arXiv subjects

Ingo Tews

Publications and source records attributed to Ingo Tews.

52 records · Page 3Linked to original sources

arXiv2021

A Detailed Examination of Astrophysical Constraints on the Symmetry Energy and the Neutron Skin of $^{208}$Pb with Minimal Modeling Assumptions

The symmetry energy and its density dependence are pivotal for many nuclear physics and astrophysics applications, as they determine properties ranging from the neutron-skin thickness of nuclei to the crust thickness and the radius of neutron stars. Recently, PREX-II reported a value of $0.283\pm0.071$ fm for the neutron-skin thickness of $^{208}$Pb, $R_{\rm skin}^{^{208}\text{Pb}}$, implying a symmetry-energy slope parameter $L$ of $106\pm37$ MeV, larger than most ranges obtained from microscopic calculations and other nuclear experiments. We use a nonparametric equation of state representation based on Gaussian processes to constrain the symmetry energy $S_0$, $L$, and $R_{\rm skin}^{^{208}\text{Pb}}$ directly from observations of neutron stars with minimal modeling assumptions. The resulting astrophysical constraints from heavy pulsar masses, LIGO/Virgo, and NICER favor smaller values of the neutron skin and $L$, as well as negative symmetry incompressibilities. Combining astrophysical data with chiral effective field theory ($χ$EFT) and PREX-II constraints yields $S_0 = 33.0^{+2.0}_{-1.8}$ MeV, $L=53^{+14}_{-15}$ MeV, and $R_{\rm skin}^{^{208}\text{Pb}} = 0.17^{+0.04}_{-0.04}$ fm. We also examine the consistency of several individual $χ$EFT calculations with astrophysical observations and terrestrial experiments. We find that there is only mild tension between $χ$EFT, astrophysical data, and PREX-II's $R_\mathrm{skin}^{^{208}\mathrm{Pb}}$ measurement ($p$-value $= 12.3\%$) and that there is excellent agreement between $χ$EFT, astrophysical data, and other nuclear experiments.

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arXiv2021

Astrophysical Constraints on the Symmetry Energy and the Neutron Skin of $^{208}$Pb with Minimal Modeling Assumptions

The symmetry energy and its density dependence are crucial inputs for many nuclear physics and astrophysics applications, as they determine properties ranging from the neutron-skin thickness of nuclei to the crust thickness and the radius of neutron stars. Recently, PREX-II reported a value of $0.283 \pm 0.071$ fm for the neutron-skin thickness of $^{208}$Pb, implying a slope parameter $L = 106 \pm 37$ MeV, larger than most ranges obtained from microscopic calculations and other nuclear experiments. We use a nonparametric equation of state representation based on Gaussian processes to constrain the symmetry energy $S_0$, $L$, and $R_\mathrm{skin}^{^{208}\mathrm{Pb}}$ directly from observations of neutron stars with minimal modeling assumptions. The resulting astrophysical constraints from heavy pulsar masses, LIGO/Virgo, and NICER clearly favor smaller values of the neutron skin and $L$, as well as negative symmetry incompressibilities. Combining astrophysical data with PREX-II and chiral effective field theory constraints yields $S_0 = 33.0^{+2.0}_{-1.8}$ MeV, $L=53^{+14}_{-15}$ MeV, and $R_\mathrm{skin}^{^{208}\mathrm{Pb}}=0.17^{+0.04}_{-0.04}$ fm.

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arXiv2021

Nuclear-Physics Multi-Messenger Astrophysics Constraints on the Neutron-Star Equation of State: Adding NICER's PSR J0740+6620 Measurement

In the past few years, new observations of neutron stars and neutron-star mergers have provided a wealth of data that allow one to constrain the equation of state of nuclear matter at densities above nuclear saturation density. However, most observations were based on neutron stars with masses of about 1.4 solar masses, probing densities up to $\sim$ 3-4 times the nuclear saturation density. Even higher densities are probed inside massive neutron stars such as PSR J0740+6620. Very recently, new radio observations provided an update to the mass estimate for PSR J0740+6620 and X-ray observations by the NICER and XMM telescopes constrained its radius. Based on these new measurements, we revisit our previous nuclear-physics multi-messenger astrophysics constraints and derive updated constraints on the equation of state describing the neutron-star interior. By combining astrophysical observations of two radio pulsars, two NICER measurements, the two gravitational-wave detections GW170817 and GW190425, detailed modeling of the kilonova AT2017gfo, as well as the gamma-ray burst GRB170817A, we are able to estimate the radius of a typical 1.4-solar mass neutron star to be $11.94^{+0.76}_{-0.87} \rm{km}$ at $90\%$ confidence. Our analysis allows us to revisit the upper bound on the maximum mass of neutron stars and disfavors the presence of a strong first-order phase transition from nuclear matter to exotic forms of matter, such as quark matter, inside neutron stars.

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arXiv2021

Modeling Neutron Star Oscillations in a Fixed General Relativistic Background Including Solid Crust Dynamics

Measurements of the gravitational-wave signals from neutron star mergers allow scientists to learn about the interior of neutron stars and the properties of dense nuclear matter. The study of neutron star mergers is usually performed with computational fluid dynamics codes, mostly in Eulerian but also in Lagrangian formulation such as smoothed particle hydrodynamics (SPH). Codes include our best knowledge of nuclear matter in the form of an equation of state as well as effects of general relativity (GR). However, one important aspect of neutron stars is usually ignored: the solid nature of their crust. The solid matter in the crust is the strongest material known in nature which could lead to a multitude of possible observational effects that have not been studied in dynamical simulations yet. The crust could change the way a neutron star deforms during a merger, leaving an imprint in the gravitational wave signal. It could even shatter during the inspiral, producing a potentially observable electromagnetic signal. Here, we present a first study of the dynamical behavior of neutron stars with a solid crust and fixed GR background with FleCSPH. FleCSPH is a general-purpose SPH code, developed at Los Alamos National Laboratory. It features an efficient algorithm for gravitational interactions via the Fast Multipole Method, which, together with the implemented nuclear equation of state, makes it appropriate for astrophysical applications. The solid material dynamics is described via the elastic-perfectly plastic model with maximum-strain breaking. Despite its simplicity, the model reproduces the stress-strain curve of crustal material as extracted from microphysical simulations very well. We present first tests of our implementation via simulations of neutron star oscillations and give an outlook on our study of the dynamical behavior of the solid crust in neutron star merger events.

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arXiv2021

On the nature of GW190814 and its impact on the understanding of supranuclear matter

The observation of a compact object with a mass of $2.50-2.67M_{\odot}$ on August 14, 2019, by the LIGO Scientific and Virgo collaborations (LVC) has the potential to improve our understanding of the supranuclear equation of state. While the gravitational-wave analysis of the LVC suggests that GW190814 likely was a binary black hole system, the secondary component could also have been the heaviest neutron star observed to date. We use our previously derived nuclear-physics-multimessenger astrophysics framework to address the nature of this object. Based on our findings, we determine GW190814 to be a binary black hole merger with a probability of $>99.9\%$. Even if we weaken previously employed constraints on the maximum mass of neutron stars, the probability of a binary black hole origin is still $\sim 81\%$. Furthermore, we study the impact that this observation has on our understanding of the nuclear equation of state by analyzing the allowed region in the mass-radius diagram of neutron stars for both a binary black hole or neutron star--black hole scenario. We find that the unlikely scenario in which the secondary object was a neutron star requires rather stiff equations of state with a maximum speed of sound $c_s\geq \sqrt{0.6}$ times the speed of light, while the binary black hole scenario does not offer any new insight.

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arXiv2020

Multi-messenger constraints on the neutron-star equation of state and the Hubble constant

Observations of neutron-star mergers based on distinct messengers, including gravitational waves and electromagnetic signals, can be used to study the behavior of matter denser than an atomic nucleus, and to measure the expansion rate of the Universe described by the Hubble constant. We perform a joint analysis of the gravitational-wave signal GW170817 with its electromagnetic counterparts AT2017gfo and GRB170817A, and the gravitational-wave signal GW190425, both originating from neutron-star mergers. We combine these with previous measurements of pulsars using X-ray and radio observations, and nuclear-theory computations using chiral effective field theory to constrain the neutron-star equation of state. We find that the radius of a $1.4$ solar mass neutron star is $11.75^{+0.86}_{-0.81}\ \rm km$ at $90\%$ confidence and the Hubble constant is $66.2^{+4.4}_{-4.2}\ \rm km \,Mpc^{-1}\, s^{-1}$ at $1σ$ uncertainty.

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arXiv2020

Combining Electromagnetic and Gravitational-Wave Constraints on Neutron-Star Masses and Radii

We perform a joint Bayesian inference of neutron-star mass and radius constraints based on GW170817, observations of quiescent low-mass X-ray binaries (QLMXBs), photospheric radius expansion X-ray bursts (PREs), and X-ray timing observations of J0030+0451. With this data set, the form of the prior distribution still has an impact on the posterior mass-radius (MR) curves and equation of state (EOS), but this impact is smaller than recently obtained when considering QLMXBs alone. We analyze the consistency of the electromagnetic data by including an "intrinsic scattering" contribution to the uncertainties, and find only a slight broadening of the posteriors. This suggests that the gravitational-wave and electromagnetic observations of neutron-star structure are providing a consistent picture of the neutron-star mass-radius curve and the EOS.

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arXiv2020

Direct Astrophysical Tests of Chiral Effective Field Theory at Supranuclear Densities

Recent observations of neutron stars with gravitational waves and X-ray timing provide unprecedented access to the equation of state (EoS) of cold dense matter at densities difficult to realize in terrestrial experiments. At the same time, predictions for the EoS with reliable uncertainty estimates from chiral effective field theory ($χ$EFT) bound our theoretical ignorance. In this work, we analyze astrophysical data using a nonparametric representation of the neutron-star EoS conditioned on $χ$EFT to directly constrain the underlying physical properties of the compact objects. We discuss how the data alone constrain the EoS at high densities when we condition on $χ$EFT at low densities. We also demonstrate how to exploit astrophysical data to directly test the predictions of $χ$EFT for the EoS up to twice nuclear saturation density, and estimate the density at which these predictions might break down. We find that the existence of massive pulsars, gravitational waves from GW170817, and NICER observations of PSR J0030+0451 favor $χ$EFT predictions for the EoS up to nuclear saturation density over a more agnostic analysis by as much as a factor of 7 for the quantum Monte Carlo (QMC) calculations used in this work. While $χ$EFT predictions using QMC are fully consistent with gravitational-wave data up to twice nuclear saturation density, NICER observations suggest that the EoS stiffens relative to these predictions at nuclear saturation density. Additionally, we marginalize over the uncertainty in the density at which $χ$EFT begins to break down, constraining the radius of a $1.4\,M_\odot$ neutron star to $R_{1.4}=11.40^{+1.38}_{-1.04}$ ($12.54^{+0.71}_{-0.63}$) km and the pressure at twice nuclear saturation density to $p(2n_\mathrm{sat})=14.2^{+18.1}_{-8.4}$ ($28.7^{+15.3}_{-15.0}$) MeV/fm$^3$ with massive pulsar and gravitational-wave (and NICER) data.

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arXiv2020

Parameter estimation for strong phase transitions in supranuclear matter using gravitational-wave astronomy

At supranuclear densities, explored in the core of neutron stars, a strong phase transition from hadronic matter to more exotic forms of matter might be present. To test this hypothesis, binary neutron-star mergers offer a unique possibility to probe matter at densities that we can not create in any existing terrestrial experiment. In this work, we show that, if present, strong phase transitions can have a measurable imprint on the binary neutron-star coalescence and the emitted gravitational-wave signal. We construct a new parameterization of the supranuclear equation of state that allows us to test for the existence of a strong phase transition and extract its characteristic properties purely from the gravitational-wave signal of the inspiraling neutron stars. We test our approach using a Bayesian inference study simulating 600 signals with three different equations of state and find that for current gravitational-wave detector networks already twelve events might be sufficient to verify the presence of a strong phase transition. Finally, we use our methodology to analyze GW170817 and GW190425, but do not find any indication that a strong phase transition is present at densities probed during the inspiral.

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arXiv2020

Stringent constraints on neutron-star radii from multimessenger observations and nuclear theory

The properties of neutron stars are determined by the nature of the matter that they contain. These properties can be constrained by measurements of the star's size. We obtain stringent constraints on neutron-star radii by combining multimessenger observations of the binary neutron-star merger GW170817 with nuclear theory that best accounts for density-dependent uncertainties in the equation of state. We construct equations of state constrained by chiral effective field theory and marginalize over these using the gravitational-wave observations. Combining this with the electromagnetic observations of the merger remnant that imply the presence of a short-lived hyper-massive neutron star, we find that the radius of a $1.4\,\rm{M}_\odot$ neutron star is $R_{1.4\,\mathrm{M}_\odot} = 11.0^{+0.9}_{-0.6}~{\rm km}$ (90% credible interval). Using this constraint, we show that neutron stars are unlikely to be disrupted in neutron-star black-hole mergers; subsequently, such events will not produce observable electromagnetic emission.

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arXiv2020

Local nucleon-nucleon and three-nucleon interactions within chiral effective field theory

To obtain an understanding of the structure and reactions of nuclear systems from first principles has been a long-standing goal of nuclear physics. In this respect, few- and many-body systems provide a unique laboratory for studying nuclear interactions. During the past decades, the development of accurate representations of the nuclear force has undergone substantial progress. Particular emphasis has been devoted to chiral effective field theory (EFT), a low-energy effective representation of quantum chromodynamics (QCD). Within chiral EFT, many studies have been carried out dealing with the construction of both the nucleon-nucleon ($N\!N$) and three-nucleon ($3N$) interactions. The aim of the present article is to give a detailed overview of the chiral interaction models that are local in configuration space, and show recent results for nuclear systems obtained by employing these local chiral forces.

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arXiv2019

Local chiral EFT potentials in nuclei and neutron matter: results and issues

In recent years, the combination of advanced quantum Monte Carlo (QMC) methods and local interactions derived from chiral effective field theory (EFT) has been shown to provide a versatile and systematic approach to nuclear systems. Calculations at next-to-next-to-leading order in chiral EFT have lead to fascinating results for nuclei and nucleonic matter. On the one hand, ground-state properties of nuclei are well reproduced up to $A\leq16$, even though these potentials have been fit to nucleon-nucleon scattering and few-body observables only. On the other hand, a reasonable description of neutron-matter properties emerges. While regulator functions applied to two- and three-nucleon forces are a necessary ingredient in these many-body calculations, the use of local regulators leads to a substantial residual regulator and cutoff dependence that increases current theoretical uncertainties. In this contribution, we review local chiral interactions, their applications, and QMC results for nuclei and neutron matter. In addition, we address regulator issues for such potentials and present a possible path forward.

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arXiv2018

Dense matter with eXTP

In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

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arXiv2018

Constraining the speed of sound inside neutron stars with chiral effective field theory interactions and observations

The dense matter equation of state (EOS) determines neutron star (NS) structure but can be calculated reliably only up to one to two times the nuclear saturation density, using accurate many-body methods that employ nuclear interactions from chiral effective field theory constrained by scattering data. In this work, we use physically motivated ansatzes for the speed of sound $c_S$ at high density to extend microscopic calculations of neutron-rich matter to the highest densities encountered in stable NS cores. We show how existing and expected astrophysical constraints on NS masses and radii from X-ray observations can constrain the speed of sound in the NS core. We confirm earlier expectations that $c_S$ is likely to violate the conformal limit of $c_S^2\leq c^2/3 $, possibly reaching values closer to the speed of light $c$ at a few times the nuclear saturation density, independent of the nuclear Hamiltonian. If QCD obeys the conformal limit, we conclude that the rapid increase of $c_S$ required to accommodate a $2 $ M$_\odot$ NS suggests a form of strongly interacting matter where a description in terms of nucleons will be unwieldy, even between one and two times the nuclear saturation density. For typical NSs with masses in the range $1.2-1.4~$ M$_\odot$, we find radii between $10$ and $14$ km, and the smallest possible radius of a $1.4$ M$_{\odot}$ NS consistent with constraints from nuclear physics and observations is $8.4$ km. We also discuss how future observations could constrain the EOS and guide theoretical developments in nuclear physics.

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arXiv2017

Symmetry Parameter Constraints From A Lower Bound On The Neutron-Matter Energy

We propose the existence of a lower bound on the energy of pure neutron matter (PNM) on the basis of unitary-gas considerations. We discuss its justification from experimental studies of cold atoms as well as from theoretical studies of neutron matter. We demonstrate that this bound results in limits to the density-dependent symmetry energy, which is the difference between the energies of symmetric nuclear matter and PNM. In particular, this bound leads to a lower limit to the volume symmetry energy parameter $S_0$. In addition, for assumed values of $S_0$ above this minimum, this bound implies both upper and lower limits to the symmetry energy slope parameter $L$, which describes the lowest-order density dependence of the symmetry energy. A lower bound on the neutron-matter incompressibility is also obtained. These bounds are found to be consistent with both recent calculations of the energies of PNM and constraints from nuclear experiments. Our results are significant because several equations of state that are currently used in astrophysical simulations of supernovae and neutron star mergers, as well as in nuclear physics simulations of heavy-ion collisions, have symmetry energy parameters that violate these bounds. Furthermore, below the nuclear saturation density, the bound on neutron-matter energies leads to a lower limit to the density-dependent symmetry energy, which leads to upper limits to the nuclear surface symmetry parameter and the neutron-star crust-core boundary. We also obtain a lower limit to the neutron-skin thicknesses of neutron-rich nuclei. Above the nuclear saturation density, the bound on neutron-matter energies also leads to an upper limit to the symmetry energy, with implications for neutron-star cooling via the direct Urca process.

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arXiv2017

Spectrum of shear modes in the neutron-star crust: Estimating the nuclear-physics uncertainties

I construct a model of the inner crust of neutron stars using interactions from chiral effective field theory (EFT) in order to calculate its equation of state (EOS), shear properties, and the spectrum of crustal shear modes. I systematically study uncertainties associated with the nuclear physics input, the crust composition, and neutron entrainment, and estimate their impact on crustal shear properties and the shear-mode spectrum. I find that the uncertainties originate mainly in two sources: The neutron-matter EOS and neutron entrainment. I compare the spectrum of crustal shear modes to observed frequencies of quasi-periodic oscillations in the afterglow of giant $γ$-ray bursts and find that all of these frequencies could be described within uncertainties, which are, however, at present too sizable to infer neutron-star properties from observations.

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