Second-generation gravitational-wave (GW) detectors, Advanced LIGO and Advanced Virgo, have opened a new window on the Universe. The first detection of GWs from a binary black hole merger in 2015 marked a major scientific breakthrough, followed by the observation of a neutron star binary coalescence accompanied by electromagnetic counterparts.
Since then, regular detections—particularly of...
Observations of the heaviest neutron stars, together with mass and radius
measurements, and gravitational wave signals from binary neutron neutron star mergers, progressively tighten the constraints on the equation-of-state of dense baryonic matter. Using the presently available data base, results are presented of detailed Bayesian inference analyses. A focus is on prerequisites and...
In this talk, I will provide a brief overview of some interesting aspects that the study of dense matter EoS may provide when combined with Beyond Standard Model ingredients. I will discuss some relevant features in the mechanisms and impact on the emission of multimessengers from dense stars. Additionally, I will highlight open questions that remain unresolved, yet are crucial for achieving a...
I will discuss the rapid recent progress made in modelling these systems and show how the gravitational signal can provide tight constraints on the equation of state, sound speed, and the occurrence of phase transitions. Finally, I will discuss a novel and tight correlation between the ratio of the energy and angular-momentum losses in the late-time portion of the post-merger signal, i.e., the...
We construct the equation of state of hypernuclear matter and study the structure of neutron stars employing
a chiral hyperon-nucleon interaction of the Julich–Bonn group tuned to femtoscopic ¨ Λp data of the ALICE Collaboration, and ΛΛ and ΞN interactions determined from lattice QCD calculations by the HAL QCD Collaboration that
reproduce the femtoscopic ΛΛ and Ξ
− p data. We employ the...
Neutron star masses and radii are observational tracers of the equation of state of cold dense matter.
One way to determine them is to model the thermal X-ray spectra of neutron stars. Here we present the results obtained by modeling the X-ray spectrum of a neutron star at the center of supernova remnant HESS J1731-347, the distance to which is 2.5(3)\,kpc, known from measurements by the Gaia...
In this presentation I will show how we can constrain Dark Matter (DM) scenarios with the supernova remnant HESS J1731-347. We assume the compact object to be an admixture of DM and Neutron Star, and presume the former to behave as a free Fermi gas. For the Neutron Star we use recently calculated regulator-independent equations of state for neutron stars obtained from first principles. Using...
The impact of dark matter (DM) on highly magnetized neutron stars (NSs) is explored using a two-fluid approach. Our model considers self-interacting, non-annihilating, asymmetric fermionic DM coupled to baryonic matter via gravity. Using the relativistic mean-field model with density-dependent magnetic fields, we examine how DM particle mass, mass fraction, and magnetic field strength affect...
The direct Urca (dUrca) process is a key mechanism driving rapid neutrino cooling in neutron stars, with its baryon density activation threshold determined by the microscopic model for nuclear matter. Understanding how nuclear interactions shape the dUrca threshold is essential for interpreting neutron star thermal evolution, particularly in light of recent studies on exceptionally cold...
We explore "ridges" in the macroscopic properties of rotating neutron stars as potential indicators of first-order phase transitions in their matter. These phase transitions induce non-analytic behavior in observables like angular momentum, moment of inertia, mass, and radii, with the intensity of this behavior directly tied to the latent heat of the transition. Notably, the Seidov limit sets...
We study the thermal index of low-density, high-temperature dense matter. We use the virial expansion to account for nuclear interaction effects. We focus on the region of validity of the expansion, which reaches $10^{-3}$ fm$^{-3}$ at $T=5$ MeV up to almost saturation density at $T=50$ MeV.
In pure neutron matter, we find an analytical expression for the thermal index, and show that it is...
In this contribution, I will start with an overview of different types of equation of state modelling in the Bayesian formalism, to demonstrate the িmpact of different experimental and observational constraints. Further, I will present equations of state at finite temperature obtained with Brussels-Skyrme-on-a-Grid (BSkG) energy density functionals developed at Brussels, which are unified...
With ongoing advancements in nuclear theory and experimentation, coupled with a growing body of neutron star (NS) observations, a wealth of information concerning the equation of state (EOS) for matter at extreme densities has become accessible. In this talk, I will discuss the present status of NS EOS by combining this information using a comprehensive Bayesian statistics. We use a hybrid EOS...
Properties of high-density strong-interaction matter of relevance for astrophysical scenarios that involve neutron stars are discussed. It is argued that theoretical insights from recent lattice comparisons of effective QCD models for vanishing baryo-chemical potential but finite isospin can guide realistic model building at high baryon density, as this regime is currently not accessible to...
The long-term evolution of neutron stars' strong internal magnetic fields requires advanced numerical modeling. Observations suggest a highly non-uniform magnetic structure, underscoring the need for three-dimensional simulations. In this talk, I will introduce MATINS, a novel three-dimensional numerical code based on a finite-volume scheme, specifically designed to simulate magneto-thermal...
The oscillation modes of neutron star (NS) merger remnants, as encoded by the kHz postmerger gravitational wave (GW) signal, hold great potential for constraining the as-yet undetermined equation of state (EOS) of dense nuclear matter. Previous works have used numerical relativity simulations to derive quasi-universal relations for the key oscillation frequencies, but most of them omit the...
We study the influence of hyperons in binary neutron star (NS) mergers, considering equations of state (EoSs) models which include hyperonic degrees of freedom and partly delta resonances. Thermally produced hyperons induce a higher heat capacity and a lower thermal index, i.e. a reduced thermal pressure for a given amount of thermal energy, compared to purely nucleonic models. These EoSs are...
I will discuss the recent work on the construction of equilibrium hybrid stars, the emergence of twins and triplets and their signatures, such as mass, radius and tidal deformability. I will further discuss the bulk viscosity of hybrid star matter which is relevant for the studies of binary neutron star mergers and their gravitational wave signals.
We study rotating hybrid stars with a particular emphasis on the effect of a deconfinement phase transition on their properties at high spin. Our analysis is based on a hybrid equation of state with a phase transition from hypernuclear matter to color-superconducting quark matter, where both phases are described within a relativistic density functional approach. By varying the vector meson and...
In this talk, I will discuss the properties of the two-flavor quark-meson diquark model as a low-energy effective model for QCD at finite density.
I will map out the phase diagram in the mu_I-mu_B plane, where mu_I and mu_B are the isospin and baryon chemical potentials, respectively.
I will present results for the speed of sound in two cases a) finite mu_I and mu_B=0, and b) finite mu_B...
The core of a neutron star is an entity of enigma due to the mystery surrounding its composition. Insights from nuclear theory and perturbative QCD suggest the possible existence of hybrid stars having a phase transition from hadronic matter to quark matter. This study presents how different types of phase transition (namely, Maxwell and Gibbs) can leave imprints on gravitational waves from...
Unlike the widely used Walecka-type models for studying nuclear matter properties and neutron star structures, we extended the linear sigma model, originally developed by Schechter and his colleagues, to the baryonic sector to explore dense nucleon systems. This extended framework is termed the baryonic extended linear sigma model (bELSM).
The bELSM incorporates 2-quark and 4-quark...
An intriguing hypothesis states that ordinary hadronic matter in bulk is a metastable state (i.e., a local minimum) of strongly interacting matter, while strange quark matter (SQM) is absolutely stable (i.e., the global minimum). These two phases would be separated by a potential barrier that prevents a spontaneous deconfinement transition of ordinary hadronic matter into SQM.
Our work aims...
Matter with extreme baryon density is believed to occur in the core of a neutron star, allowing for various novel phases, from conventional nucleon superfluid phase to exotic high baryon density QCD phases. Here, we point out a unique phenomenon associated with phase transitions to a superfluid phase, which may be the nucleon superfluid phase, or a phase like CFL phase, allowing for superfluid...
While symmetric nuclear matter has been studied in laboratories, neutron star matter is characterized by high asymmetry. Therefore, by examining the strongly interacting matter properties in a wide range of densities and isospin asymmetry we confront two regimes to understand how the enforced electric neutrality and beta equilibrium alter the onset density of quark matter. Particularly, we...
The influence of modified gravity on the bulk properties of neutron stars can be constrained through integration of the Tolman–Oppenheimer–Volkoff (TOV) equations, provided the input equation of state (EoS) is derived solely from microscopic physics. This becomes particularly challenging in the intermediate to high baryon-density regime, where neither nuclear nor chiral effective field...
The ab-initio determination of the nuclear response to small perturbation is of fundamental importance to predict transport properties of neutron star matter and cross sections for electro-weak processes in neutron stars. The direct calculation of dynamical properties however poses serious challenges to traditional many-body methods. In this talk I will discuss how quantum computing could help...
Abstract: Understanding the interactions between nucleons in dense matter is one of the outstanding challenges of theoretical physics. Effective field theories have emerged as the dominant approach to address this problem at low energies, with many successful applications to the structure of nuclei and the properties of dense nucleonic matter. However, how far into the interior of neutron...
The scale invariance of QCD at classical level in chiral limit is broken by quantum effect. When matching trace anomaly of QCD to the effective theory of strong interaction at low energy, one can setup a framework including the scalar meson field in chiral effective theory---scale-chiral EFT. In this talk, I will present the essence of the scale-chiral EFT and discuss the behavior of the trace...
The composition of the core of neutron stars is still under debate. One possibility is that matter could be deconfined into quark matter due to the high densities reached in their cores. The possible existence of hadronic and hybrid stars is studied using microscopic models to describe the different phases of matter, including RMF models, chiral symmetric models, and quark models. Within...
Precise and reliable measurements of neutron star radii are essential to our understanding of cold, catalyzed matter beyond nuclear saturation density. After reviewing the methodology used to measure these quantities from NICER X-ray, I will present our current mass and radius measurements and discuss their implications for the matter in the cores of neutron stars.
Light nuclear clusters, such as H and He isotopes, are expected to be present in astrophysical environments and play an important role in different astrophysical phenomena involving ultra-dense baryonic matter: in binary neutron star mergers, the abundance of light clusters has a direct influence on the fraction of the ejecta, or on the viscous evolution of the accretion disk after the...
In this work, we perform a Bayesian analysis putting together the available knowledge from the nuclear physics experiments and astrophysical observations to explore the equation of state of supranuclear matter. In particular, we employ a relativistic metamodeling technique to nuclear matter to cover the uncertainties in the parameter space of the saturation properties of nuclear matter, both...
Formed in the aftermath of gravitational core-collapse supernova explosions, neutron stars are the most compact observed stars. Their average density exceeds that found inside atomic nuclei. Neutron stars are also endowed with the highest magnetic fields known, which can reach millions of billion times that of the Earth. According to our current understanding, a neutron star is stratified into...
A large sample of equation of state (EOS) models built within a Bayesian inference is used to investigate the thermal behavior of homogeneous matter with Brussels extended Skyrme interactions over wide ranges of density and temperature, in both symmetric nuclear matter and pure neutron matter. Our key finding is that, via a lower limit on the effective mass, the condition imposed on neutron...
Bayesian inference offers a powerful framework for constraining the nuclear equation of state (EoS) across a wide range of densities by combining information from astrophysical observations, ab-initio nuclear theory, and heavy-ion collisions. In this work, we refine a unified meta-modeling framework for the EoS by incorporating low-density corrections based on energy density functionals...
We investigate the properties of non-accreted crusts of neo-neutron stars, i.e., of inhomogeneous subsaturation warm dense matter in beta equilibrium. We present two novel results and one known, but frequently ignored property of such matter. The new features include: the presence of an exotic light nucleus, $^{14}$He, starting from the baryon density of $\approx~0.01~$fm$^{-3}$ and up to the...
Matching the equation of state (EoS) of pure neutron matter (PNM) with the EoS of the crust (outer and inner) of a neutron star (NS) is fundamental to understanding the properties of neutron stars, but remains a major challenge.
Our starting point here is the EoS band obtained for PNM at zero temperature and very low densities, expressed in terms of the nucleon scattering data [1], and...
The Equation of State (EoS) of nuclear matter is related to many topics in nuclear physics. In particular, it is crucial for understanding the structure of compact objects such as neutron stars. In the conservative hypothesis of a purely nucleonic composition of neutron star matter, the EoS is fully determined in terms of the so-called nuclear matter parameters (NMPs), which, in principle, can...
This talk investigates the impact of assuming a barotropic equation of state (EoS) for neutron stars, which neglects potential out-of-β-equilibrium effects, on the calculation of oscillation modes. We focus on how the assumption of β-equilibrated EoS influences the frequencies of the non-radial fundamental (f) and first pressure (p1) modes. These calculations are performed using a wide set of...
We investigate the hyperonic equation of state (EoS) within the non-linear derivative model that incorporates a momentum dependence on the interactions, with a special emphasis on properly establishing the conditions for hyperon appearance in neutron star matter. We demonstrate that hyperons can appear at finite momentum, forming a so-called “moat” region, even when they are
absent at zero...
We present a holographic framework designed to model strongly interacting matter relevant for the interiors of neutron stars. This approach captures key features of QCD at finite density and provides a controlled approximation for the nonperturbative regime inaccessible to standard methods. By calibrating the model to lattice data at low density, we obtain predictions for the equation of state...
Estimating the value of bulk viscosity in neutron stars is crucial to determine the dampening of oscillations and gravitational wave emission in pulsars and binary mergers. The main contribution originates from weak processes through beta equilibration rates and depends both on the equation of state and weak equilibration rates. We use symmetry arguments in the form of Ward identities to...
We propose a three-flavor nonlocal Nambu–Jona-Lasinio model of quark matter with attractive scalar and diquark, and repulsive vector interaction channels to study the question whether an approximately conformal behavior of the strongly interacting quark matter in neutron star interiors is possible. The model qualitatively agrees with the perturbative quantum chromodynamics (pQCD), which...
