Astrophysical measurement of the equation of state of neutron star matter (original) (raw)

Low‐Mass Neutron Stars and the Equation of State of Dense Matter

The Astrophysical Journal, 2003

Neutron-star radii provide useful information on the equation of state of neutron rich matter. Particularly interesting is the density dependence of the equation of state (EOS). For example, the softening of the EOS at high density, where the pressure rises slower than anticipated, could signal a transition to an exotic phase. However, extracting the density dependence of the EOS requires measuring the radii of neutron stars for a broad range of masses. A "normal" 1.4M⊙ (M⊙=solar mass) neutron star has a central density of a few times nuclear-matter saturation density (ρ0). In contrast, low mass (≃ 0.5M⊙) neutron stars have central densities near ρ0 so its radius provides information on the EOS at low density. Unfortunately, low-mass stars are rare because they may be hard to form. Instead, a precision measurement of nuclear radii on atomic nuclei may contain similar information. Indeed, we find a strong correlation between the neutron radius of 208 Pb and the radius of a 0.5M⊙ neutron star. Thus, the radius of a 0.5M⊙ neutron star can be inferred from a measurement of the the neutron radius of 208 Pb. Comparing this value to the measured radius of a ≃ 1.4M⊙ neutron star should provide the strongest constraint to date on the density dependence of the equation of state.

Neutron stars: Cosmic laboratories for matter under extreme conditions

EPJ Web of Conferences, 2016

The true nature and the internal constitution of the compact stars known as neutron stars (NSs) is one of the most fascinating enigma in modern astrophysics. We discuss some of the present models for the internal structure of NSs and the connection with the properties of ultra dense hadronic matter. In particular, we discuss the role of strangeness on the equation of state and the implications of the measurement of 2 solar mass NSs in PSR J1614-2230 and PSR J0348+0432. Neutron stars (NSs) are incomparable natural laboratories that allow us to investigate the fundamental constituents of matter and their interactions under extreme conditions that cannot be reproduced in terrestrial laboratories. The bulk properties and the internal constitution of NSs primarily depend on the equation of state (EoS) of strong interacting matter [1], i.e. on the thermodynamical relation between the matter pressure, energy density and temperature. Determining the correct EoS model describing NS is a fundamental problem of nuclear physics and astrophysics, and a major effort has been made during the last few decades to solve it by measuring different NS properties using the data collected by various generations of X-ray and γ-ray satellites and by ground-based radio telescopes. The rather recent accurate measurement of the masses, M = 1.97 ± 0.04 M sun [2] and M = 2.01 ± 0.04 M sun [3], of the neutron stars in PSR J1614-2230 and PSR J0348+0432 respectively, has ruled out all the EoS models which cannot support such high values of stellar masses.

The Dense Matter Equation of State from Neutron Star Radius and Mass Measurements

The Astrophysical Journal, 2016

We present a comprehensive study of spectroscopic radius measurements of twelve neutron stars obtained during thermonuclear bursts or in quiescence. We incorporate, for the first time, a large number of systematic uncertainties in the measurement of the apparent angular sizes, Eddington fluxes, and distances, in the composition of the interstellar medium, and in the flux calibration of X-ray detectors. We also take into account the results of recent theoretical calculations of rotational effects on neutron star radii, of atmospheric effects on surface spectra, and of relativistic corrections to the Eddington critical flux. We employ Bayesian statistical frameworks to obtain neutron star radii from the spectroscopic measurements as well as to infer the equation of state from the radius measurements. Combining these with the results of experiments in the vicinity of nuclear saturation density and the observations of ∼ 2 M neutron stars, we place strong and quantitative constraints on the properties of the equation of state between ≈ 2 − 8 times the nuclear saturation density. We find that around M = 1.5 M , the preferred equation of state predicts radii between 10.1 − 11.1 km. When interpreting the pressure constraints in the context of high density equations of state based on interacting nucleons, our results suggest a relatively weak contribution of the three-body interaction potential.

Neutron star interiors and the equation of state of ultra-dense matter

2006

Neutron stars contain matter in one of the densest forms found in the Universe. This feature, together with the unprecedented progress in observational astrophysics, makes such stars superb astrophysical laboratories for a broad range of exciting physical studies. This paper gives an overview of the phases of dense matter predicted to make their appearance in the cores of neutron stars. Particular emphasis is put on the role of strangeness. Net strangeness is carried by hyperons, K-mesons, H-dibaryons, and strange quark matter, and may leave its mark in the masses, radii, moment of inertia, dragging of local inertial frames, cooling behavior, surface composition, and the spin evolution of neutron stars. These observables play a key role for the exploration of the phase diagram of dense nuclear matter at high baryon number density but low temperature, which is not accessible to relativistic heavy ion collision experiments.

Dense Stellar Matter and Structure of Neutron Stars

Eprint Arxiv Astro Ph 9609074, 1996

After giving an overview of the history and idea of neutron stars, I shall discuss, in part one of my lectures, the physics of dense neutron star matter. In this connection concepts like chemical equilibrium and electric charge neutrality will be outlined, and the baryonic and mesonic degrees of freedom of neutron star matter as well as the transition of confined hadronic matter into quark matter at supernuclear densities are studied. Special emphasis is put onto the possible absolute stability of strange quark matter. Finally models for the equation of state of dense neutron star matter, which account for various physical possibilities concerning the unknown behavior of superdense neutron star matter, will be derived in the framework of relativistic nuclear field theory. Part two of my lectures deals with the construction of models of static as well as rapidly rotating neutron stars and an investigation of the cooling behavior of such objects. Both is performed in the framework of Einstein's theory of general relativity. For this purpose the broad collection of models for the equation of state, derived in part one, will be used as an input. Finally, in part three a comparison of the theoretically computed neutron star properties (e.g., masses, radii, moments of inertia, red and blueshifts, limiting rotational periods, cooling behavior) with the body of observed data will be performed. From this conclusions about the interior structure of neutron stars-and thus the behavior of matter at extreme densities-will be drawn.

Properties of high-density matter in neutron stars

This short review aims at giving a brief overview of the various states of matter that have been suggested to exist in the ultra-dense centers of neutron stars. Particular emphasis is put on the role of quark deconfinement in neutron stars and on the possible existence of compact stars made of absolutely stable strange quark matter (strange stars). Astrophysical phenomena, which distinguish neutron stars from quark stars, are discussed and the question of whether or not quark deconfinement may occur in neutron stars is investigated. Combined with observed astrophysical data, such studies are invaluable to delineate the complex structure of compressed baryonic matter and to put firm constraints on the largely unknown equation of state of such matter.

GW170817: Measurements of Neutron Star Radii and Equation of State

Physical Review Letters, 2018

GW170817: Measurements of neutron star radii and equation of state The LIGO Scientific Collaboration and The Virgo Collaboration On August 17, 2017, the LIGO and Virgo observatories made the first direct detection of gravitational waves from the coalescence of a neutron star binary system. The detection of this gravitational wave signal, GW170817, offers a novel opportunity to directly probe the properties of matter at the extreme conditions found in the interior of these stars. The initial, minimal-assumption analysis of the LIGO and Virgo data placed constraints on the tidal effects of the coalescing bodies, which were then translated to constraints on neutron star radii. Here, we expand upon previous analyses by working under the hypothesis that both bodies were neutron stars that are described by the same equation of state and have spins within the range observed in Galactic binary neutron stars. Our analysis employs two methods: the use of equation-of-state-insensitive relations between various macroscopic properties of the neutron stars and the use of an efficient parameterization of the defining function p(ρ) of the equation of state itself. From the LIGO and Virgo data alone and the first method, we measure the two neutron star radii as R 1 = 10.8 +2.0 −1.7 km for the heavier star and R 2 = 10.7 +2.1 −1.5 km for the lighter star at the 90% credible level. If we additionally require that the equation of state supports neutron stars with masses larger than 1.97 M as required from electromagnetic observations and employ the equation of state parametrization, we further constrain R 1 = 11.9 +1.4 −1.4 km and R 2 = 11.9 +1.4 −1.4 km at the 90% credible level. Finally, we obtain constraints on p(ρ) at supranuclear densities, with pressure at twice nuclear saturation density measured at 3.5 +2.7 −1.7 × 10 34 dyn cm −2 at the 90% level.

Neutron-rich nuclei and the equation of state of stellar matter

Physica Scripta, 2013

In this contribution we will review our present understanding of the matter equation of state in the density and temperature conditions where it can be described by nucleonic degrees of freedom. At zero temperature, all the information is contained in the nuclear energy functional in its isoscalar and isovector channels. At finite temperature, particular emphasis will be given to the specificity of the thermodynamics in the nucleonic regime, with the simultaneous presence of long range electromagnetic and short range nuclear interactions. The astrophysical implications of the resulting phase diagram, as well as of different observables of exotic nuclei on the neutron-rich side, will be touched upon.