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

2018 ◽  
Vol 616 ◽  
pp. A105 ◽  
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.

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

2020 ◽  
Vol 642 ◽  
pp. A78 ◽  
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.

scholarly journals Speed of sound and quark confinement inside neutron stars

2020 ◽  
Vol 229 (22-23) ◽  
pp. 3651-3661
Author(s):  
Michał Marczenko
Keyword(s):  
Neutron Star ◽  
Equation Of State ◽  
Neutron Stars ◽  
Speed Of Sound ◽  
Dense Matter ◽  
High Mass ◽  
Quark Confinement ◽  
Binary Neutron Star ◽  
Upper Limit ◽  
Unified Description

AbstractSeveral observations of high-mass neutron stars (NSs), as well as the first historic detection of the binary neutron star merger GW170817, have delivered stringent constraints on the equation of state (EoS) of cold and dense matter. Recent studies suggest that, in order to simultaneously accommodate a 2M⊙ NS and the upper limit on the compactness, the pressure has to swiftly increase with density and the corresponding speed of sound likely exceeds the conformal limit. In this work, we employ a unified description of hadron-quark matter, the hybrid quark-meson-nucleon (QMN) model, to investigate the EoS under NS conditions. We show that the dynamical confining mechanism of the model plays an important role in explaining the observed properties of NSs.

scholarly journals Roles of crust and core in the tidal deformability of neutron stars

2019 ◽  
Vol 28 (09) ◽  
pp. 1950081 ◽  
Author(s):  
A. M. Kalaitzis ◽  
T. F. Motta ◽  
A. W. Thomas
Keyword(s):  
Neutron Star ◽  
Equation Of State ◽  
Neutron Stars ◽  
Dense Matter ◽  
Moment Of Inertia ◽  
Recent Measurement ◽  
Low Density ◽  
Important Constraint ◽  
The Moment

With the recent measurement of GW170817 providing constraints on the tidal deformability (TD) of a neutron star, it is very important to understand what features of the equation of state (EoS) have the biggest effect on it. We therefore study the contribution of the crust to the TD and the moment of inertia (MoI) of a neutron star for a variety of well-known EoS. It is found that the contributions to these quantities from the low-density crust are typically quite small and as a result the determination of the TD provides an important constraint on the EoS of dense matter.

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

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.

scholarly journals Probing the Nuclear Equation of State from the Existence of a ∼2.6 M⊙ Neutron Star: The GW190814 Puzzle

Symmetry ◽  
2021 ◽  
Vol 13 (2) ◽  
pp. 183
Author(s):  
Alkiviadis Kanakis-Pegios ◽  
Polychronis S. Koliogiannis ◽  
Charalampos C. Moustakidis
Keyword(s):  
Black Hole ◽  
Neutron Star ◽  
Equation Of State ◽  
Speed Of Sound ◽  
Compact Object ◽  
Dense Matter ◽  
High Mass ◽  
Nuclear Equation Of State ◽  
Dense Nuclear Matter ◽  
Low Mass

On 14 August 2019, the LIGO/Virgo collaboration observed a compact object with mass ∼2.59−0.09+0.08M⊙, as a component of a system where the main companion was a black hole with mass ∼23M⊙. A scientific debate initiated concerning the identification of the low mass component, as it falls into the neutron star–black hole mass gap. The understanding of the nature of GW190814 event will offer rich information concerning open issues, the speed of sound and the possible phase transition into other degrees of freedom. In the present work, we made an effort to probe the nuclear equation of state along with the GW190814 event. Firstly, we examine possible constraints on the nuclear equation of state inferred from the consideration that the low mass companion is a slow or rapidly rotating neutron star. In this case, the role of the upper bounds on the speed of sound is revealed, in connection with the dense nuclear matter properties. Secondly, we systematically study the tidal deformability of a possible high mass candidate existing as an individual star or as a component one in a binary neutron star system. As the tidal deformability and radius are quantities very sensitive on the neutron star equation of state, they are excellent counters on dense matter properties. We conjecture that similar isolated neutron stars or systems may exist in the universe and their possible future observation will shed light on the maximum neutron star mass problem.

Super-critical column accretion on to strongly magnetized neutron stars in ULX pulsars

2020 ◽  
Vol 72 (1) ◽  
Author(s):  
Tomohisa Kawashima ◽  
Ken Ohsuga
Keyword(s):  
Magnetic Field ◽  
Neutron Star ◽  
Neutron Stars ◽  
Dense Matter ◽  
High Density ◽  
High Mass ◽  
Hollow Cone ◽  
Radial Magnetic Field ◽  
Accretion Rates ◽  
Hydrodynamical Simulations

Abstract We carry out axisymmetric two-dimensional radiation hydrodynamical simulations of super-critical accretion columns on to strongly magnetized neutron stars. The effect of the strong magnetic field is taken into account by inhibiting the fluid motion across the radial magnetic field of the neutron stars. It is found that the high-density matter falls on to the neutron star along the sidewall of the column. Within the column, two high-density inflow regions shaped like a hollow cone are found for the case of extremely high mass accretion rates, ${\dot{M}}/(L_{\rm Edd}/c{\,}^2) \sim 5\times 10^{2}$, where $\dot{M}$, $L_{\rm Edd}$, and $c$ are the mass accretion rate on to the neutron star, the Eddington luminosity, and the speed of light, respectively. The less dense matter in the gap between the high density inflow regions is blown away via the radiative force. The resultant structure of the inflow looks like a triple hollow cone. Matter falls on to the neutron star only through the sidewall for the case of moderately high mass accretion rates, ${\dot{M}}/(L_{\rm Edd}/c{\,}^2)\, {\sim 3 \times 10^1}$. A low-density outflow fills the interior of the column. In this case, the inflow structure looks like a single hollow cone. Although the copious photons are generated in the inflow regions via a shock, the photons escape from the sidewall of the column and the radiation force does not prevent inflow. The resulting luminosity of the sidewall exceeds $\sim\! 30$ times the Eddington luminosity for neutron stars, which is consistent with the observed luminosity of ultra-luminous X-ray pulsars.

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

2020 ◽  
Vol 235 ◽  
pp. 07002
Author(s):  
Ingo Tews
Keyword(s):  
Neutron Star ◽  
Equation Of State ◽  
Neutron Stars ◽  
Nuclear Physics ◽  
Nuclear Astrophysics ◽  
Dense Matter ◽  
Effective Field ◽  
Chiral Effective Field Theory ◽  
Astrophysical Objects ◽  
R Process

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.

scholarly journals Tidal deformability and other global parameters of compact stars with strong phase transitions

2019 ◽  
Vol 622 ◽  
pp. A174 ◽  
Author(s):  
M. Sieniawska ◽  
W. Turczański ◽  
M. Bejger ◽  
J. L. Zdunik
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Neutron Star ◽  
Neutron Stars ◽  
Equations Of State ◽  
Dense Matter ◽  
Compact Stars ◽  
High Mass ◽  
Density Jump ◽  
Strong Phase

Context. Using parametric equations of state (relativistic polytropes and a simple quark bag model) to model dense-matter phase transitions, we study global, measurable astrophysical parameters of compact stars such as their allowed radii and tidal deformabilities. We also investigate the influence of stiffness of matter before the onset of the phase transitions on the parameters of the possible exotic dense phase. Aims. The aim of our study is to compare the parameter space of the dense matter equation of state permitting phase transitions to a sub-space compatible with current observational constraints such as the maximum observable mass, tidal deformabilities of neutron star mergers, radii of configurations before the onset of the phase transition, and to give predictions for future observations. Methods. We studied solutions of the Tolman-Oppenheimer-Volkoff equations for a flexible set of parametric equations of state, constructed using a realistic description of neutron-star crust (up to the nuclear saturation density), and relativistic polytropes connected by a density-jump phase transition to a simple bag model description of deconfined quark matter. Results. In order to be consistent with recent observations of massive neutron stars, a compact star with a strong high-mass phase transition cannot have a radius smaller than 12 km in the range of masses 1.2 − 1.6 M⊙. We also compare tidal deformabilities of stars with weak and strong phase transitions with the results of the GW170817 neutron star merger. Specifically, we study characteristic phase transition features in the Λ1 − Λ2 relation, and estimate the deviations of our results from the approximate formulæ for Λ∼ − R (M1) and Λ-compactness proposed in the literature. We find constraints on the hybrid equations of state to produce stable neutron stars on the twin branch. For the exemplary equations of state most of the high-mass twins occur for the minimum values of the density jump λ = 1.33 − 1.54; corresponding values of the square of the speed of sound are α = 0.7 − 0.37. We compare results with observations of gravitational waves and with the theoretical causal limit and find that the minimum radius of a twin branch is between 9.5 and 10.5 km, and depends on the phase transition baryon density. For these solutions the phase transition occurs below 0.56 fm−3.

scholarly journals Testing Pulsar Thermal Evolution Theories with Observation

2004 ◽  
Vol 218 ◽  
pp. 21-28
Author(s):  
Sachiko Tsuruta
Keyword(s):  
Neutron Star ◽  
Equation Of State ◽  
Neutron Stars ◽  
Thermal Evolution ◽  
Dense Matter ◽  
Stellar Radius ◽  
Neutron Star Cooling ◽  
Successful Launch ◽  
Careful Comparison ◽  
Insight Into

With the successful launch of Chandra and XMM-Newton, the time has arrived when careful comparison of thermal evolution theories of isolated neutron stars with observations will offer a better hope for distinguishing among various competing neutron star cooling theories. For instance, the latest theoretical and observational developments may already exclude both nucleon and kaon direct Urea cooling. In this way we can now have a realistic hope for determining various important properties, such as the composition, superfluidity, the equation of state and stellar radius. These developments should help us obtain deeper insight into the properties of dense matter.

scholarly journals ULTRA-DENSE NEUTRON STAR MATTER, STRANGE QUARK STARS, AND THE NUCLEAR EQUATION OF STATE

2007 ◽  
Vol 16 (04) ◽  
pp. 1165-1180 ◽  
Author(s):  
FRIDOLIN WEBER ◽  
MATTHEW MEIXNER ◽  
RODRIGO P. NEGREIROS ◽  
MANUEL MALHEIRO
Keyword(s):  
Neutron Star ◽  
Equation Of State ◽  
Neutron Stars ◽  
Strange Quark ◽  
Dense Matter ◽  
Neutron Star Matter ◽  
Nuclear Equation Of State ◽  
Atomic Nuclei ◽  
Quark Stars ◽  
The Universe

With central densities way above the density of atomic nuclei, neutron stars contain matter in one of the densest forms found in the universe. Depending of the density reached in the cores of neutron stars, they may contain stable phases of exotic matter found nowhere else in space. This article gives a brief overview of the phases of ultra-dense matter predicted to exist deep inside neutron stars and discusses the equation of state (EoS) associated with such matter.

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