Magnetohydrodynamic simulations of binary neutron star mergers in general relativity: Effects of magnetic field orientation on jet launching

Abstract

Binary neutron star mergers can be sources of gravitational waves coincident with electromagnetic counterpart emission across the spectrum. To solidify their role as multimessenger sources, we present fully 3D, general relativistic, magnetohydrodynamic simulations of highly spinning binary neutrons stars initially on quasicircular orbits that merge and undergo delayed collapse to a black hole. The binaries consist of two identical stars modeled as Γ = 2 polytropes with spin χNS = 0.36 aligned along the direction of the total orbital angular momentum L. Each star is initially threaded by a dynamical unimportant interior dipole magnetic field. The field is extended into the exterior where a nearly force-free magnetosphere resembles that of a pulsar. The magnetic dipole moment µ is either aligned or perpendicular to L and has the same initial magnitude for each orientation. For comparison, we also impose symmetry across the orbital plane in one case where µ in both stars is aligned along L. We find that the lifetime of the transient hypermassive neutron star remnant, the jet launching time, and the ejecta (which can give rise to a detectable kilonova) are very sensitive to the magnetic field orientation. By contrast, the physical properties of the black hole + disk remnant, such as the mass and spin of the black hole, the accretion rate, and the electromagnetic (Poynting) luminosity, are roughly independent of the initial magnetic field orientation. In addition, we find imposing symmetry across the orbital plane does not play a significant role in the final outcome of the mergers. Our results suggest that, as in the black hole-neutron star merger scenario, an incipient jet emerges only when the seed magnetic field has a sufficiently large-scale poloidal component aligned to the initial orbital angular momentum. The lifetime [∆t 140(MNS/1.625M )ms] and Poynting luminosities [LEM 10 52 erg/s] of the jet, when it forms, are consistent with typical short gamma ray bursts, as well as with the Blandford-Znajek mechanism for launching jets.

Turbulent magnetic-field amplification in the first 10 milliseconds after a binary neutron star merger: Comparing high-resolution and large-eddy simulations

“…Each star initially has a purely poloidal magnetic field confined in its interior, calculated from a vector potential A ϕ ∝ r 2 ðP − P cut Þ, where P cut is a hundred times the pressure of the atmosphere and r the distance to the axis perpendicular to the orbital plane passing through the center of each star. The maximum magnetic intensity (at the centres) is 5 × 10 11 G, orders of magnitude lower than the large initial fields of other simulations (e.g., [18,[21][22][23][24][25]) and compatible with the upper range of the expected realistic intensities for old NSs. Such values are also at the lower border of the computational feasibility, since the accurate evolution for too small ratios of magnetic-tokinetic pressure is hampered by round-off errors.…”

“…In the absence of computationally viable DNS to consistently evolve all the phases of the magnetic dynamics described above, different approaches were considered. Many studies have imposed rather large initial premerger (e.g., [21][22][23][24][25]) or postmerger ( [26]) magnetic field strengths ∼10 [14][15][16] G, to compensate the inability to capture the KHI amplification. However, the quantitative results may not be fully reliable, since the amplification via KHI happens over a broad range of scales and does not preserve a large-scale ordered field.…”

Multimessenger Binary Mergers Containing Neutron Stars: Gravitational Waves, Jets, and γ-Ray Bursts

“…spin, the shorter the time delay between the peak GW and the emergence of the incipient jet. On the other hand, the simulations of Ruiz et al (2020b) suggest that there is a threshold value of the inclination of magnetic dipole moment with respect to the orbital angular momentum L of the binary beyond which jet launching is suppressed. A jet is launched whenever a net poloidal magnetic flux with a consistent sign along L is accreted onto the BH once B 2 /8πρ 0 ≫ 1 above the BH poles.…”

“…To further assess the robustness of the emergence of the incipient jet in NSNS mergers, numerical studies by Ruiz et al (2019Ruiz et al ( , 2020b probed the impact of the NS spin and the orientation of the seed poloidal magnetic field on the formation and lifetime of the HMNS, BH + disk remnant, and the jet launching time. Ruiz et al (2019) found that the larger the co-rotating NS spin, the more massive the accretion disk, and hence the longer the jet's lifetime.…”

Neutron star merger remnants

“…However, the changes in maximum mass are moderate and up to 15-30% for extreme values of the magnetic field B ∼ 10 18 G [149]. In merger remnants, magnetohydrodynamics instabilities and magnetic-field amplifications can lead to global-scale magnetic effects and angular momentum redistribution [17,35,[150][151][152][153]. These instabilities operate on length scales of meters to centimeters, and it is presently impossible to perform fully-resolved, global merger simulations with realistic initial conditions.…”