)$ field (log scale), the magnetic lines emanating from the remnant poles, and the fluid velocity flow vectors. The field lines form a tightly wound helical funnel and drive a jet in delayed collapse, but not in the other two cases. Here $M=0.0136(M_{\mathrm{tot}}/2.74M_{\odot })\mathrm{ms}=4.07(M_{\mathrm{tot}}/2.74M_{\odot })\mathrm{km}$.