arXiv subjects

Carolyn A. Raithel

Publications and source records attributed to Carolyn A. Raithel.

At least 19 recordsLinked to original sources

arXiv2026

Stiffness corrections for f-mode neutron star universal relations

Excitations of the neutron star (NS) fundamental mode (f-mode) during a binary NS inspiral or from a glitching pulsar can produce gravitational waves (GWs) that will be detectable with the next generation of detectors, opening the door to GW asteroseismology of neutron stars. There exist approximately equation-of-state (EOS)-insensitive, or "universal," relations that relate the f-mode to other NS properties. These universal relations have typically been identified with relatively small, selective EOS samples. To more thoroughly explore the parameter space, we examine the f-modes for a large sample of piecewise-parametric EOSs by solving the linearized fluid perturbation equations for the f-mode frequency and damping time in full general relativity. We utilize our sample to evaluate the EOS-sensitivity of previously-proposed universal relations (URs) for both the f-mode frequency and damping time. We find that URs that depend on combinations of only NS mass and radius have significant residuals, as previously reported; but that these residuals are strongly correlated with the overall stiffness of the EOS. We account for this residual dependence on the EOS by introducing linear correction terms that depend on the characteristic radius predicted by the EOS and the slope of the mass-radius curve at intermediate masses. To assess the impact of our new correction terms on observables, we consider the case of GW emission due to a glitch in a Vela-like pulsar and demonstrate that the predicted signal-to-noise ratio for such an event is over/under-estimated by up to ~10% when using an existing, single-parameter UR, but is recovered nearly exactly with our improved 3-parameter UR. In contrast, we find that URs between the f-mode and tidal deformability exhibit only minimal EOS-dependent correlations in their residuals and overall remain highly EOS-insensitive.

gr-qc↗

arXiv2026

Impact of thermal effects on prompt-collapse binary neutron star mergers

The fate of the remnant following the merger of two neutron stars initially on quasicircular orbits depends primarily on the mass of the initial neutron stars, the mass ratio, and the still-uncertain dense-matter equation of state (EoS). Previous works studying the threshold mass for prompt collapse to a black hole have primarily focused on the uncertainties in the zero-temperature EoS, which are parametrized by a macroscopic quantity such as the characteristic neutron star radius. However, prompt collapse can take place either with or without a core bounce during the merger. In the bounce-collapse scenario, shocks can produce additional thermal support, potentially altering the threshold for collapse. In this work, we investigate the impact of the uncertainties in the finite-temperature part of the nuclear EoS on the threshold mass for prompt collapse in equal mass mergers. Using two cold EoSs, combined with four parametrizations of the finite-temperature part of the EoS, we find that the threshold mass is insensitive to realistic variations of the thermal prescription, at sub-percent accuracy. We report on the thermal properties and ejecta of mergers with masses just above the threshold mass, i.e., which experience a single core-bounce before collapsing. During the bounce, the thermal pressure can reach )(1-10)% of the cold pressure at supranuclear densities, depending on the thermal treatment, leading to modest differences in the dynamical ejecta that are launched and in the remnant disk mass as a result.

astro-ph.HE↗

arXiv2024

Detectability of Finite-Temperature Effects From Neutron Star Mergers with Next-Generation Gravitational Wave Detectors

Observations of the high-frequency gravitational waves (GWs) emitted by the hot and massive remnant of a binary neutron star merger will provide new probes of the dense-matter equation of state (EoS). We show that current uncertainties in the thermal physics can cause the emergent GW spectum to differ by a degree comparable to changing the cold EoS by $\pm\sim120$ m in the characteristic radius of a neutron star. Unless a very close binary neutron star merger takes place, these effects are unlikely to be measurable with current GW detectors. However, with proposed next-generation detectors such as Cosmic Explorer or Einstein Telescope, the effects can be distinguished for events at distances of up to ~80-200 Mpc, if the cold EoS is sufficiently well constrained.

astro-ph.HE↗

arXiv2023

Influence of stellar compactness on finite-temperature effects in neutron star merger simulations

Binary neutron star mergers probe the dense-matter equation of state (EoS) across a wide range of densities and temperatures, from the cold conditions of the inspiral to the high-temperature matter of the massive neutron star remnant. In this paper, we explore the sensitivity of neutron star mergers to uncertainties in the finite-temperature part of the EoS with a series of merger simulations performed in full general relativity. We expand on our previous work to explore the interplay between the thermal prescription and the stiffness of the zero-temperature EoS, which determines the compactness of the initial neutron stars. Using a phenomenological model of the particle effective mass, $M^*$, to calculate the finite-temperature part of the EoS, we perform merger simulations for a range of thermal prescriptions, together with two cold EoSs that predict either compact or large-radius initial neutron stars. We report on how the choice of $M^*$-parameters influences the thermal properties of the post-merger remnant, and how this varies for stars with different initial stellar compactness. We characterize the post-merger gravitational wave signals, and find differences in the peak frequencies of up to 190 Hz depending on the choice of $M^*$-parameters. Finally, we find that the total dynamical ejecta is in general only weakly sensitive to the thermal prescription, but that a particular combination of $M^*$-parameters, together with a soft cold EoS, can lead to significant enhancements in the ejecta.

astro-ph.HE↗

arXiv2023

Tidal Deformability Doppelgangers: Implications of a low-density phase transition in the neutron star equation of state

Studying the properties of ultra-dense matter is one of the key goals of modern neutron star research. The measurement of the tidal deformability from the inspiral of a binary neutron star merger offers one promising method for constraining the equation of state (EoS) of cold, dense matter. In this work, we report on a new class of EoSs which have significantly different pressures at nuclear densities and large differences in stellar radii, but that predict surprisingly similar tidal deformabilities across the entire range of astrophysically-observed neutron star masses. Using a survey of 5 million piecewise polytropic EoSs, subject to five different sets of nuclear priors, we demonstrate that these "tidal deformability doppelgangers" occur generically. We find that they can differ substantially in the pressure (by up to a factor of 3 at nuclear densities) and in the radius of intermediate-mass neutron stars (by up to 0.5 km), but are observationally indistinguishable in their tidal deformabilities ($ΔΛ< 30$) with the sensitivity of current gravitational wave detectors. We demonstrate that this near-degeneracy in the tidal deformability is a result of allowing for a phase transition at low densities. We show that a combination of input from nuclear theory (e.g., from chiral effective field theory), X-ray observations of neutron star radii, and/or the next generation of gravitational wave detectors will be able to significantly constrain these tidal deformability doppelgangers.

astro-ph.HE↗

arXiv2023

Degeneracy in the inference of phase transitions in the neutron star equation of state from gravitational wave data

Gravitational wave (GW) detections of binary neutron star inspirals will be crucial for constraining the dense matter equation of state (EoS). We demonstrate a new degeneracy in the mapping from tidal deformability data to the EoS, which occurs for models with strong phase transitions. We find that there exists a new family of EoS with phase transitions that set in at different densities and that predict neutron star radii that differ by up to ~500m, but that produce nearly identical tidal deformabilities for all neutron star masses. Next generation GW detectors and advances in nuclear theory may be needed to resolve this degeneracy.

astro-ph.HE↗

arXiv2022

Long Range Plan: Dense matter theory for heavy-ion collisions and neutron stars

Since the release of the 2015 Long Range Plan in Nuclear Physics, major events have occurred that reshaped our understanding of quantum chromodynamics (QCD) and nuclear matter at large densities, in and out of equilibrium. The US nuclear community has an opportunity to capitalize on advances in astrophysical observations and nuclear experiments and engage in an interdisciplinary effort in the theory of dense baryonic matter that connects low- and high-energy nuclear physics, astrophysics, gravitational waves physics, and data science

nucl-th↗

arXiv2022

Finite-temperature effects in dynamical-spacetime binary neutron star merger simulations: Validation of the parametric approach

Parametric equations of state (EoSs) provide an important tool for systematically studying EoS effects in neutron star merger simulations. In this work, we perform a numerical validation of the M*-framework for parametrically calculating finite-temperature EoS tables. The framework, introduced in Raithel et al. (2019), provides a model for generically extending any cold, beta-equilibrium EoS to finite-temperatures and arbitrary electron fractions. In this work, we perform numerical evolutions of a binary neutron star merger with the SFHo finite-temperature EoS, as well as with the M*-approximation of this same EoS, where the approximation uses the zero-temperature, beta-equilibrium slice of SFHo and replaces the finite-temperature and composition-dependent parts with the M*-model. We find that the approximate version of the EoS is able to accurately recreate the temperature and thermal pressure profiles of the binary neutron star remnant, when compared to the results found using the full version of SFHo. We additionally find that the merger dynamics and gravitational wave signals agree well between both cases, with differences of ~1-2% introduced into the post-merger gravitational wave peak frequencies by the approximations of the EoS. We conclude the M*-framework can be reliably used to probe neutron star merger properties in numerical simulations.

astro-ph.HE↗

arXiv2022

Characterizing the breakdown of quasi-universality in the post-merger gravitational waves from binary neutron star mergers

The post-merger gravitational wave (GW) emission from a binary neutron star merger is expected to provide exciting new constraints on the dense-matter equation of state (EoS). Such constraints rely, by and large, on the existence of quasi-universal relations, which relate the peak frequencies of the post-merger GW spectrum to properties of the neutron star structure in a model-independent way. In this work, we report on violations of existing quasi-universal relations between the peak spectral frequency, f_2, and the stellar radius, for EoSs models with backwards-bending slopes in their mass-radius relations (such that the radius increases at high masses). The violations are extreme, with variations in f_2 of up to ~600 Hz between EoSs that predict the same radius for a 1.4 Msun neutron star, but that have significantly different radii at higher masses. Quasi-universality can be restored by adding in a second parameter to the fitting formulae that depends on the slope of the mass-radius curve. We further find strong evidence that quasi-universality is better maintained for the radii of very massive stars (with masses 2 Msun). Both statements imply that f_2 is mainly sensitive to the high-density EoS. Combined with observations of the binary neutron star inspiral, these generalized quasi-universal relations can be used to simultaneously infer the characteristic radius and slope of the neutron star mass-radius relation.

astro-ph.HE↗

arXiv2022

Improving the convergence order of binary neutron star merger simulations in the Baumgarte-Shapiro-Shibata-Nakamura formulation

High-accuracy numerical relativity simulations of binary neutron star mergers are a necessary ingredient for constructing gravitational waveform templates to analyze and interpret observations of compact object mergers. Numerical convergence in the post-merger phase of such simulations is challenging to achieve with many modern codes. In this paper, we study two ways of improving the convergence properties of binary neutron star merger simulations within the Baumgarte-Shapiro-Shibata-Nakamura formulation of Einstein's equations. We show that discontinuities in a particular constraint damping scheme in this formulation can destroy the post-merger convergence of the simulation. A continuous prescription, in contrast, ensures convergence until late times. We additionally study the impact of the equation of state parametrization on the pre- and post-merger convergence properties of the simulations. In particular, we compare results for a piecewise polytropic parametrization, which is commonly used in merger simulations but suffers unphysical discontinuities in the sound speed, with results using a "generalized" piecewise polytropic parametrization, which was designed to ensure both continuity and differentiability of the equation of state. We report on the differences in the gravitational waves and any spurious pre-merger heating, depending on which equation of state parametrization is used.

gr-qc↗

arXiv2021

Impact of the nuclear symmetry energy on the post-merger phase of a binary neutron star coalescence

The nuclear symmetry energy plays a key role in determining the equation of state of dense, neutron-rich matter, which governs the properties of both terrestrial nuclear matter as well as astrophysical neutron stars. A recent measurement of the neutron skin thickness from the PREX collaboration has lead to new constraints on the slope of the nuclear symmetry energy, L, which can be directly compared to inferences from gravitational-wave observations of the first binary neutron star merger inspiral, GW170817 In this paper, we explore a new regime for potentially constraining the slope, L, of the nuclear symmetry energy with future gravitational wave events: the post-merger phase a binary neutron star coalescence. In particular, we go beyond the inspiral phase, where imprints of the slope parameter L may be inferred from measurements of the tidal deformability, to consider imprints on the post-merger dynamics, gravitational wave emission, and dynamical mass ejection. To this end, we perform a set of targeted neutron star merger simulations in full general relativity using new finite-temperature equations of state, which systematically vary L. We find that the post-merger dynamics and gravitational wave emission are mostly insensitive to the slope of the nuclear symmetry energy. In contrast, we find that dynamical mass ejection contains a weak imprint of L, with large values of L leading to systematically enhanced ejecta.

astro-ph.HE↗

arXiv2020

Markov Chains for Horizons (MARCH). I. Identifying Biases in Fitting Theoretical Models to Event Horizon Telescope Observations

We introduce a new Markov Chain Monte Carlo (MCMC) algorithm with parallel tempering for fitting theoretical models of horizon-scale images of black holes to the interferometric data from the Event Horizon Telescope (EHT). The algorithm implements forms of the noise distribution in the data that are accurate for all signal-to-noise ratios. In addition to being trivially parallelizable, the algorithm is optimized for high performance, achieving 1 million MCMC chain steps in under 20 seconds on a single processor. We use synthetic data for the 2017 EHT coverage of M87 that are generated based on analytic as well as General Relativistic Magnetohydrodynamic (GRMHD) model images to explore several potential sources of biases in fitting models to sparse interferometric data. We demonstrate that a very small number of data points that lie near salient features of the interferometric data exert disproportionate influence on the inferred model parameters. We also show that the preferred orientations of the EHT baselines introduce significant biases in the inference of the orientation of the model images. Finally, we discuss strategies that help identify the presence and severity of such biases in realistic applications.

astro-ph.IM↗

arXiv2019

Measurement of the nuclear symmetry energy parameters from gravitational wave events

The nuclear symmetry energy plays a role in determining both the nuclear properties of terrestrial matter as well as the astrophysical properties of neutron stars. The first measurement of the neutron star tidal deformability, from gravitational wave event GW170817, provides a new way of probing the symmetry energy. In this work, we report on new constraints on the symmetry energy from GW170817. We focus in particular on the low-order coefficients: namely, the value of the symmetry energy at the nuclear saturation density, S_0, and the slope of the symmetry energy, L_0. We find that the gravitational wave data are relatively insensitive to S_0, but that they depend strongly on L_0 and point to lower values of L_0 than have previously been reported, with a peak likelihood near L_0 ~ 20 MeV. Finally, we use the inferred posteriors on L_0 to derive new analytic constraints on higher-order nuclear terms.

astro-ph.HE↗

arXiv2019

Constraints on the Neutron Star Equation of State from GW170817

The first detection of gravitational waves from a neutron star-neutron star merger, GW170817, has opened up a new avenue for constraining the ultradense-matter equation of state (EOS). The deviation of the observed waveform from a point-particle waveform is a sensitive probe of the EOS controlling the merging neutron stars' structure. In this topical review, I discuss the various constraints that have been made on the EOS in the year following the discovery of GW170817. In particular, I review the surprising relationship that has emerged between the effective tidal deformability of the binary system and the neutron star radius. I also report new results that make use of this relationship, finding that the radius inferred from GW170817 lies between 9.8 and 13.2 km at 90% confidence, with distinct likelihood peaks at 10.8 and 12.3 km. I compare these radii, as well as those inferred in the literature, to X-ray measurements of the neutron star radius. I also summarize the various maximum mass constraints, which point towards a maximum mass < 2.3 M_sun, depending on the fate of the remnant, and which can be used to additionally constrain the high-density EOS. I review the constraints on the EOS that have been performed directly, through Bayesian inference schemes. Finally, I comment on the importance of disentangling thermal effects in future EOS constraints from neutron star mergers.

astro-ph.HE↗

arXiv2019

Finite-temperature extension for cold neutron star equations of state

Observations of isolated neutron stars place constraints on the equation of state (EOS) of cold, neutron-rich matter, while nuclear physics experiments probe the EOS of hot, symmetric matter. Many dynamical phenomena, such as core-collapse supernovae, the formation and cooling of proto-neutron stars, and neutron star mergers, lie between these two regimes and depend on the EOS at finite temperatures for matter with varying proton fractions. In this paper, we introduce a new framework to accurately calculate the thermal pressure of neutron-proton-electron matter at arbitrary density, temperature, and proton fraction. This framework can be expressed using a set of five physically-motivated parameters that span a narrow range of values for realistic EOS and are able to capture the leading-order effects of degenerate matter on the thermal pressure. We base two of these parameters on a new approximation of the Dirac effective mass, with which we reproduce the thermal pressure to within <~30% for a variety of realistic EOS at densities of interest. Three additional parameters, based on the behavior of the symmetry energy near the nuclear saturation density, allow for the extrapolation of any cold EOS in beta-equilibrium to arbitrary proton fractions. Our model thus allows a user to extend any cold nucleonic EOS, including piecewise-polytropes, to arbitrary temperature and proton fraction, for use in calculations and numerical simulations of astrophysical phenomena. We find that our formalism is able to reproduce realistic finite-temperature EOS with errors of <~20% and offers a 1-3 orders-of-magnitude improvement over existing ideal-fluid models.

astro-ph.HE↗

arXiv2017

Confronting Models of Massive Star Evolution and Explosions with Remnant Mass Measurements

The mass distribution of compact objects provides a fossil record that can be studied to uncover information on the late stages of massive star evolution, the supernova explosion mechanism, and the dense matter equation of state. Observations of neutron star masses indicate a bimodal Gaussian distribution, while the observed black hole mass distribution decays exponentially for stellar-mass black holes. We use these observed distributions to directly confront the predictions of stellar evolution models and the neutrino-driven supernova simulations of Sukhbold et al. (2016). We find excellent agreement between the black hole and low-mass neutron star distributions created by these simulations and the observations. We show that a large fraction of the stellar envelope must be ejected, either during the formation of stellar-mass black holes or prior to the implosion through tidal stripping due to a binary companion, in order to reproduce the observed black hole mass distribution. We also determine the origins of the bimodal peaks of the neutron star mass distribution, finding that the low-mass peak (centered at ~1.4 M_sun) originates from progenitors with M_zams ~ 9-18 M_sun. The simulations fail to reproduce the observed peak of high-mass neutron stars (centered at ~1.8 M_sun) and we explore several possible explanations. We argue that the close agreement between the observed and predicted black hole and low-mass neutron star mass distributions provides new promising evidence that these stellar evolution and explosion models are accurately capturing the relevant stellar, nuclear, and explosion physics involved in the formation of compact objects.

astro-ph.HE↗

arXiv2017

From Neutron Star Observables to the Equation of State. II. Bayesian Inference of Equation of State Pressures

One of the key goals of observing neutron stars is to infer the equation of state (EoS) of the cold, ultradense matter in their interiors. We present here a Bayesian statistical method of inferring the pressures at five fixed densities, from a sample of mock neutron star masses and radii. We show that while five polytropic segments are needed for maximum flexibility in the absence of any prior knowledge of the EoS, regularizers are also necessary to ensure that simple underlying EoS are not over-parametrized. For ideal data with small measurement uncertainties, we show that the pressure at roughly twice the nuclear saturation density, rho_sat, can be inferred to within 0.3 dex for many realizations of potential sources of uncertainties. The pressures of more complicated EoS with significant phase transitions can also be inferred to within ~30%. We also find that marginalizing the multi-dimensional parameter space of pressure to infer a mass-radius relation can lead to biases of nearly 1 km in radius, towards larger radii. Using the full, five-dimensional posterior likelihoods avoids this bias.

astro-ph.HE↗

arXiv2016

From Neutron Star Observables to the Equation of State. I. An Optimal Parametrization

The increasing number and precision of measurements of neutron star masses, radii, and, in the near future, moments of inertia offer the possibility of precisely determining the neutron star equation of state. One way to facilitate the mapping of observables to the equation of state is through a parametrization of the latter. We present here a generic method for optimizing the parametrization of any physically allowed EoS. We use mock equations of state that incorporate physically diverse and extreme behavior to test how well our parametrization reproduces the global properties of the stars, by minimizing the errors in the observables mass, radius, and the moment of inertia. We find that using piecewise polytropes and sampling the EoS with five fiducial densities between ~1-8 times the nuclear saturation density results in optimal errors for the smallest number of parameters. Specifically, it recreates the radii of the assumed EoS to within less than 0.5 km for the extreme mock equations of state and to within less than 0.12 km for 95% of a sample of 42 proposed, physically-motivated equations of state. Such a parametrization is also able to reproduce the maximum mass to within 0.04 M_sun and the moment of inertia of a 1.338 M_sun neutron star to within less than 10% for 95% of the proposed sample of equations of state.

astro-ph.HE↗