arXiv subjects

Spencer J. Magnall

Publications and source records attributed to Spencer J. Magnall.

5 recordsLinked to original sources

arXiv2026

Realistic Equations of State Informing Neutron Star Post-Merger Gravitational-Wave Frequencies

Binary neutron star mergers are thought to produce hot, rapidly rotating neutron stars with masses that can far exceed their Tolman-Oppenheimer-Volkoff mass. The gravitational-wave emission from such remnants provides a unique opportunity to measure the nuclear equation of state at densities and temperatures not available to terrestrial experiments. Current detector design is informed by gravitational-wave signals from general relativistic hydrodynamics simulations of neutron star mergers, typically with hybrid thermal treatments for the equation of state, where a cold equation of state is modified by adding a thermal component. We use realistic equations of state based on the relativistic mean field model with consistent treatment of thermal effects to compute the distribution of expected peak gravitational-wave frequencies. Marginalising over equation of state and progenitor neutron star masses, we show the peak frequency of emission ranges from $\sim2.5$ to 4 kHz. The width of this distribution suggests the need for broadband observatories with kHz sensitivity, and calls into question some of the so-called post-merger optimised configurations. We show the proposed KAGRA high-frequency design is well-suited to measuring post-merger remnants when compared to the KAGRA broadband design.

astro-ph.HE↗

arXiv2025

The origin of the coherent radio flash potentially associated with GRB 201006A

Rowlinson et al. 2023 recently claimed the detection of a coherent radio flash 76.6 minutes after a short gamma-ray burst. They proposed that the radio emission may be associated with a long-lived neutron star engine. We show through theoretical and observational arguments that the coherent radio emission, if real and indeed associated with GRB 201006A and at the estimated redshift, is unlikely to be due to the collapse of the neutron star, ruling out a blitzar-like mechanism. Instead, we show if a long-lived engine was created, it must have been stable with the radio emission likely linked to the intrinsic magnetar activity. However, we find that the optical upper limits require fine-tuning to be consistent with a magnetar-driven kilonova: we show that neutron-star engines that do satisfy the optical constraints would have produced a bright kilonova afterglow that should already be observable by the VLA or MeerKAT (for ambient densities typical for short GRBs). Given the optical limits and the current lack of a kilonova afterglow, we instead posit that no neutron star survived the merger, and the coherent radio emission was produced far from a black hole central engine via mechanisms such as synchrotron maser or magnetic reconnection in the jet -- a scenario consistent with all observations. We encourage future radio follow-up to probe the engine of this exciting event and continued prompt radio follow-up of short GRBs.

astro-ph.HE↗

arXiv2025

Physics-Informed Priors Improve Gravitational-Wave Constraints on Neutron-Star Matter

Gravitational-wave astronomy shows great promise in determining nuclear physics in a regime not accessible to terrestrial experiments. We introduce physics-informed priors constrained by nuclear theory and perturbative Quantum Chromodynamics calculations, as well as astrophysical measurements of neutron-star masses and radii. When these priors are used in gravitational-wave astrophysical inference, we show a significant improvement on nuclear equation of state constraints. Applying these to the first observed gravitational-wave binary neutron-star merger GW170817, the constraints on the radius of a $1.4\,M_\odot$ neutron star improve from $R_{1.4} ={12.54^{+1.05}_{-1.54}} \, {\rm km}$ to $R_{1.4} = 12.11^{+0.91}_{-1.11} \,{\rm km}$ and those on the tidal deformability from $\tildeΛ_{1.186} < 720$ to $\tildeΛ_{1.186} = 384^{+306}_{-158}$ ($90\%$ confidence intervals) at the events measured chirp mass $\mathcal{M}=1.186\,M_\odot$. We also show these priors can be used to perform model selection between binary neutron star and neutron star-black hole mergers; in the case of GW190425, the results provide only marginal evidence with a Bayes factor $\mathcal{BF}=1.33$ in favour of the binary neutron star merger hypothesis. Given their ability to improve the astrophysical inference of binary mergers involving neutron stars, we advocate for these physics-informed priors to be used as standard in the literature and provide open-source code for reproducibility and adaptation of the method.

astro-ph.HE↗

arXiv2024

Directly inferring cosmology and the neutron-star equation of state from gravitational-wave mergers

Upgrades to existing gravitational-wave observatories have the potential to simultaneously constrain the nuclear equation of state and Hubble's constant $H_0$ to percent level with merging neutron star binaries. In practice, performing simultaneous inference of $H_0$ and the equation of state is limited computationally by the requirement to solve the equations of general-relativistic hydrostatic equilibrium millions of times. We develop a machine-learning model to solve the Tolman-Oppenheimer-Volkoff equations in less than a millisecond, and demonstrate its utility by performing direct inference of both equation of state and Hubble's constant for synthetic neutron star merger signals with LIGO-Virgo-KAGRA operating at A+ sensitivities. We show that a population of fifteen mergers observed with A+ allows for the radius of a $1.4\,M_{\odot}$ neutron star and $H_0$ to be constrained to $R_{1.4} = 11.74^{+0.35}_{-0.28}$ km and $H_0 = 68^{+17}_{-13} \rm \ km \ s^{-1} \ Mpc^{-1}$, at 90% credible interval and 68% credible interval respectively. These constraints utilise only the gravitational-wave information to infer cosmological parameters; such numbers will be further improved with the addition of electromagnetic counterparts and/or galaxy catalogues.

gr-qc↗

arXiv2023

Inhomogeneous Cosmology using General Relativistic Smoothed Particle Hydrodynamics coupled to Numerical Relativity

We perform three-dimensional simulations of homogeneous and inhomogeneous cosmologies via the coupling of a numerical relativity code for spacetime evolution and smoothed particle hydrodynamics (SPH) code. Evolution of a flat dust and radiation dominated Friedmann-Lemaître-Roberston-Walker (FLRW) spacetime shows an agreement of exact solutions with residuals on the order $10^{-6}$ and $10^{-3}$ respectively, even at low grid resolutions. We demonstrate evolution of linear perturbations of density, velocity and metric quantities to the FLRW with residuals of only $10^{-2}$ compared to exact solutions. Finally, we demonstrate the evolution of non-linear perturbations of the metric past shell-crossing, such that dark matter halo formation is possible. We show that numerical relativistic smoothed particle hydrodynamics is a viable method for understanding non-linear effects in cosmology.

gr-qc↗