Observations Of Neutron Stars Research Articles

Neural simulation-based inference of the neutron star equation of state directly from telescope spectra

Neutron stars provide a unique opportunity to study strongly interacting matter under extreme density conditions. The intricacies of matter inside neutron stars and their equation of state are not directly visible, but determine bulk properties, such as mass and radius, which affect the star's thermal X-ray emissions. However, the telescope spectra of these emissions are also affected by the stellar distance, hydrogen column, and effective surface temperature, which are not always well-constrained. Uncertainties on these nuisance parameters must be accounted for when making a robust estimation of the equation of state. In this study, we develop a novel methodology that, for the first time, can infer the full posterior distribution of both the equation of state and nuisance parameters directly from telescope observations. This method relies on the use of neural likelihood estimation, in which normalizing flows use samples of simulated telescope data to learn the likelihood of the neutron star spectra as a function of these parameters, coupled with Hamiltonian Monte Carlo methods to efficiently sample from the corresponding posterior distribution. Our approach surpasses the accuracy of previous methods, improves the interpretability of the results by providing access to the full posterior distribution, and naturally scales to a growing number of neutron star observations expected in the coming years.

The Neutron Star Mass, Distance, and Inclination from Precision Timing of the Brilliant Millisecond Pulsar J0437-4715

The observation of neutron stars enables the otherwise impossible study of fundamental physical processes. The timing of binary radio pulsars is particularly powerful, as it enables precise characterization of their (three-dimensional) positions and orbits. PSR J0437–4715 is an important millisecond pulsar for timing array experiments and is also a primary target for the Neutron Star Interior Composition Explorer (NICER). The main aim of the NICER mission is to constrain the neutron star equation of state by inferring the compactness (M p /R) of the star. Direct measurements of the mass M p from pulsar timing therefore substantially improve constraints on the radius R and the equation of state. Here we use observations spanning 26 yr from Murriyang, the 64 m Parkes radio telescope, to improve the timing model for this pulsar. Among the new precise measurements are the pulsar mass M p = 1.418 ± 0.044 M ⊙, distance D = 156.96 ± 0.11 pc, and orbital inclination angle i = 137.°506 ± 0.°016, which can be used to inform the X-ray pulse profile models inferred from NICER observations. We demonstrate that these results are consistent between multiple data sets from the Parkes Pulsar Timing Array (PPTA), each modeled with different noise assumptions. Using the longest available PPTA data set, we measure an apparent second derivative of the pulsar spin frequency and discuss how this can be explained either by kinematic effects due to the proper motion and radial velocity of the pulsar or excess low-frequency noise such as a gravitational-wave background.

Bayesian inference of neutron-skin thickness and neutron-star observables based on effective nuclear interactions

Bayesian inference of neutron-skin thickness and neutron-star observables based on effective nuclear interactions

Probing outbursts of the transient neutron star low-mass X-ray binary Aql X-1 with NICER: a study of spectral evolution

ABSTRACT X-ray observations of neutron star (NS) low-mass X-ray binaries (LMXBs) are useful for probing the physical processes close to the NS and for constraining source parameters. Aql X-1 is a transient NS LMXB that frequently undergoes outbursts and provides an excellent opportunity to study source properties and accretion mechanisms in a strong-gravity regime over a wide range of accretion rates. In this work, we systematically investigate the spectral evolution of Aql X-1 usingNICER observations during the source outbursts in 2019 and 2020. The NICER observations cover the complete transition of the source from its canonical hard state to a soft state and back. The spectra extracted from most observations can be explained by a partially Comptonized accretion disc. We find that the system can be described by an accretion disc with an inner temperature of $\sim 0.7$ keV and a Comptonizing medium of thermal electrons at $\sim 2$ keV, while the photon index is strongly degenerate with the covering fraction of the medium. We also find evidence of Fe K$\alpha$ fluorescence emission in the spectra, indicating reprocessing of the Comptonized photons. We observe an absorption column density higher than the Galactic column density for most of the observations, indicating a significant local absorption. For some of the observations in the 2020 outburst, however, the local absorption is negligible.

From neutron star observations to nuclear matter properties: A machine learning approach

From neutron star observations to nuclear matter properties: A machine learning approach

Relativistic description of dense matter equation of state and neutron star observables constrained by recent astrophysical observations

In the present work, we investigate the bulk properties of nuclear matter and neutron stars with the newly proposed relativistic interaction NL-RS which provides an opportunity to readjust the coupling constants keeping in view the properties of finite nuclei, nuclear matter, PREX-II results for neutron skin thickness in 208Pb and astrophysical observations. The NL-RS model interaction has been proposed by fitting the ground state properties (binding energies and charge radii) of finite nuclei, bulk nuclear matter properties, and PREX-II results for neutron skin thickness of 208Pb. The relativistic interaction has been generated by including nonlinear self-interactions of σ and ω μ -mesons and mixed interactions of ω μ , and ρ μ -meson up to the quartic order. The proposed interaction harmonizes with the finite nuclei, bulk nuclear matter, and neutron star properties. A covariance analysis is performed to assess the statistical uncertainties on the model parameters and nuclear observables of interest along with correlations amongst them. The equation of state (EoS) composed of nucleons and leptons in β-equilibrium is computed with the proposed parameter set and used to study the neutron star structure. The maximum mass of the neutron star by employing the EoS computed with the NL-RS parameter set is 2.04 ± 0.03M ⊙ and the radius of a canonical mass neutron star (R 1.4) comes out to be equal to 13.06 ± 0.16 Km. The value of dimensionless tidal deformability, for canonical mass, is 602.23 ± 33.13 which satisfies the constraints of waveform models analysis of GW170817 within 90% confidence level.

Constraints on the isovector properties of finite nuclei from neutron star observations

Constraints on the isovector properties of finite nuclei from neutron star observations

On the sound velocity bound in neutron stars

It has been suggested in the literature that the sound velocity of the nuclear matter vs exceeds the so-called sound velocity bound vs≤c/3 at high density, where c is the speed of light. In this paper, we revisit the issue of neutron star properties, comparing current measurements of mass, radius, and tidal deformability with 105 different equations of state. These equations are parametrized at low density and saturate the sound velocity bound beyond twice the saturation density. This allows us to conservatively determine the maximum mass compatible with observed neutron star properties and the sound velocity bound. We find that majority of the models are eliminated by the incompatibility with the observations and, especially, the recently detected massive pulsar (2.35±0.17M⊙) is hardly realized by our simulations. Our study strongly supports that the sound velocity bound is not satisfied in nature.

Implications on Skyrme equations of state from neutron skin measurements

Abstract The recent measurements on neutron skins via parity violation in electron scattering have extracted an abnormal thick neutron skin for $^{208}$Pb, which has significant consequences in nuclear equation of state (EoS) and neutron star observations. This work performs optimizations of extended Skyrme forces in a consistent manner by including neutron skin thicknesses from PREX-II and CREX experiments
and investigates nuclear EoS and neutron stars in the GW170817 event. By varying the fitting weights of neutron skins, several new Skyrme parameterizations are obtained. Our results show the competition in the fitting procedure to simultaneously describe neutron skins, other properties of finite nuclei and neutron star observations. The prospects in resolving the neutron skin issues are also discussed.

Vortex creep heating in neutron stars

Recent observations of old warm neutron stars suggest the presence of a heating source in these stars, requiring a paradigm beyond the standard neutron-star cooling theory. In this work, we study the scenario where this heating is caused by the friction associated with the creep motion of neutron superfluid vortex lines in the crust. As it turns out, the heating luminosity in this scenario is proportional to the time derivative of the angular velocity of the pulsar rotation, and the proportionality constant J has an approximately universal value for all neutron stars. This J parameter can be determined from the temperature observation of old neutron stars because the heating luminosity is balanced with the photon emission at late times. We study the latest data of neutron star temperature observation and find that these data indeed give similar values of J, in favor of the assumption that the frictional motion of vortex lines heats these neutron stars. These values turn out to be consistent with the theoretical calculations of the vortex-nuclear interaction.

Probing Dark Sectors with Neutron Stars

Tensions in the measurements of neutron and kaon weak decays, such as of the neutron lifetime, may speak to the existence of new particles and dynamics not present in the Standard Model (SM). In scenarios with dark sectors, particles that couple feebly to those of the SM appear. We offer a focused overview of such possibilities and describe how the observations of neutron stars, which probe either their structure or dynamics, limit them. In realizing these constraints, we highlight how the assessment of particle processes within dense baryonic matter impacts the emerging picture—and we emphasize both the flavor structure of the constraints and their broader connections to cogenesis models of dark matter and baryogenesis.

What Can We Learn about the Unstable Equation-of-state Branch from Neutron Star Mergers?

The equation of state (EOS) of dense strongly interacting matter can be probed by astrophysical observations of neutron stars (NS), such as X-ray detections of pulsars or the measurement of the tidal deformability of NSs during the inspiral stage of NS mergers. These observations constrain the EOS at most up to the density of the maximum-mass configuration, n TOV, which is the highest density that can be explored by stable NSs for a given EOS. However, under the right circumstances, binary neutron star (BNS) mergers can create a postmerger remnant that explores densities above n TOV. In this work, we explore whether the EOS above n TOV can be measured from gravitational-wave or electromagnetic observations of the postmerger remnant. We perform a total of 25 numerical-relativity simulations of BNS mergers for a range of EOSs and find no case in which different descriptions of the matter above n TOV have a detectable impact on postmerger observables. Hence, we conclude that the EOS above n TOV can likely not be probed through BNS merger observations for the current and next generation of detectors.

Targeted search for gravitational waves from highly spinning light compact binaries

Abstract Searches for gravitational waves from compact-binary mergers, which to date have reported ∼100 observations, have previously ignored binaries whose components are both consistent with the mass of neutron stars (1M⊙ to 2M⊙) and have high dimensionless spin >0.05. While previous searches targeted sources that are representative of observed neutron star binaries in the galaxy, it is already known that neutron stars can regularly be spun up to a dimensionless spin of ∼0.4, and in principle reach up to ∼0.7 before breakup would occur. Furthermore, there may be primordial black hole binaries or exotic formation mechanisms to produce light black holes. In these cases, it is possible for the binary constituent to be spun up beyond that achievable by a neutron star. A single detection of this type of source would reveal a novel formation channel for compact-binaries. To determine if there is evidence for any such sources, we use PyCBC to conduct a targeted search of LIGO and Virgo data for light compact objects with high spin. Our analysis detects previously known observations GW170817 and GW200115; however, we report no additional mergers. The most significant candidate, not previously known, is consistent with the noise distribution, and so we constrain the merger rate of spinning light binaries.

Determination of the equation of state from nuclear experiments and neutron star observations

Determination of the equation of state from nuclear experiments and neutron star observations

Isovector properties of finite nuclei: Constraints from neutron stars observations

The nuclear symmetry energy plays a crucial role in the structure of finite nuclei and the bulk properties of neutron stars. However, its values at high densities are highly uncertain, and the corresponding experimental data have large errors. One way to determine or at least estimate these high-density values is through neutron star observations. Recently, observations of gravitational waves from binary neutron star mergers have provided useful information on their radius and tidal deformability, which are directly related to the symmetry energy. This work attempts to use recent observations to constrain the structure of finite nuclei. Specifically, we parameterize the equation of state (EoS) describing asymmetric and symmetric nuclear matter using the parameter η = (K0L2)1/3, where K0 is the incompressibility and L is the slope parameter, The parameter η regulates the stiffness of the EoS, and we expect its values to affect both finite nuclei and neutron star properties, especially given the important role of isovector interactions. It is natural to expect that constraints on η for finite nuclei will also imply constraints on neutron star properties and vice versa. In light of the above, we propose a simple yet self-consistent method to simultaneously examine the effects of η on the properties of finite nuclei and neutron stars. We found constraints on these systems by combining recent experimental data (PREX-2) and observational data from the LIGO and Virgo detectors.

HESS J1731-347 and the existence of exotic matter: Kaon condensation in neutron stars

The recent observation of a compact star with the lowest observable mass of 0.77+0.20 –0.17 M⊙ within the supernova remnant HESS J1731-347 has redefined our understanding for the dense nuclear matter equation of state and enhances the existence of exotic matter. In the present work, we investigate the possible existence of kaon condensation in hadronic neutron stars through the framework of the Momentum-Dependent Interaction nuclear model describing the dense nuclear matter. We concentrate on the strangeness content of the proton and its implications on both the microscopic and macroscopic properties of neutron stars. The aforementioned quantity is of interest as it is also directly related to the location of the phase transition to kaons. The simultaneous description of the compact object within the HESS J1731-347 and the maximum observable neutron star mass imposes severe constraints on the strangeness content of the proton and the critical density for kaon condensation. The present study along with observations of neutron stars, may help to provide tighter constraints on the equation of state of dense nuclear matter and, even more, to shed light to the existence of an exotic core in neutron stars.

Bayesian analysis of nontrivial features in the speed of sound inside neutron stars in light of astrophysical and pQCD constraints

Functional forms of the neutron star Equation of State (EoS) are required to extract the viable EoS band from neutron star observations. Realistic nuclear EoS, containing deconfined quarks or hyperons, present nontrivial features in the speed of sound such as bumps, kinks, and plateaus. Using modified Gaussian processes to model EoS with nontrivial features, we show in a fully Bayesian analysis incorporating measurements from X-ray sources, gravitational wave observations, and perturbative QCD results that these features are compatible with current constraints. We find that nontrivial behavior in the EoS plays a role in understanding the possible phase structure of neutron stars at densities around 2 nsat.

Quark matter and nuclear astrophysics: Recent developments

Does deconfined cold quark matter occur in nature? This is currently one of the fundamental open questions in nuclear astrophysics. In these proceedings, I review the current state-of-the-art techniques to address this question in a model-agnostic manner, by synthesizing inputs from astrophysical observations of neutron stars and their binary mergers, and first-principles calculations within nuclear and particle theory. I highlight recent improvements in perturbative calculations in asymptotically dense cold quark matter, as well as compelling evidence for a conformalizing transition within the cores of massive neutron stars.

Emergent conformality in high-density matter

This contribution discusses baryon-rich matter in the context of recent astrophysical observations of neutron stars (NSs) and the structure of the equation of state that arises from these observations. We show that, generating an ensemble of equations of state that fulfill multimessenger constraints, the speed of sound and trace anomaly can reach their conformal values at the center of maximally massive NSs. The local peak of the speed of sound appears at values of the energy and particle densities being consistent with deconfinement and percolation conditions of QCD phase transition. Some implications of advocating a pseudo-conformal symmetry in dense matter are presented.

Heaven and Earth: Nuclear Astrophysics after GW170817

The historical detection of gravitational waves from the binary neutron star merger GW170817 is providing fundamental new insights into the astrophysical site for the creation of the heaviest elements in the cosmos and the equation of state of neutron-rich matter. Shortly after this historical detection, electromagnetic observations of neutron stars together with measurements of the properties of neutron-rich nuclei at terrestrial facilities have placed additional constraints on the dynamics of neutron-rich matter. It is this unique synergy between heaven and earth that is the focus of this article.