Postmerger Multimessenger Analysis of Binary Neutron Stars: Effect of the Magnetic Field Strength and Topology

Jamie Bamber,Antonios Tsokaros,Milton Ruiz,Stuart L. Shapiro

arXiv · （2024）

Cited 0|Views4

Abstract

The oscillation modes of neutron star (NS) merger remnants, as encoded by the kHz postmerger gravitational wave (GW) signal, hold great potential for constraining the as-yet undetermined equation of state (EOS) of dense nuclear matter. Previous works have used numerical relativity simulations to derive quasi-universal relations for the key oscillation frequencies, but most of them omit the effects of a magnetic field. We conduct full general-relativistic magnetohydrodynamics simulations of NSNS mergers with two different masses and two different EOSs (SLy and ALF2) with three different initial magnetic field topologies (poloidal and toroidal only, confined to the interior, and "pulsar-like": dipolar poloidal extending from the interior to the exterior), with four different magnetic field strengths with maximum values ranging from from 5.5× 10^15G to 2.2× 10^17G at the time of insertion. We find that magnetic braking and magnetic effective turbulent viscosity drives the merger remnants towards uniform rotation and increases their overall angular momentum loss. As a result, the f_2 frequency of the dominant postmerger GW mode shifts upwards over time. The overall shift is up to ∼ 200Hz for the strongest magnetic field we consider and ∼ 50Hz for the median case and is therefore detectable in principle by future GW observatories, which should include the magnetic field in their analyses. We also explore the impact of the magnetic field on the postmerger electromagnetic emission, and demonstrate that an extremely large magnetic field, or alternatively a significant shear viscosity mechanism, can cause a supramassive NS remnant to collapse to a BH in less than 100ms and lead to jet formation, although we do not expect the conditions for such an outcome to be realistic.

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【要点】：本文通过全广义相对论磁流体动力学模拟研究了双中子星合并后磁场的强度和拓扑对引力波信号的影响，发现了磁场对中子星合并遗迹的振荡模式及电磁辐射的显著效应。

【方法】：文章使用全广义相对论磁流体动力学方法，模拟了两种不同质量的双中子星合并，两种不同的物态方程（SLy和ALF2），三种不同的初始磁场拓扑（仅限于内部的多极子和环状，以及从内部延伸到外部的“脉冲星式”偶极子多极子），以及四种不同强度的磁场。

【实验】：通过模拟，研究了磁场对合并遗迹的旋转、角动量损失以及引力波f_2频率的影响，并使用的数据集包括了不同磁场强度和拓扑条件下的模拟结果。结果表明，磁场导致的磁阻尼和有效湍流粘度使得遗迹趋向于均匀旋转，增加了角动量损失，导致引力波f_2频率随时间上升，最强磁场条件下频率整体上升约200Hz，中等情况下上升约50Hz，这在未来引力波观测中是可检测的。同时，研究还发现极高磁场或显著剪切粘度机制可能导致超质量中子星遗迹在100ms内坍缩成黑洞并形成喷流。

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