Session D05: Simulations of Neutron Stars

Jet-like structures in low-mass binary neutron star merger remnants.

GW170817 and GRB 170817A provided direct evidence that binary neutron star (NSNS) mergers can produce short gamma-ray bursts (sGRBs). However, questions remain about the nature of the central engine. Depending on the mass, the merger remnant may promptly collapse to a black hole (BH), form a hypermassive star which undergoes a delayed collapse to a BH after several tens of rotation periods, a supramassive star with a much longer lifetime, or an indefinitely stable sub-supramassive NS. We have performed general relativistic magnetohydrodynamics (GRMHD) simulations of the merger of both irrotational and spinning, equal-mass NSNSs constructed from a realistic SLy equation of state, with a range of gravitational (ADM) masses from 1.96 to 2.70 M_Sun. Each NS is endowed with a dipolar magnetic field extending from the interior into the exterior, as in a radio pulsar. The remnants are sub-supramassive for the lowest mass case, supramassive for the intermediate cases and hypermassive for the highest mass case. The latter is the only case that collapses to a BH before our simulation ends. We observe helical magnetic field structures and mildly relativistic outflow from the poles for both the supramassive and hypermassive remnants, and are investigating whether these jet-like structures can be the progenitors of sGRBs.   

Exploring Dynamical Ejecta in Black Hole-Neutron Star Binaries: Neutron Star Disruption and a Dual Potential for Kilonovae and Compact Binary Gamma-Ray Bursts

Merging black hole–neutron star (BHNS) binaries serve as an important production mechanism in the creation of gravitational waves (GWs) and are also a proposed origin of UV/optical/infrared kilonova signals and gamma-ray bursts (GRBs). For a BHNS merger to produce one of these electromagnetic (EM) transients, the spin of the black hole must be sufficiently high, creating a tidal field strong enough to disrupt the neutron star. With spins higher than this threshold, a disrupting BHNS binary may eject a few percent of a solar mass of matter, leading to observable kilonovae driven by radioactive decays in the ejecta, and/or a compact-binary GRB (cbGRB) resulting from the formation of a massive, hot accretion disk. In this talk, I summarize new and updated results from simulations of BHNS binaries with black hole spins above the predicted spin limit for disruption. These simulations serve as an exploration into the behavior of dynamical ejecta close to this disruption threshold using the Spectral Einstein Code (SpEC) for binaries described by SFHo and DD2 equations of state.   

Impact of a mean field dynamo on jet and wind launching from neutron-star merger remnants

We present a mean field model for the αΩ-dynamo potentially active in the post-merger phase of a binary neutron star coalescence. We introduce simplified equations for ideal general relativistic magnetohydrodynamics (GRMHD) with an additional α−term, which closely resemble their Newtonian counterpart, but remain compatible with standard numerical relativity simulations. We propose a heuristic dynamo closure relation for the magnetorotational instability-driven turbulent dynamo in the outer layers of a differentially rotating magnetar remnant and its accretion disk. We demonstrate that depending on the strength of the dynamo action, magnetically driven outflows from the magnetar remnant can be present with their amount of baryon loading correlating with the magnetic field amplification. Our results are consistent with the expectation that substantial dynamo amplification (either during or after the merger) may be required for neutron-star remnants to power short gamma-ray bursts or precursors thereof. Finally, we provide an outlook into the subsequent evolution of the system, including novel radiative gamma-ray burst signatures.   

Evolution of Neutron Stars with a Dark Matter Core

We use the consistent quasi-equilibrium initial data implementation of the Sgrid program for dark matter admixed neutron stars to create initial data with the baryonic matter and the dark matter spinning independently. The baryonic matter is modeled with the SLy equation of state, and the dark matter is treated as a degenerate Fermi gas, with the two fluids coupled minimally through gravity without any other kind of interaction. In these cases, the dark matter forms a core inside the baryonic matter, and then different independent spins are imparted to the two fluids. We then study the properties of these configurations, such as binding energy and deformation. We further utilize the corresponding implementation in the BAM program to evolve these neutron stars with dark matter cores and study the effects on gravitational wave signal and matter behavior, such as fluid spin interactions, etc.   

Self-gravitating neutron star-disks in general relativity

Binary neutron star mergers or accreting compact objects other than black holes can lead to a self-gravitating neutron star-disk system. Using the COCAL initial value code, we compute self-gravitating neutron star-disk spacetimes for a variety of neutron star equations of state and rotation parameters, as well as for different disk angular velocity profiles. We make direct comparisons with similar self-gravitating disks around black holes.   

Validating Binary Neutron Star Simulations with Insights from GW170817 and GW190425

Binary neutron star mergers are key to understanding extreme physics, the origins of gamma-ray bursts, fast radio bursts and heavy elements. With limited observations, we rely on numerical relativity simulations for advancing our knowledge. However, accurately modeling BNS collisions require ongoing refinement due to complex multi-physics, numerical methods, and high-performance computing.

Existing simulations show similarities in convergence properties and dynamics, but differ in predictions like merger timing and gravitational wave characteristics. This leads us to compare results obtained by various codes simulating the real-world events GW170817 and GW190425. We collect simulations results and compare them, focusing on four sets of indicators: initial conditions, collision time, gravitational wave and matter output and remnant fate. If we find significant differences between the results, we investigate the sources of disagreement, which  allow us to identify the role played by the numerical and analytical methods implemented. Our aim is to distinguish what are the numerical differences affecting these simulations.

This study is vital for future gravitational wave astronomy, considering the upcoming generation of GW observatories are promising a surge in BNS systems detections. Our work will enhance simulation reliability, paving the way for significant astronomical discoveries and improved gravitational wave templates.   

Stability study of differentially rotating relativistic stars

Stability properties have been examined in detail in the past for uniformly rotating stars. One of the important results for testing the stability of relativistic stars is the turning point criterion. According to the criterion, along a constant angular momentum sequence of mass-central energy density curve of equilibrium stars, instability occurs before the turning point. While this criterion has been investigated for differentially rotating stars in recent years, it has primarily been studied with non-realistic rotation laws (such as the j-constant law) and isentropic stars. Our study aims to extend this criterion to two different rotation laws, one of which is similar to the post-merger neutron star rotation profile, and considers different entropy profiles, including non-isentropic ones.   

Accelerating Binary Neutron Star Merger Simulations with Artificial Neural Networks

Artificial neural networks (ANNs) are fast becoming an important element in the toolkit we use to tackle complex computational problems, with GPUs making up the majority of available horsepower on many of the latest generation of computing clusters. In this talk I will discuss how ANNs can be used to accelerate a modern general-realtivistic radiation magneto-hydrodynamics code aimed at exploring binary neutron star (BNS) mergers. In particular I will give a brief description of ANNs and how they work, explore where ANNs may be useful in the context of simulating BNS mergers, and demonstrate that ANNs are able to increase the speed of even our CPU based code.