arXiv subjects

Alexander Knight

Publications and source records attributed to Alexander Knight.

4 recordsLinked to original sources

arXiv2023

Gravitational Waves from Binary Neutron Star Mergers with a Spectral Equation of State

In numerical simulations of binary neutron star systems, the equation of state of the dense neutron star matter is an important factor in determining both the physical realism and the numerical accuracy of the simulations. Some equations of state used in simulations are $C^2$ or smoother in the pressure/density relationship function, such as a polytropic equation of state, but may not have the flexibility to model stars or remnants of different masses while keeping their radii within known astrophysical constraints. Other equations of state, such as tabular or piece-wise polytropic, may be flexible enough to model additional physics and multiple stars' masses and radii within known constraints, but are not as smooth, resulting in additional numerical error. We will study in this paper a recently developed family of equation of state, using a spectral expansion with sufficient free parameters to allow for a larger flexibility than current polytropic equations of state, and with sufficient smoothness to reduce numerical errors compared to tabulated or piece-wise polytropic equations of state. We perform simulations at three mass ratios with a common chirp mass, using two distinct spectral equations of state, and at multiple numerical resolutions. We evaluate the gravitational waves produced from these simulations, comparing the phase error between resolutions and equations of state, as well as with respect to analytical models. From our simulations we estimate that the phase difference at merger for binaries with a dimensionless weighted tidal deformability difference greater than $\Delta \tilde{\Lambda} = 55$ can be captured by the SpEC code for these equations of state.

astro-ph.HE↗

arXiv2023

Van der Waals Nanoantennas on Gold as Hosts for Hybrid Mie-Plasmonic Resonances

Dielectric nanoresonators have been shown to circumvent the heavy optical losses associated with plasmonic devices, however they suffer from less confined resonances. By constructing a hybrid system of both dielectric and metallic materials, one can retain the low losses of dielectric resonances, whilst gaining additional control over the tuning of the modes with the metal, and achieving stronger mode confinement. In particular, multi-layered van der Waals materials are emerging as promising candidates for integration with metals owing to their weak attractive forces, which enable deposition onto such substrates without the requirement of lattice matching. Here we use layered, high refractive index WS$_2$ exfoliated on gold, to fabricate and optically characterize a hybrid nanoantenna-on-gold system. We experimentally observe a hybridization of Mie resonances, Fabry-P\'erot modes, and surface plasmon-polaritons launched from the nanoantennas into the substrate. We achieve experimental quality factors of Mie-plasmonic modes of up to 20 times that of Mie resonances in nanoantennas on silica, and observe signatures of a supercavity mode with a Q factor of 263 $\pm$ 28, resulting from strong mode coupling between a higher-order anapole and Fabry-P\'erot-plasmonic mode. We further simulate WS$_2$ nanoantennas on gold with an hBN spacer, resulting in calculated electric field enhancements exceeding 2600, and a Purcell factor of 713. Our results demonstrate dramatic changes in the optical response of dielectric nanophotonic structures placed on gold, opening new possibilities for nanophotonics and sensing with simple-to-fabricate devices.

cond-mat.mes-hall↗

arXiv2020

A comparison of momentum transport models for numerical relativity

The main problems of nonvacuum numerical relativity, compact binary mergers and stellar collapse, involve hydromagnetic instabilities and turbulent flows, so that kinetic energy at small scales have mean effects at large scale that drive the secular evolution. Notable among these effects is momentum transport. We investigate two models of this transport effect, a relativistic Navier-Stokes system and a turbulent mean stress model, that are similar to all of the prescriptions that have been attempted to date for treating subgrid effects on binary neutron star mergers and their aftermath. Our investigation involves both stability analysis and numerical experimentation on star and disk systems. We also begin the investigation of the effects of particle and heat transport on post-merger simulations. We find that correct handling of turbulent heating can be important for avoiding unphysical instabilities. Given such appropriate handling, the evolution of a differentially rotating star and the accretion rate of a disk are reassuringly insensitive to the choice of prescription. However, disk outflows can be sensitive to the choice of method, even for the same effective viscous strength. We also consider the effects of eddy diffusion in the evolution of an accretion disk and show that it can interestingly affect the composition of outflows.

gr-qc↗

arXiv2019

Intuitive joint priors for variance parameters

Variance parameters in additive models are typically assigned independent priors that do not account for model structure. We present a new framework for prior selection based on a hierarchical decomposition of the total variance along a tree structure to the individual model components. For each split in the tree, an analyst may be ignorant or have a sound intuition on how to attribute variance to the branches. In the former case a Dirichlet prior is appropriate to use, while in the latter case a penalised complexity (PC) prior provides robust shrinkage. A bottom-up combination of the conditional priors results in a proper joint prior. We suggest default values for the hyperparameters and offer intuitive statements for eliciting the hyperparameters based on expert knowledge. The prior framework is applicable for R packages for Bayesian inference such as INLA and RStan. Three simulation studies show that, in terms of the application-specific measures of interest, PC priors improve inference over Dirichlet priors when used to penalise different levels of complexity in splits. However, when expressing ignorance in a split, Dirichlet priors perform equally well and are preferred for their simplicity. We find that assigning current state-of-the-art default priors for each variance parameter individually is less transparent and does not perform better than using the proposed joint priors. We demonstrate practical use of the new framework by analysing spatial heterogeneity in neonatal mortality in Kenya in 2010-2014 based on complex survey data.

stat.ME↗