Binary Neutron Star Mergers and Short Gamma-Ray Bursts: Effects of Magnetic Field Orientation, Equation of State, and Mass Ratio

TL;DR

This study uses fully GRMHD simulations to explore how magnetic field orientation, equation of state, and mass ratio affect binary neutron star mergers and their potential to produce short gamma-ray bursts, finding organized magnetic fields but no relativistic jets.

Contribution

It demonstrates that organized magnetic field structures form regardless of EOS, mass ratio, and magnetic orientation, and highlights the impact of HMNS lifetime on magnetic field amplification.

Findings

Organized magnetic fields form independently of initial conditions.

Longer-lived HMNSs lead to stronger magnetic fields before collapse.

No relativistic outflows observed in simulations.

Abstract

We present fully GRMHD simulations of the merger of binary neutron star (BNS) systems. We consider BNSs producing a hypermassive neutron star (HMNS) that collapses to a spinning black hole (BH) surrounded by a magnetized accretion disk in a few tens of ms. We investigate whether such systems may launch relativistic jets and power short gamma-ray bursts. We study the effects of different equations of state (EOSs), different mass ratios, and different magnetic field orientations. For all cases, we present a detailed investigation of the matter dynamics and of the magnetic field evolution, with particular attention to its global structure and possible emission of relativistic jets. The main result of this work is that we found the formation of an organized magnetic field structure. This happens independently of EOS, mass ratio, and initial magnetic field orientation. We also show that those models that produce a longer-lived HMNS lead to a stronger magnetic field before collapse to BH. Such larger fields make it possible, for at least one of our models, to resolve the MRI and hence further amplify the magnetic field. However, by the end of our simulations, we do not observe a magnetically dominated funnel and hence neither a relativistic outflow. With respect to the recent simulations of Ruiz et al 2016, we evolve models with lower and more realistic initial magnetic field strengths and, because of computational reasons, we do not evolve the accretion disk for the long timescales that seem to be required in order to see a relativistic outflow. Since all our models produce a similar ordered magnetic field structure, we expect that the results found in Ruiz et al 2016, where they only considered an equal-mass system with an ideal fluid EOS, should be general and, at least from a qualitative point of view, independent from mass-ratio, magnetic field orientation, and EOS.