Abstract➲ We implement a code to find the gravitational News at future null infinity.➲ We use dat... more Abstract➲ We implement a code to find the gravitational News at future null infinity.➲ We use data from a Cauchy code as a boundary data for a characteristic code.➲ The Cauchy-characteristic extraction (CCE) technique allows the extraction of gravitational waves from numerical simulation.➲ We test this method on flat and static black hole spacetimes in various gauges.➲ We show the convergence of the algorithm and illustrate its succes in cases where other wave extraction methods fail.
MoL can be used for hyperbolic, parabolic and even elliptic problems (although I definitely don't... more MoL can be used for hyperbolic, parabolic and even elliptic problems (although I definitely don't recommend the latter). As it currently stands it is set up for systems of equations in the first order type form of equation (2). If you want to implement a multilevel scheme such as leapfrog it is not obvious to me that MoL is the thing to use. However if you have lots of thorns that you want to interact, for example ADM BSSN and a hydro code plus maybe EM or a scalar field, and they can easily be written in this sort of form, then you probably want to use MoL.
There exists plenty of evidence that supermassive black holes (SMBHs) exist in the center of gala... more There exists plenty of evidence that supermassive black holes (SMBHs) exist in the center of galaxies, but their actual formation process has been a mystery for many decades [1]. Several different scenarios have been proposed, some based on stellar dynamics, others on gas hydrodynamics, and still others that combine these processes. There are three major routes to form a SMBH.
Abstract The advanced interferometer network will herald a new era in observational astronomy. Th... more Abstract The advanced interferometer network will herald a new era in observational astronomy. There is a very strong science case to go beyond the advanced detector network and build detectors that operate in a frequency range from 1 Hz to 10 kHz, with sensitivity a factor 10 better in amplitude. Such detectors will be able to probe a range of topics in nuclear physics, astronomy, cosmology and fundamental physics, providing insights into many unsolved problems in these areas.
We perform 3 1 general relativistic simulations of rotating core collapse in the context of the c... more We perform 3 1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase.
ABSTRACT We use time evolutions of the linear perturbation equations to study the oscillations of... more ABSTRACT We use time evolutions of the linear perturbation equations to study the oscillations of rapidly rotating neutrons stars. Our models account for the buoyancy due to composition gradients and we study, for the first time, the nature of the resultant g modes in a fast spinning star. We provide detailed comparisons of non-stratified and stratified models. This leads to an improved understanding of the relationship between the inertial modes of a non-stratified star and the g modes of a stratified system.
Abstract We consider models of relativistic matter containing sharp interfaces across which the m... more Abstract We consider models of relativistic matter containing sharp interfaces across which the matter model changes. These models will be relevant for neutron stars with crusts, phase transitions or for viscous boundaries where the length scale is too short to be modelled smoothly. In particular we look at the numerical techniques that allow us to evolve stable interfaces, for the interfaces to merge, and for strong waves and shocks to interact with the interfaces.
Abstract: We investigate the role of rotational instabilities in the context of black hole format... more Abstract: We investigate the role of rotational instabilities in the context of black hole formation in relativistic stars. In addition to the standard scenario-an axially symmetric dynamical instability forming a horizon at the star's center-the recently found low-$ T/| W| $ instabilities are shown to lead to fragmentation and off-center horizon formation in differentially rotating stars. This process might be an alternative pathway to produce SMBHs from supermassive stars with inefficient angular momentum transport.
We present the first fully relativistic calculations of the crustal strain induced in a neutron s... more We present the first fully relativistic calculations of the crustal strain induced in a neutron star by a binary companion at the late stages of inspiral, employing realistic equations of state for the fluid core and the solid crust. We show that while the deep crust is likely to fail only shortly before coalescence, there is a large variation in elastic strain, with the outermost layers failing relatively early on in the inspiral. We discuss the significance of the results for both electromagnetic and gravitational-wave astronomy.
We present a practical framework for ideal hyperelasticity in numerical relativity. For this purp... more We present a practical framework for ideal hyperelasticity in numerical relativity. For this purpose, we recast the formalism of Carter and Quintana as a set of Eulerian conservation laws in an arbitrary 3+1 split of spacetime. The resulting equations are presented as an extension of the standard Valencia formalism for a perfect fluid, with additional terms in the stress-energy tensor, plus a set of kinematic conservation laws that evolve a configuration gradient. We prove that the equations can be made symmetric hyperbolic by suitable constraint additions, at least in a neighbourhood of the unsheared state. We discuss the Newtonian limit of our formalism and its relation to a second formalism also used in Newtonian elasticity. We validate our framework by numerically solving a set of Riemann problems in Minkowski spacetime, as well as Newtonian ones from the literature.
We discuss the response of neutron stars to the tidal interaction in a compact binary system, as ... more We discuss the response of neutron stars to the tidal interaction in a compact binary system, as encoded in the Love number associated with the induced deformation. This problem is of interest for gravitational-wave astronomy as there may be a detectable imprint on the signal from the late stages of binary coalescence. Previous work has focussed on simple barotropic neutron star models, providing an understanding of the role of the stellar compactness and overall density profile. We add realism to the discussion by developing the framework required to model stars with varying composition and an elastic crust. These effects are not expected to be significant for the next generation of detectors but it is nevertheless useful to be able to quantify them. Our results show that (perhaps surprisingly) internal stratification has no impact whatsoever on the Love number. We also show that crust elasticity provides a (predictably) small correction to existing models.
Abstract➲ We implement a code to find the gravitational News at future null infinity.➲ We use dat... more Abstract➲ We implement a code to find the gravitational News at future null infinity.➲ We use data from a Cauchy code as a boundary data for a characteristic code.➲ The Cauchy-characteristic extraction (CCE) technique allows the extraction of gravitational waves from numerical simulation.➲ We test this method on flat and static black hole spacetimes in various gauges.➲ We show the convergence of the algorithm and illustrate its succes in cases where other wave extraction methods fail.
MoL can be used for hyperbolic, parabolic and even elliptic problems (although I definitely don't... more MoL can be used for hyperbolic, parabolic and even elliptic problems (although I definitely don't recommend the latter). As it currently stands it is set up for systems of equations in the first order type form of equation (2). If you want to implement a multilevel scheme such as leapfrog it is not obvious to me that MoL is the thing to use. However if you have lots of thorns that you want to interact, for example ADM BSSN and a hydro code plus maybe EM or a scalar field, and they can easily be written in this sort of form, then you probably want to use MoL.
There exists plenty of evidence that supermassive black holes (SMBHs) exist in the center of gala... more There exists plenty of evidence that supermassive black holes (SMBHs) exist in the center of galaxies, but their actual formation process has been a mystery for many decades [1]. Several different scenarios have been proposed, some based on stellar dynamics, others on gas hydrodynamics, and still others that combine these processes. There are three major routes to form a SMBH.
Abstract The advanced interferometer network will herald a new era in observational astronomy. Th... more Abstract The advanced interferometer network will herald a new era in observational astronomy. There is a very strong science case to go beyond the advanced detector network and build detectors that operate in a frequency range from 1 Hz to 10 kHz, with sensitivity a factor 10 better in amplitude. Such detectors will be able to probe a range of topics in nuclear physics, astronomy, cosmology and fundamental physics, providing insights into many unsolved problems in these areas.
We perform 3 1 general relativistic simulations of rotating core collapse in the context of the c... more We perform 3 1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase.
ABSTRACT We use time evolutions of the linear perturbation equations to study the oscillations of... more ABSTRACT We use time evolutions of the linear perturbation equations to study the oscillations of rapidly rotating neutrons stars. Our models account for the buoyancy due to composition gradients and we study, for the first time, the nature of the resultant g modes in a fast spinning star. We provide detailed comparisons of non-stratified and stratified models. This leads to an improved understanding of the relationship between the inertial modes of a non-stratified star and the g modes of a stratified system.
Abstract We consider models of relativistic matter containing sharp interfaces across which the m... more Abstract We consider models of relativistic matter containing sharp interfaces across which the matter model changes. These models will be relevant for neutron stars with crusts, phase transitions or for viscous boundaries where the length scale is too short to be modelled smoothly. In particular we look at the numerical techniques that allow us to evolve stable interfaces, for the interfaces to merge, and for strong waves and shocks to interact with the interfaces.
Abstract: We investigate the role of rotational instabilities in the context of black hole format... more Abstract: We investigate the role of rotational instabilities in the context of black hole formation in relativistic stars. In addition to the standard scenario-an axially symmetric dynamical instability forming a horizon at the star's center-the recently found low-$ T/| W| $ instabilities are shown to lead to fragmentation and off-center horizon formation in differentially rotating stars. This process might be an alternative pathway to produce SMBHs from supermassive stars with inefficient angular momentum transport.
We present the first fully relativistic calculations of the crustal strain induced in a neutron s... more We present the first fully relativistic calculations of the crustal strain induced in a neutron star by a binary companion at the late stages of inspiral, employing realistic equations of state for the fluid core and the solid crust. We show that while the deep crust is likely to fail only shortly before coalescence, there is a large variation in elastic strain, with the outermost layers failing relatively early on in the inspiral. We discuss the significance of the results for both electromagnetic and gravitational-wave astronomy.
We present a practical framework for ideal hyperelasticity in numerical relativity. For this purp... more We present a practical framework for ideal hyperelasticity in numerical relativity. For this purpose, we recast the formalism of Carter and Quintana as a set of Eulerian conservation laws in an arbitrary 3+1 split of spacetime. The resulting equations are presented as an extension of the standard Valencia formalism for a perfect fluid, with additional terms in the stress-energy tensor, plus a set of kinematic conservation laws that evolve a configuration gradient. We prove that the equations can be made symmetric hyperbolic by suitable constraint additions, at least in a neighbourhood of the unsheared state. We discuss the Newtonian limit of our formalism and its relation to a second formalism also used in Newtonian elasticity. We validate our framework by numerically solving a set of Riemann problems in Minkowski spacetime, as well as Newtonian ones from the literature.
We discuss the response of neutron stars to the tidal interaction in a compact binary system, as ... more We discuss the response of neutron stars to the tidal interaction in a compact binary system, as encoded in the Love number associated with the induced deformation. This problem is of interest for gravitational-wave astronomy as there may be a detectable imprint on the signal from the late stages of binary coalescence. Previous work has focussed on simple barotropic neutron star models, providing an understanding of the role of the stellar compactness and overall density profile. We add realism to the discussion by developing the framework required to model stars with varying composition and an elastic crust. These effects are not expected to be significant for the next generation of detectors but it is nevertheless useful to be able to quantify them. Our results show that (perhaps surprisingly) internal stratification has no impact whatsoever on the Love number. We also show that crust elasticity provides a (predictably) small correction to existing models.
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Papers by Ian Hawke