arXiv subjects

Jamie Bamber

Publications and source records attributed to Jamie Bamber.

15 recordsLinked to original sources

arXiv2025

Impact of Magnetic Field Topology on Electromagnetic and Gravitational Waves from Binary Neutron Star Merger Remnants

We perform general relativistic magnetohydrodynamic (GRMHD) simulations of binary neutron star (BNS) mergers with four distinct magnetic field topologies: (i) a dipole pulsar-like configuration, (ii) a mixed linear superposition of poloidal and toroidal components inside the star, and (iii-iv) two topologies featuring a smooth transition from a confined mixed core to a pulsar-like structure at radii $0.95\,R_{\rm NS}$ and $0.5\,R_{\rm NS}$, with $R_{\rm NS}$ the radius of the star. The latter topologies are explored in BNS merger studies for the first time. We evolve systems with two equations of state (EoS), SLy and WFF1, with ADM masses 2.7 and 2.6, respectively, and include an additional lower-mass SLy binary to probe the behavior of long-lived remnants. We perform an extensive analysis of the emission properties of the systems, both electromagnetic and gravitational waves, and of the properties of the remnants, namely their frequency modes, density eigenfunctions, rotation, temperature, and convective stability. We report three key results: (1) for the first time, we assess the convective stability of magnetized remnants, extending previous unmagnetized analyses; (2) we identify a clear secondary peak in the gravitational-wave spectrum of pulsar-like configurations, consistent with the nonlinear coupling of the $m=0$ and $m=2$ modes, which is absent in other topologies; and (3) the magnetic field topology strongly influences the gravitational wave emission properties to the extent that nearby ($<50\,{\rm Mpc}$) events could allow one to observationally distinguish between different field structures with future gravitational-wave detectors. Across all models, we obtain luminosities compatible with short gamma-ray bursts (sGRBs), with purely poloidal configurations being the most efficient in driving possible relativistic jets.

astro-ph.HE↗

arXiv2025

The Gravitational Wave Memory from Binary Neutron Star Mergers

The gravitational wave signal produced by the merger of two compact objects includes both an oscillatory transient and a non-oscillatory part, the so-called memory effect. This produces a permanent displacement of test masses and has not yet been measured. We use general relativistic magnetohydrodynamic simulations, including neutrinos, with several representative viable equations of state, to quantify--for the first time--the effects of the neutron star magnetic field, neutrino emission, and the ejected mass on the linear and nonlinear displacement memory in binary neutron star mergers. We find that the additional contributions due to the emission of electromagnetic radiation, neutrinos and baryonic ejecta can be ~15% of the total memory for moderate magnetic fields and up to ~50% for extreme magnetic fields. The memory is most affected by changes in the equation of state, the binary mass, and the magnetic field. In particular, for moderate premerger field strengths, the dominant impact of the electromagnetic field is the change in the gravitational wave luminosity, and the associated gravitational wave null memory, due to the unstable growth of the magnetic field and the resulting redistribution of angular momentum it induces in the remnant. While the direct electromagnetic contribution to the null memory is additive, the change in the gravitational wave null memory can--in some cases--result in the total memory being smaller than that from the corresponding nonmagnetized binary. Furthermore, in contrast to binary black hole mergers, the growth of the memory in binary neutron star mergers is extended due to the long emission timescale of electromagnetic fields, neutrinos, and ejecta. These results necessitate the consideration of the magnetic field, as well as the equation of state, for accurate parameter estimation in future analyses of gravitational wave memory data.

gr-qc↗

arXiv2025

Evolution of a black hole cluster in full general relativity

We evolve for the first time in full general relativity a small, collisional N-body black hole cluster of arbitrary total mass M. The bound cluster is initially compact (radius R/M~10), stable, and consists of 25 equal-mass, nonspinning black holes. The dynamical interactions of compact objects in N-body clusters is of great interest for the formation of black holes in the upper mass gap as well as intermediate and supermassive black holes. These are potential sources of gravitational waves that may be detected by both current and future observatories. Unlike previous N-body Newtonian and post-Newtonian simulations, no "subgrid physics" is required to handle collisions and mergers. We can therefore confirm in full general relativity several predictions from these simulations and analytic estimates: the runaway growth of a large black hole via repeated mergers; spindown of the central black hole with increasing captures; the ejection of a black hole with a large asymptotic velocity due to a several-body interaction; and a regime where mergers occur primarily via direct collisions on highly eccentric orbits instead of quasicircular inspirals. We extract the gravitational wave signal and find it has several distinct features associated with the compact cluster regime. Our results suggest the signal is sufficiently loud that next generation observatories would likely be able to detect similar events across most of the observable universe. This work is a preliminary proof-of-principle study that we hope will open up a new arena for numerical relativity and the study of N-body compact systems.

gr-qc↗

arXiv2025

GRTresna: An open-source code to solve the initial data constraints in numerical relativity

GRTresna is a multigrid solver designed to solve the constraint equations for the initial data required in numerical relativity simulations. In particular, it is focussed on scenarios with fundamental fields around black holes and inhomogeneous cosmological spacetimes. The code is based on the formalism in Aurrekoetxea, Clough \& Lim arXiv:2207.03125 and can be found at https://github.com/GRTLCollaboration/GRTresna

gr-qc↗

arXiv2024

Masking equation of state effects in binary neutron star mergers

Recent nonmagnetized studies of binary neutron star mergers have indicated the possibility of identifying equation of state features, such as a phase transition or a quark-hadron crossover, based on the frequency shift of the main peak in the postmerger gravitational wave spectrum. By performing a series of general relativistic, magnetohydrodynamic simulations we show that similar frequency shifts can be obtained due to the effect of the magnetic field. The existing degeneracy can either mask or nullify a shift due to a specific equation of state feature, and therefore the interpretation of observational data is more complicated than previously thought, requiring a more complete treatment that would necessarily include the neutron star's magnetic field.

gr-qc↗

arXiv2024

Postmerger multimessenger analysis of binary neutron stars: Effect of the magnetic field strength and topology

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 $5.5\times 10^{15}G$ to $2.2\times 10^{17}G$ 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 $\sim 200$Hz for the strongest magnetic field we consider and $\sim 50$Hz 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.

gr-qc↗

arXiv2024

Jetlike 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 remnant from a NSNS merger may promptly collapse to a black hole (BH), form a hypermassive neutron star (HMNS) which undergoes a delayed collapse to a BH, a supramassive neutron star (SMNS) with a much longer lifetime, or an indefinitely stable NS. There is strong evidence that a BH with an accretion disk can launch a sGRB-compatible jet via the Blandford-Znajek mechanism, but whether a supramassive star can do the same is less clear. We have performed general relativistic magnetohydrodynamics simulations of the merger of both irrotational and spinning, equal-mass NSNSs constructed from a piecewise polytropic representation of the SLy equation of state, with a range of gravitational masses that yield remnants with mass above and below the supramassive limit. Each NS is endowed with a dipolar magnetic field extending from the interior into the exterior, as in a radio pulsar. We examine cases with different initial binary masses, including a case which produces a HMNS which collapses to a BH, and lower mass binaries that produce SMNS remnants. We find similar jetlike structures for both the SMNS and HMNS remnants that meet our basic critera for an incipient jet. The outflow for the HMNS case is consistent with a Blandford-Znajek (BZ) jet. There is sufficient evidence that such BZ-powered outflows can break out and produce ulrarelativistic jets so that we can describe the HMNS system as a sGRB progenitor. However, the incipient jets from the SMNS remnants have much more baryon pollution and we see indications of inefficient acceleration and mixing with the surrounding debris. Therefore, we cannot conclude that SMNS outflows are the progenitors of sGRBs.

astro-ph.HE↗

arXiv2023

The effect of wave dark matter on equal mass black hole mergers

For dark matter to be detectable with gravitational waves from binary black holes, it must reach higher than average densities in their vicinity. In the case of light (wave-like) dark matter, the density of dark matter between the binary can be significantly enhanced by accretion from the surrounding environment. Here we show that the resulting dephasing effect on the last ten orbits of an equal mass binary is maximized when the Compton wavelength of the scalar particle is comparable to the orbital separation, $2\pi/\mu\sim d$. The phenomenology of the effect is different from the channels that are usually discussed, where dynamical friction (along the orbital path) and radiation of energy and angular momentum drive the dephasing, and is rather dominated by the radial force (the spacetime curvature in the radial direction) towards the overdensity between the black holes. Whilst our numerical studies limit us to scales of the same order, this effect may persist at larger separations and/or particle masses, playing a significant role in the merger history of binaries.

gr-qc↗

arXiv2023

GRDzhadzha: A code for evolving relativistic matter on analytic metric backgrounds

GRDzhadzha is an open-source code for relativistic simulations of matter fields on curved spacetimes that admit an analytic description (e.g. stationary black holes). It is based on the publicly available 3+1D numerical relativity code GRChombo. Such a description is valid where the density of the matter is small compared to the curvature scale of the spacetime, which is the case for many physical scenarios - for example, dark matter environments. The approach offers significant savings on memory and speed compared to running full numerical relativity simulations, since the metric variables and their derivatives are calculated analytically, and therefore are not evolved or stored on the grid. This brief paper introduces the code and gives details of some applications for which it has already been used.

gr-qc↗

arXiv2022

Black hole merger simulations in wave dark matter environments

The interaction of binary black hole mergers with their environments can be studied using numerical relativity simulations. These start only a short finite time before merger, at which point appropriate initial conditions must be imposed. A key task is therefore to identify the configuration that is appropriate for the binary and its environment at this stage of the evolution. In this work we study the behaviour of wave dark matter around equal mass black hole binaries, finding that there is a preferred, quasi-stationary profile that persists and grows over multiple orbits, in contrast to heavier mass dark matter where any overdensity tends to be dispersed by the binary motion. Whilst different initial configurations converge to the preferred quasi-stationary one after several orbits, unwanted transient oscillations are generated in the process, which may impact on the signal in short simulation runs. We also point out that naively superimposing the matter onto a circular binary results in artificially eccentric orbits due to the matter backreaction, which is an effect of the initial conditions and not a signature of dark matter. We discuss the further work required so that comparison of waveforms obtained with environments to vacuum cases can be done in a meaningful way.

gr-qc↗

arXiv2022

Fifth forces and frame invariance

I discuss how one can apply the covariant formalism developed by Vilkovisky and DeWitt to obtain frame invariant fifth force calculations for scalar-tensor theories. Fifth forces are severely constrained by astrophysical measurements. It was shown previously that for scale-invariant Higgs-dilaton gravity, in a particular choice of Jordan frame, the dilaton fifth force is dramatically suppressed, evading the observational constraints. Using a geometric approach I extend this result to all frames, and show that the usual dichotomy of "Jordan frame" versus "Einstein frame" is better understood as a continuum of frames: submanifold slices of a more general field space.

gr-qc↗

arXiv2022

GRChombo: An adaptable numerical relativity code for fundamental physics

GRChombo is an open-source code for performing Numerical Relativity time evolutions, built on top of the publicly available Chombo software for the solution of PDEs. Whilst GRChombo uses standard techniques in NR, it focusses on applications in theoretical physics where adaptability, both in terms of grid structure, and in terms of code modification, are key drivers.

gr-qc↗

arXiv2021

The quasi normal modes of growing dirty black holes

The ringdown of a perturbed black hole contains fundamental information about space-time in the form of Quasi Normal Modes (QNM). Modifications to general relativity, or extended profiles of other fields surrounding the black hole, so called "black hole hair", can perturb the QNM frequencies. Previous works have examined the QNM frequencies of spherically symmetric "dirty" black holes - that is black holes surrounded by arbitrary matter fields. Such analyses were restricted to static systems, making the assumption that the metric perturbation was independent of time. However, in most physical cases such black holes will actually be growing dynamically due to accretion of the surrounding matter. Here we develop a perturbative analytic method that allows us to compute for the first time the time dependent QNM deviations of such growing dirty black holes. Whilst both are small, we show that the change in QNM frequency due to the accretion can be of the same order or larger than the change due to the static matter distribution itself, and therefore should not be neglected in such calculations. We present the case of spherically symmetric accretion of a complex scalar field as an illustrative example, but the method has the potential to be extended to more complicated cases.

gr-qc↗

arXiv2020

Growth of accretion driven scalar hair around Kerr black holes

Scalar fields around compact objects are of interest for scalar-tensor theories of gravity and dark matter models consisting of a massive scalar, e.g. axions. We study the behaviour of a scalar field around a Kerr black hole with non trivial asymptotic boundary conditions - both non zero density and non zero angular momentum. Starting from an initial radially homogeneous configuration, a scalar cloud is accreted, which asymptotes to known stationary configurations over time. We study the cloud growth for different parameters including black hole spin, scalar field mass, and the scalar field density and angular momentum far from the black hole. We characterise the transient growth of the mass and angular momentum in the cloud, and the spatial profile of the scalar around the black hole, and relate the results of fully non-linear simulations to an analytic perturbative expansion. We also highlight the potential for these accreted clouds to create monochromatic gravitational wave signals - similar to the signals from superradiant clouds, although significantly weaker in amplitude.

gr-qc↗

arXiv2019

Beyond the Runge-Kutta-Wentzel-Kramers-Brillouin method

We explore higher-dimensional generalizations of the Runge-Kutta-Wentzel-Kramers-Brillouin method for integrating coupled systems of first-order ordinary differential equations with highly oscillatory solutions. Such methods could improve the performance and adaptability of the codes which are used to compute numerical solutions to the Einstein-Boltzmann equations. We test Magnus expansion-based methods on the Einstein-Boltzmann equations for a simple universe model dominated by photons with a small amount of cold dark matter. The Magnus expansion methods achieve an increase in run speed of about 50% compared to a standard Runge-Kutta integration method. A comparison of approximate solutions derived from the Magnus expansion and the Wentzel-Kramers-Brillouin (WKB) method implies the two are distinct mathematical approaches. Simple Magnus expansion solutions show inferior long range accuracy compared to WKB. However we also demonstrate how one can improve on the standard Magnus approach to obtain a new "Jordan-Magnus" method. This has a WKB-like performance on simple two-dimensional systems, although its higher-dimensional generalization remains elusive.

physics.comp-ph↗