arXiv subjects

Peter Hammond

Publications and source records attributed to Peter Hammond.

11 recordsLinked to original sources

arXiv2026

Transport Properties of the MRI in Differentially Rotating Neutron Stars

We perform three-dimensional, high-resolution, general-relativistic magnetohydrodynamics (GRMHD) simulations of the magnetorotational instability (MRI) in differentially rotating neutron stars. We consider both high-mass models which collapse either promptly, or due to the outward transport of angular momentum removing rotational support, and lower-mass models, which remain stable even after solid-body rotation has been achieved. We measure effective transport coefficients of the resulting turbulent flow, shear-viscosity $\alpha_{\rm vis}$, mixing-length $\ell_{\rm mix}$, and mean-field dynamo $\alpha_{\rm DYN}$-parameter, and their correlations with mean-flow parameters, such as rest-mass density $\rho$, characteristic wavelength of the MRI $\lambda_{\rm MRI}$, and vertical Alfv\'en velocity $v_A$. At saturation, $\ell_{\rm mix}$ and $\alpha_{\rm DYN}$ correlate with $\lambda_{\rm MRI}$ and $v_A$ respectively, as expected from dimensional analysis. However, both quantities show deviations of roughly an order of magnitude from the values predicted on the basis of these correlations. They are also highly intermittent in space and time. The mixing length is found to be largely independent of density, calling into question the $\ell_{\rm mix}(\rho)$ ansatz used in many general-relativistic large-eddy simulations. Our results show that, while the effects of MRI-induced turbulence might be qualitatively reproduced by simulations that employ transport coefficients chosen using dimensional-analysis considerations, fully-resolved GRMHD simulations are needed to make quantitative predictions.

astro-ph.HE↗

arXiv2026

GR-Athena++: Binary Neutron Star Merger Simulations with Neutrino Transport

We present general-relativistic radiation magnetohydrodynamics simulations of binary neutron star mergers performed with GR-Athena++. Neutrino transport is treated using a moment-based, energy-integrated scheme (M1), augmented by neutrino number density evolution (N0). Our implementation is validated through an extensive suite of standard tests and demonstrated to perform robustly under adaptive mesh refinement. As a first application, we simulate the gravitational collapse of a uniformly rotating, magnetized neutron star, demonstrating stable radiation evolution through apparent-horizon formation using a novel excision technique based on the tapering of state vector evolution inside the horizon. To further test robustness in highly dynamic environments, we apply our code to two demanding binary neutron star merger scenarios. We investigate a long-lived remnant with the DD2 equation of state, evolved with full general-relativistic magnetohydrodynamics and M1 neutrino transport. Following this, a gravitational collapse scenario with the SFHo equation of state is explored. We showcase long-term stable evolution on neutrino cooling time-scales, demonstrating robust handling of excision and stable evolution of the post-collapse accretion phase in three-dimensional mergers with magnetic fields and neutrino radiation.

gr-qc↗

arXiv2025

Magnetic Field Configurations in Binary Neutron Star Mergers II: Inspiral, Merger and Ejecta

We perform a series of simulations of magnetised Binary Neutron Star mergers, with varying magnetic field topologies in the initial data, as well as varying Equations of State, and mass ratios. In this paper, a companion paper to arXiv:2506.18995, we analyse the impact of the initial field configuration on the gravitational wave signal, the amplification of the magnetic field, and the ejected material. We investigate the dependence of the phase evolution of the gravitational wave in the post-merger on the initial magnetic field, finding that dephasing between the $(\ell=2,m=2)$ mode of the gravitational wave, and the $(2,1)$ and $(3,3)$ modes may be strongly impacted by the numerical reconstruction scheme. The magnetic field amplification during the Kelvin-Helmholtz dominated phase may be considerably enhanced by anti-aligned fields, or suppressed by toroidal fields. The post-merger amplification of the field due to winding may be suppressed by toroidal fields, and enhanced by asymmetries or mixtures of poloidal and toroidal fields. The field strength in the ejecta may be impacted by the initial magnetic field, with configurations which lead to large amplifications and those with mixtures of poloidal and toroidal fields preferentially emitting highly magnetised material in the polar regions, showing a weaker dependence of the magnetic field on the density of the ejecta than in cases that amplify the magnetic field less. We find that the magnetic field is largely randomly oriented in the ejected material, supporting such models used to estimate thermalisation timescales of ejected material. We find that configurations which begin with an initial bitant symmetry break this symmetry uniformly, independent of the initial configuration, when evolved without an enforced symmetry. This behaviour suggests the presence of a spontaneous symmetry breaking bifurcation in the solution.

astro-ph.HE↗

arXiv2025

Magnetic Flux Emergence in Binary Neutron Star Remnants

Using high-resolution AthenaK simulations of a twisted toroidal flux tube, we study the flux emergence of magnetic structures in the shear layer of a hot massive neutron star typical of a binary neutron star remnant. High-resolution simulations demonstrate that magnetic buoyant instabilities allow for emergence only for extremely large magnetic fields significantly exceeding $10^{17}~\mathrm{G}$, and more typical fields around $10^{16}~\mathrm{G}$ are instead dominated by hydrodynamic effects. Because merger remnants tend to be stable against hydrodynamic convection, our work places strong limitations on the mechanisms by which massive binary neutron star remnants can produce the magnetically-driven outflows needed to power jets.

astro-ph.HE↗

arXiv2025

Recurring region for neutron-star observables

In this letter, we report a novel, somewhat analytical way to produce equations of state (EOSs) that generate particular values of neutron star mass, radius, and tidal deformability. This is possible because our description for the EoS of dense matter can produce recurring regions, small areas where several EoSs cross in the mass-radius and mass-tidal deformability diagrams. We can place recurring regions in desired locations of these diagrams, corresponding e.g., to a given observation. Our prescription is versatile, in the sense that different microscopic models can be used for the low density hadronic phase and high density quark phase, as long as they are connected by a percolation, a description that mimics quark deconfinement and is motivated by QCD. The several EoSs that pass by a recurring region can present different thresholds for the boundaries of the percolation region (different beginning and ending for the quark deconfinement region), as well as different orders for the phase transition at the boundaries. When combining all these features, our prescription allows one not only to produce an EoS that matches an observation, but also one that matches specific chosen criteria for the EoS. The EoSs produced by this new method will be specially suitable for the study of dense-matter properties in future gravitational-wave observations, when both the inspiral and post-merger phase signals will become available. Our numerical code that calculates recurring regions using CompOSE microscopic EoSs is open source and publicly available.

astro-ph.HE↗

arXiv2025

Magnetic Field Configurations in Binary Neutron Star Mergers I: Post-merger Remnant and Disk

We present a suite of general relativistic magnetohydrodynamic (GRMHD) simulations of binary neutron star (BNS) mergers performed with the code GR-Athena++. We investigate how a different initial magnetic field configuration, nuclear equation of state, or binary mass ratio affects the magnetic and thermodynamic evolution of the post-merger remnant and disk. We also analyze the impact of the commonly-assumed reflection (bitant) symmetry across the equatorial plane. Magnetic field amplification occurs shortly after the merger due to the Kelvin-Helmholtz instability; later, the field keeps evolving with a predominantly toroidal configuration due to winding and turbulence. The initial magnetic field topology leaves an imprint on the field structure and affects magnetic field amplification for the initial magnetic field values commonly assumed in the literature and the limited resolution of the simulations. Enforcing equatorial reflection symmetry partially suppresses the development of turbulence near the equatorial plane and impacts the post-merger magnetic field evolution. Stiffer EOSs produce larger, less compact remnants that may retain memory of the pre-merger strong poloidal field.

astro-ph.HE↗

arXiv2024

Numerical relativity simulations of compact binaries: comparison of cell- and vertex-centered adaptive meshes

Given the compact binary evolution problem of numerical relativity, in the finite-difference, block-based, adaptive mesh refinement context, choices must be made on how evolved fields are to be discretized. In GR-Athena++, the space-time solver was previously fixed to be vertex-centered. Here, our recent extensions to a cell-centered treatment, are described. Simplifications in the handling of variables during the treatment of general relativistic magneto-hydrodynamical (GRMHD) evolution are found. A novelty is that performance comparison for the two choices of grid sampling is made within a single code-base. In the case of a binary black hole inspiral-merger problem, by evolving geometric fields on vertex-centers, an average $\sim 20\%$ speed increase is observed, when compared against cell-centered sampling. The opposite occurs in the GRMHD setting. A binary neutron star inspiral-merger-collapse problem, representative of typical production simulations is considered. We find that cell-centered sampling for the space-time solver improves performance, by a similar factor.

gr-qc↗

arXiv2023

GR-Athena++: General-relativistic magnetohydrodynamics simulations of neutron star spacetimes

We present the extension of GR-Athena++ to general-relativistic magnetohydrodynamics (GRMHD) for applications to neutron star spacetimes. The new solver couples the constrained transport implementation of Athena++ to the Z4c formulation of the Einstein equations to simulate dynamical spacetimes with GRMHD using oct-tree adaptive mesh refinement. We consider benchmark problems for isolated and binary neutron star spacetimes demonstrating stable and convergent results at relatively low resolutions and without grid symmetries imposed. The code correctly captures magnetic field instabilities in non-rotating stars with total relative violation of the divergence-free constraint of $10^{-16}$. It handles evolutions with a microphysical equation of state and black hole formation in the gravitational collapse of a rapidly rotating star. For binaries, we demonstrate correctness of the evolution under the gravitational radiation reaction and show convergence of gravitational waveforms. We showcase the use of adaptive mesh refinement to resolve the Kelvin-Helmholtz instability at the collisional interface in a merger of magnetised binary neutron stars. GR-Athena++ shows strong scaling efficiencies above $80\%$ in excess of $10^5$ CPU cores and excellent weak scaling is shown up to $\sim 5 \times 10^5$ CPU cores in a realistic production setup. GR-Athena++ allows for the robust simulation of GRMHD flows in strong and dynamical gravity with exascale computers.

gr-qc↗

arXiv2023

Neutrino trapping and out-of-equilibrium effects in binary neutron star merger remnants

We study out-of-thermodynamic equilibrium effects in neutron star mergers with 3D general-relativistic neutrino-radiation large-eddy simulations. During merger, the cores of the neutron stars remain cold ($T \sim$ a few MeV) and out of thermodynamic equilibrium with trapped neutrinos originating from the hot collisional interface between the stars. However, within ${\sim}2{-}3$ milliseconds matter and neutrinos reach equilibrium everywhere in the remnant. Our results show that dissipative effects, such as bulk viscosity, if present, are only active for a short window of time after the merger.

astro-ph.HE↗

arXiv2022

Detecting the impact of nuclear reactions on neutron star mergers through gravitational waves

Nuclear reactions may affect gravitational-wave signals from neutron-star mergers, but the impact is uncertain. In order to quantify the effect, we compare two numerical simulations representing intuitive extremes. In one case reactions happen instantaneously. In the other case, they occur on timescales much slower than the evolutionary timescale. We show that, while the differences in the two gravitational-wave signals are small, they should be detectable by third-generation observatories. To avoid systematic errors in equation of state parameters inferred from observed signals, we need to accurately implement nuclear reactions in future simulations.

astro-ph.HE↗

arXiv2021

Thermal aspects of neutron star mergers

In order to extract maximal information from neutron-star merger signals, both gravitational and electromagnetic, we need to ensure that our theoretical models/numerical simulations faithfully represent the extreme physics involved. This involves a range of issues, with the finite temperature effects regulating many of the relevant phenomena. As a step towards understanding these issues, we explore the conditions for $β$-equilibrium in neutron star matter for the densities and temperatures reached in a binary neutron star merger. Using the results from our out-of-equilibrium merger simulation, we consider how different notions of equilibrium may affect the merger dynamics, raising issues that arise when attempting to account for these conditions in future simulations. These issues are both computational and conceptual. We show that the effects lead to, in our case, a softening of the equation of state in some density regions, and to composition changes that affect processes that rely on deviation from equilibrium, such as bulk viscosity, both in terms of the magnitude and the equilibration timescales inherent to the relevant set of reactions. We also demonstrate that it is difficult to determine exactly which equilibrium conditions are relevant in which regions of the matter due to the dependence on neutrino absorption, further complicating the calculation of the reactions that work to restore the matter to equilibrium.

astro-ph.HE↗