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 sph max 2.74/β, where β 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 β to be as high as 1.27, while most realistic candidate equations of state predict β to be closer to 1.2, yielding M sph max in the range 2.16 − 2.28M . A minimal set of assumptions based on these simulations distinguishes this analysis from previous ones, but leads to a 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.

We perform magnetohydrodynamic simulations in full general relativity (GRMHD) of quasi-circular, equal-mass, binary neutron stars that undergo merger. The initial stars are irrotational, = 1 polytropes and are magnetized. We explore two types of magnetic-field geometries: one where each star is endowed with a dipole magnetic field extending from the interior into the exterior, as in a pulsar, and the other where the dipole field is initially confined to the interior. In both cases the adopted magnetic fields are initially dynamically unimportant. The merger outcome is a hypermassive neutron star that undergoes delayed collapse to a black hole (spin parameter/ ~ 0.74) immersed in a magnetized accretion disk. About 4000 ~ 60(/1.625) ms following merger, the region above the black hole poles becomes strongly magnetized, and a collimated, mildly relativistic outflow-an incipient jet-is launched. The lifetime of the accretion disk, which likely equals the lifetime of the jet, is Δ ~ 0.1 (/1.625) s. In contrast to black hole-neutron star mergers, we find that incipient jets are launched even when the initial magnetic field is confined to the interior of the stars.

We perform magnetohydrodynamic simulations in full general relativity (GRMHD) of a binary black hole-neutron star on a quasicircular orbit that undergoes merger. The binary mass ratio is 3 : 1, the black hole initial spin parameter a/m = 0.75 (m is the black hole Christodoulou mass) aligned with the orbital angular momentum, and the neutron star is an irrotational Γ = 2 polytrope. About two orbits prior to merger (at time t = t B ), we seed the neutron star with a dynamically weak interior dipole magnetic field that extends into the stellar exterior. At t = t B the exterior has a low-density atmosphere with constant plasma parameter β ≡ P gas /P mag . Varying β at t B in the exterior from 0.1 to 0.01, we find that at a time ∼ 4000M ∼ 100(M NS /1.4M )ms [M is the total (ADM) mass] following the onset of accretion of tidally disrupted debris, magnetic winding above the remnant black hole poles builds up the magnetic field sufficiently to launch a mildly relativistic, collimated outflowan incipient jet. The duration of the accretion and the lifetime of the jet is ∆t ∼ 0.5(M NS /1.4M )s. Our simulations furnish the first explicit examples in GRMHD which show that a jet can emerge following a black hole -neutron star merger.

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