What can neutron stars reveal about the equation of state of dense matter?

Neutron stars are astrophysical objects of extremes, reaching the highest densities we can observe in the cosmos, and probing matter under conditions that cannot be recreated in terrestrial experiments. In August 2017, the first neutron-star merger has been observed, which provided compelling evidence that these events are an important site for r-process nucleosynthesis. Furthermore, the gravitational-wave signal of such events might shed light upon the nature of strongly interacting matter in the neutron-star core. To understand these remarkable events, reliable nuclear physics input is essential. In this contribution, I explain how to use chiral effective field theory and advanced many-body methods to provide a consistent and systematic approach to strongly inter- acting systems from nuclei to neutron stars with controlled theoretical uncertainties. I will discuss recent results for the equation of state relevant for the nuclear astrophysics of neutron stars and neutron-star mergers.

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Impact of chiral hyperonic three-body forces on neutron stars

The European Physical Journal A ◽

10.1140/epja/i2019-12909-9 ◽

2019 ◽

Vol 55 (11) ◽

Cited By ~ 11

Author(s):

Domenico Logoteta ◽

Isaac Vidaña ◽

Ignazio Bombaci

Keyword(s):

Neutron Star ◽

Equation Of State ◽

Neutron Stars ◽

Nucleon Nucleon ◽

Effective Field ◽

Many Body ◽

Chiral Effective Field Theory ◽

Hartree Fock ◽

The Many ◽

Three Body

Abstract.We study the effects of the nucleon-nucleon-lambda (NN$ \Lambda$Λ three-body force on neutron stars. In particular, we consider the NN$ \Lambda$Λ force recently derived by the Jülich-Bonn-Munich group within the framework of chiral effective field theory at next-to-next-to-leading order. This force, together with realistic nucleon-nucleon, nucleon-nucleon-nucleon and nucleon-hyperon interactions, is used to calculate the equation of state and the structure of neutron stars within the many-body non-relativistic Brueckner-Hartree-Fock approach. Our results show that the inclusion of the NN$ \Lambda$Λ force leads to an equation of state stiff enough such that the resulting neutron star maximum mass is compatible with the largest currently measured ( $ \sim 2 M_\odot$∼2M⊙ neutron star masses. Using a perturbative many-body approach we calculate also the separation energy of the $ \Lambda$Λ in some hypernuclei finding that the agreement with the experimental data improves for the heavier ones when the effect of the NN$ \Lambda$Λ force is taken into account.

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Chiral Effective Field Theory and the High-Density Nuclear Equation of State

Annual Review of Nuclear and Particle Science ◽

10.1146/annurev-nucl-102419-041903 ◽

2021 ◽

Vol 71 (1) ◽

Author(s):

C. Drischler ◽

J.W. Holt ◽

C. Wellenhofer

Keyword(s):

Field Theory ◽

Equation Of State ◽

Neutron Stars ◽

Nuclear Matter ◽

Effective Field Theory ◽

Effective Field ◽

High Density ◽

Chiral Effective Field Theory ◽

Nuclear Equation Of State ◽

Density Regime

Born in the aftermath of core-collapse supernovae, neutron stars contain matter under extraordinary conditions of density and temperature that are difficult to reproduce in the laboratory. In recent years, neutron star observations have begun to yield novel insights into the nature of strongly interacting matter in the high-density regime where current theoretical models are challenged. At the same time, chiral effective field theory has developed into a powerful framework to study nuclear matter properties with quantified uncertainties in the moderate-density regime for modeling neutron stars. In this article, we review recent developments in chiral effective field theory and focus on many-body perturbation theory as a computationally efficient tool for calculating the properties of hot and dense nuclear matter. We also demonstrate how effective field theory enables statistically meaningful comparisons among nuclear theory predictions, nuclear experiments, and observational constraints on the nuclear equation of state. Expected final online publication date for the Annual Review of Nuclear and Particle Science, Volume 71 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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Thermal properties of hot and dense matter: Influence of rapid rotation on protoneutron stars, hot neutron stars, and neutron star merger remnants

EPJ Web of Conferences ◽

10.1051/epjconf/202125205004 ◽

2021 ◽

Vol 252 ◽

pp. 05004

Author(s):

Polychronis Koliogiannis ◽

Charalampos Moustakidis

Keyword(s):

Neutron Star ◽

Equation Of State ◽

Neutron Stars ◽

Nuclear Matter ◽

Equations Of State ◽

Interaction Model ◽

Dense Matter ◽

Baryon Density ◽

Dense Nuclear Matter ◽

Protoneutron Stars

The knowledge of the equation of state is a key ingredient for many dynamical phenomena that depend sensitively on the hot and dense nuclear matter, such as the formation of protoneutron stars and hot neutron stars. In order to accurately describe them, we construct equations of state at FInite temperature and entropy per baryon for matter with varying proton fractions. This procedure is based on the momentum dependent interaction model and state-of-the-art microscopic data. In addition, we investigate the role of thermal and rotation effects on microscopic and macroscopic properties of neutron stars, including the mass and radius, the frequency, the Kerr parameter, the central baryon density, etc. The latter is also connected to the hot and rapidly rotating remnant after neutron star merger. The interplay between these quantities and data from late observations of neutron stars, both isolated and in matter of merging, could provide useful insight and robust constraints on the equation of state of nuclear matter.

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Hyperons in hot dense matter: what do the constraints tell us for equation of state?

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2018.32 ◽

2018 ◽

Vol 35 ◽

Cited By ~ 12

Author(s):

M. Fortin ◽

M. Oertel ◽

C. Providência

Keyword(s):

Neutron Star ◽

Equation Of State ◽

Symmetry Energy ◽

Nuclear Physics ◽

Equations Of State ◽

Dense Matter ◽

Baryon Octet ◽

Thermodynamic Quantities ◽

Binary Neutron Star ◽

Hot Matter

AbstractFor core-collapse and neutron star merger simulations, it is important to have adequate equations of state which describe dense and hot matter as realistically as possible. We present two newly constructed equations of state including the entire baryon octet, compatible with the main constraints coming from nuclear physics, both experimental and theoretical. One of the equations of state describes cold β-equilibrated neutron stars with a maximum mass of 2 Msun. Results obtained with the new equations of state are compared with the ones of DD2Y, the only existing equation of state containing the baryon octet and satisfying the above constraints. The main difference between our new equations of state and DD2Y is the harder symmetry energy of the latter. We show that the density dependence of the symmetry energy has a direct influence on the amount of strangeness inside hot and dense matter and, consequently, on thermodynamic quantities. We expect that these differences affect the evolution of a proto-neutron star or binary neutron star mergers. We propose also several parameterisations based on the DD2 and SFHo models calibrated to Lambda hypernuclei that satisfy the different constraints.

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Constraints on the speed of sound of dense nuclear matter through the tidal deformability of neutron stars

EPJ Web of Conferences ◽

10.1051/epjconf/202125205005 ◽

2021 ◽

Vol 252 ◽

pp. 05005

Author(s):

Alkiviadis Kanakis-Pegios ◽

Polychronis Koliogiannis ◽

Charalampos Moustakidis

Keyword(s):

Neutron Star ◽

Equation Of State ◽

Neutron Stars ◽

Nuclear Matter ◽

Speed Of Sound ◽

Nuclear Physics ◽

Equations Of State ◽

Open Problems ◽

Binary Neutron Star ◽

Dense Nuclear Matter

One of the greatest interest and open problems in nuclear physics is the upper limit of the speed of sound in dense nuclear matter. Neutron stars, both in isolated and binary system cases, constitute a very promising natural laboratory for studying this kind of problem. This present work is based on one of our recent study, regarding the speed of sound and possible constraints that we can obtain from neutron stars. To be more speciﬁc, in the core of our study lies the examination of the speed of sound through the measured tidal deformability of a binary neutron star system (during the inspiral phase). The relation between the maximum neutron star mass scenario and the possible upper bound on the speed of sound is investigated. The approach that we used follows the contradiction between the recent observations of binary neutron star systems, in which the effective tidal deformability favors softer equations of state, while the high measured masses of isolated neutron stars favor stiffer equations of state. In our approach, we parametrized the stiffness of the equation of state by using the speed of sound. Moreover, we used the two recent observations of binary neutron star mergers from LIGO/VIRGO, so that we can impose robust constraints on the speed of sound. Furthermore, we postulate the kind of future measurements that could be helpful by imposing more stringent constraints on the equation of state.

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Progress in nuclear astrophysics of east and southeast Asia

AAPPS Bulletin ◽

10.1007/s43673-021-00018-z ◽

2021 ◽

Vol 31 (1) ◽

Author(s):

Azni Abdul Aziz ◽

Nor Sofiah Ahmad ◽

S. Ahn ◽

Wako Aoki ◽

Muruthujaya Bhuyan ◽

...

Keyword(s):

Southeast Asia ◽

Nuclear Physics ◽

Nuclear Reactions ◽

Nuclear Astrophysics ◽

Dense Matter ◽

Research Field ◽

Research Activities ◽

East And Southeast Asia ◽

R Process ◽

Nuclear Processes

AbstractNuclear astrophysics is an interdisciplinary research field of nuclear physics and astrophysics, seeking for the answer to a question, how to understand the evolution of the universe with the nuclear processes which we learn. We review the research activities of nuclear astrophysics in east and southeast Asia which includes astronomy, experimental and theoretical nuclear physics, and astrophysics. Several hot topics such as the Li problems, critical nuclear reactions and properties in stars, properties of dense matter, r-process nucleosynthesis, and ν-process nucleosynthesis are chosen and discussed in further details. Some future Asian facilities, together with physics perspectives, are introduced.

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Neural network reconstruction of the dense matter equation of state derived from the parameters of neutron stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038130 ◽

2020 ◽

Vol 642 ◽

pp. A78 ◽

Cited By ~ 2

Author(s):

F. Morawski ◽

M. Bejger

Keyword(s):

Neural Network ◽

Machine Learning ◽

Neural Networks ◽

Neutron Star ◽

Equation Of State ◽

Neutron Stars ◽

Equations Of State ◽

Dense Matter ◽

Data Set ◽

Global Parameters

Context. Neutron stars are currently studied with an rising number of electromagnetic and gravitational-wave observations, which will ultimately allow us to constrain the dense matter equation of state and understand the physical processes at work within these compact objects. Neutron star global parameters, such as the mass and radius, can be used to obtain the equation of state by directly inverting the Tolman-Oppenheimer-Volkoff equations. Here, we investigate an alternative approach to this procedure. Aims. The aim of this work is to study the application of the artificial neural networks guided by the autoencoder architecture as a method for precisely reconstructing the neutron star equation of state, using their observable parameters: masses, radii, and tidal deformabilities. In addition, we study how well the neutron star radius can be reconstructed using only the gravitational-wave observations of tidal deformability, that is, using quantities that are not related in any straightforward way. Methods. The application of an artificial neural network in the equation-of-state reconstruction exploits the non-linear potential of this machine learning model. Since each neuron in the network is basically a non-linear function, it is possible to create a complex mapping between the input sets of observations and the output equation-of-state table. Within the supervised training paradigm, we construct a few hidden-layer deep neural networks on a generated data set, consisting of a realistic equation of state for the neutron star crust connected with a piecewise relativistic polytropes dense core, with its parameters representative of state-of-the art realistic equations of state. Results. We demonstrate the performance of our machine-learning implementation with respect to the simulated cases with a varying number of observations and measurement uncertainties. Furthermore, we study the impact of the neutron star mass distributions on the results. Finally, we test the reconstruction of the equation of state trained on parametric polytropic training set using the simulated mass–radius and mass–tidal-deformability sequences based on realistic equations of state. Neural networks trained with a limited data set are capable of generalising the mapping between global parameters and equation-of-state input tables for realistic models.

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Estimating the equation of state from measurements of neutron star radii with 5% accuracy

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833071 ◽

2018 ◽

Vol 616 ◽

pp. A105 ◽

Cited By ~ 8

Author(s):

M. Sieniawska ◽

M. Bejger ◽

B. Haskell

Keyword(s):

Neutron Star ◽

Equation Of State ◽

Neutron Stars ◽

Dense Matter ◽

High Energy ◽

High Mass ◽

Recent Measurement ◽

Accuracy Of Measurements ◽

Observational Errors ◽

Global Parameters

Context. Observations of heavy (⋍2 M⊙) neutron stars, such as PSR J1614−2230 and PSR J0348+0432, in addition to the recent measurement of tidal deformability from the binary neutron-star merger GW170817, place interesting constraints on theories of dense matter. Currently operating and future observatories, such as the Neutron star Interior Composition Explorer (NICER) and the Advanced Telescope for High ENergy Astrophysics (ATHENA), are expected to collect information on the global parameters of neutron stars, namely masses and radii, with an accuracy of a few percent. Such accuracy will allow for precise comparisons of measurements to models of compact objects and significantly improve our understanding of the physics of dense matter. Aims. The dense-matter equation of state is still largely unknown. We investigate how the accuracy of measurements expected from the NICER and ATHENA missions will improve our understanding of the dense-matter interior of neutron stars. Methods. We compared global parameters of stellar configurations obtained using three different equations of state: a reference (SLy4 EOS) and two piecewise polytropes manufactured to produce mass-radius relations indistinguishable from an observational point of view, i.e. within the predicted error of radius measurement. We assumed observational errors on the radius determination corresponding to the accuracies expected for the NICER and ATHENA missions. The effect of rotation was examined using high-precision numerical relativity computations. Because masses and rotational frequencies might be determined very precisely in the most optimistic scenario, only the influence of observational errors on radius measurements was investigated. Results. We show that ±5% errors in radius measurement lead to ~10% and ~40% accuracy in central parameter estimations for low-mass and high-mass neutron stars, respectively. Global parameters, such as oblateness and surface area, can be established with 8–10% accuracy, even if only compactness (instead of mass and radius) is measured. We also report on the range of tidal deformabilities corresponding to the estimated masses of GW170817 for the assumed uncertainty in radius.

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Neutron Star Mass and Radius Measurements

Universe ◽

10.3390/universe5070159 ◽

2019 ◽

Vol 5 (7) ◽

pp. 159 ◽

Cited By ~ 9

Author(s):

James M. Lattimer

Keyword(s):

Neutron Star ◽

Equation Of State ◽

Nuclear Physics ◽

Structural Parameters ◽

Dense Matter ◽

Saturation Density ◽

Maximum Mass ◽

Binary Neutron Star ◽

Pulsar Timing ◽

Stellar Masses

Constraints on neutron star masses and radii now come from a variety of sources: theoretical and experimental nuclear physics, astrophysical observations including pulsar timing, thermal and bursting X-ray sources, and gravitational waves, and the assumptions inherent to general relativity and causality of the equation of state. These measurements and assumptions also result in restrictions on the dense matter equation of state. The two most important structural parameters of neutron stars are their typical radii, which impacts intermediate densities in the range of one to two times the nuclear saturation density, and the maximum mass, which impacts the densities beyond about three times the saturation density. Especially intriguing has been the multi-messenger event GW170817, the first observed binary neutron star merger, which provided direct estimates of both stellar masses and radii as well as an upper bound to the maximum mass.

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