arXiv subjects

Rodrigo Negreiros

Publications and source records attributed to Rodrigo Negreiros.

At least 19 recordsLinked to original sources

arXiv2026

Neutron Star Equation of State via Physics Informed Neural Network

We present the first application, to the best of our knowledge, of Physics-Informed Neural Networks (PINNs) to the neutron star equation-of-state (EOS) inverse problem. Two interacting networks -- one representing the EOS $P(\varepsilon)$ as a continuous, non-parametric function, the other solving the Tolman-Oppenheimer-Volkoff (TOV) equations -- are trained jointly on NICER X-ray timing posteriors and pulsar mass measurements. The TOV equations enter as a mean-square ODE residual enforced via automatic differentiation at every training step, rooted in the Neural Differential Equation framework. The inferred EOS satisfies nuclear saturation properties, causality, and perturbative QCD bounds simultaneously; $\chi$EFT consistency at $1$--$2\rhoz$ emerges without explicit enforcement, providing a non-trivial self-consistency check. Across $N=15$ independent training runs, we find a neutron star maximum mass $M_\mathrm{max}=2.06^{+0.07}_{-0.09}$ and radius and tidal deformability of a 1.4 $M_\odot$ star $R_{1.4}=12.85^{+0.03}_{-0.06}$~km and $\Lambda_{1.4}=684$, respectively, with 68\% CI, in agreement with recent Bayesian analyses. Most interestingly, the speed of sound exhibits a reproducible softening at $2$--$4\,\rhoz$, consistent with a quark-hadron crossover.

astro-ph.HE↗

arXiv2026

Sub-stellar Strange Quark Matter Objects: Predicting a New Class of Highly-Compact Candidates

We investigate the existence and stability of highly-compact sub-stellar objects composed of strange quark matter (SQM), focusing on finite-size strangelets with baryon number $A \leq 100$. Motivated by the emergence of mass--radius outliers in the \textit{Gaia} DR3 era, we employ a Bayesian exploration of the MIT bag-model parameter space, explicitly accounting for finite-size surface and curvature contributions that become relevant at low baryon number. Enforcing the bulk absolute-stability requirement for SQM ($E/A < 930~\mathrm{MeV}$), we find that self-gravitating equilibrium sequences are confined to the sub-stellar regime, with typical masses $M \simeq 10^{-2}$--$10^{-1}\,M_{\odot}$ and characteristic radii of order $10^{3}$--$10^{4}$ km. We further show that rapid rotation, treated through a self-consistent framework that incorporates relativistic thermodynamics, can substantially inflate the equatorial radius and extend the accessible mass--radius domain. While rotation does not eliminate the intrinsic high-density compactness of these configurations, it shifts the most extended models closer to the observational parameter space of massive exoplanets. A comparison with objects from the NASA Exoplanet Archive reveals a pronounced density gap separating standard atomic-matter planets and brown dwarfs from the strangelet-rich branch predicted here. We conclude that light strangelets cannot account for solar-mass white dwarfs, but they robustly predict a previously unexplored population of ultra-compact sub-stellar objects, offering testable targets for future microlensing searches and high-cadence photometric surveys.

astro-ph.HE↗

arXiv2026

S-wave kaon condensation in neutron-star matter within a chiral model framework with dynamical meson masses

We investigate s-wave kaon condensation in dense matter and neutron stars within the updated Chiral Mean Field model with an improved meson description (mCMF), which incorporates dynamically generated in-medium meson masses arising from explicit chiral symmetry breaking and vector-meson self-interactions. In contrast to conventional relativistic mean-field descriptions with constant meson masses, the mCMF framework introduces a self-consistent feedback between the meson sector and the dense-matter equations of motion. The kaon dispersion relation is derived from the nonlinear SU(3) Lagrangian, including the Weinberg-Tomozawa interaction and additional baryon-pseudoscalar couplings, and the onset of condensation is determined under conditions of charge neutrality and $\beta$ equilibrium. Our calculations include the full baryon octet together with electrons and muons at zero temperature. We analyze the impact of hyperons, muons, and kaon condensation on the equation of state, on neutron-star mass--radius relations, and neutron-star thermal evolution, and examine the sensitivity of the onset density and stellar properties to variations in the nucleon--kaon scattering length and to different model vector parameters and vector self-interactions. We find that $K^{-}$ condensation sets in between $n \sim (2-8)\, n_0$ (in units of nuclear saturation density) and leads to a moderate to strong softening (in one case, a slight stiffening of the equation of state), depending on the interplay of kaons and hyperons, while remaining compatible with current $2\,M_\odot$ and small-radius neutron-star observational constraints and producing distinguishable behavior in the neutron-star cooling. This work provides an improved and thermodynamically consistent framework for studying exotic degrees of freedom in neutron-star matter.

nucl-th↗

arXiv2026

Enhanced Neutrino Cooling from Parity-Doubled Nucleons in Neutron Star Cooling Simulations

Although restoration of chiral symmetry is predicted by quantum chromodynamics to take place at high baryon density, most modeling of neutron star interiors disregards a chiral phase transition. We model neutron star cores with a parity doublet model, which allows for dynamical chiral symmetry restoration and predicts the appearance of the parity partners of nucleons and hyperons at large densities, as well as deconfined quark matter. We study the thermal evolution of neutron stars, focusing for the first time on the impact of Urca processes involving the parity partners in neutron star cooling simulations. We find that Urca processes for the parity partners of the nucleons significantly affect the thermal evolution of massive stars and allow for improved agreement with observed surface temperature and ages.

astro-ph.HE↗

arXiv2025

Axisymmetric Cooling of Neutron Stars with Strong Magnetic Fields

We study the cooling evolution of neutron stars with strong poloidal magnetic fields (with strength not far from observed values) using the full general relativity 2-dimensional \textit{Astreus} code, which solves consistently Einstein's and Maxwell's equations. We find that central magnetic fields with strengths $3-4\times10^{17}$ G, corresponding to surface magnetic fields $7-8\times10^{16}$, can significantly modify the cooling behavior of neutron stars, leading to stars with similar masses but different magnetic fields to exhibit different thermal evolution. We show a non-linear increase in the thermal relaxation time with increasing magnetic fields and that this behavior is associated with the reduction of the Direct Urca process in stars with strong magnetic fields. This is a novel result in which we can observe the magnetic field influence on the thermal evolution of stars, even if it is not strong enough to affect the Fermi distribution of particles.

astro-ph.HE↗

arXiv2024

Can the central compact object in HESS J1731--347 be indeed the lightest neutron star observed?

The exceptionally low mass of $0.77_{-0.17}^{+0.2} M_{\odot}$ for the central compact object (CCO) XMMU J173203.3 -- 344518 (XMMU J1732) in the supernova remnant (SNR) HESS J1731 -- 347 challenges standard neutron star (NS) formation models. The nearby post-AGB star IRAS 17287 -- 3443 ($\approx 0.6 M_\odot$), also within the SNR, enriches the scenario. To address this puzzle, we advance the possibility that the gravitational collapse of a rotating pre-SN iron core ($\approx 1.2 M_\odot$) could result in a low-mass NS. We show that angular momentum conservation during the collapse of an iron core rotating at $\approx 45\%$ of the Keplerian limit results in a mass loss of $\approx 0.3 M_\odot$, producing a stable newborn NS of $\approx 0.9 M_\odot$. Considering the possible spin-down, this indicates that the NS is now slowly rotating, thus fulfilling the observed mass-radius relation. Additionally, the NS's surface temperature ($\approx 2 \times 10^6$ K) aligns with canonical thermal evolution for its $\approx 4.5$ kyr age. We propose the pre -- SN star, likely an ultra-stripped core of $\approx 4.2 M_\odot$, formed a tidally locked binary with IRAS 17287 -- 3443, having a 1.43-day orbital period. The supernova led to a $\approx 3 M_\odot$ mass loss, imparting a kick velocity $\lesssim 670$ km s$^{-1}$, which disrupted the binary. This scenario explains the observed 0.3 pc offset between XMMU J1732 and IRAS 17287 -- 3443 and supports the possibility of CCOs forming in binaries, with rotation playing a key role in core-collapse, and the CCO XMMU J1732 being the lightest NS ever observed.

astro-ph.HE↗

arXiv2024

Rotational Properties of Inverted Hybrid Stars

We study the rotational properties of inverted hybrid stars (also termed cross stars), which have been recently proposed as a possible new class of compact stars characterized by an outer layer of quark matter and a core of hadrons, in an inverted structure compared to traditional hybrid stars. We analyze distinct models representing varying depths of quark-hadron phase transitions. Our findings reveal that, while cross stars rotating at their Kepler frequencies typically exhibit a significantly higher mass and larger circumferential radius as anticipated, interestingly, there is a significant increase in potential twin configurations in the case of rapid rotations. We further study sequences of constant baryonic mass, representing potential paths of rotational evolution. Our results indicate that not all stars in these sequences are viable due to the onset of phase transitions during spin-down, leading to possible mini-collapses. We also investigate the phenomenon of ``back-bending" during spin-down sequences, which is manifested in a rather different shape for cross stars due to their inverted structure and the large density discontinuity caused by the strong phase transition, in contrast to traditional hybrid stars. Our research enriches existing studies by introducing the significant aspect of rotation, unveiling intr

astro-ph.HE↗

arXiv2024

Fast Pulsars, Neutron Stars, and Astrophysical Strange Quark Matter Objects

This book chapter explores key aspects of neutron stars, pulsar glitches, tidal deformability, fast pulsars, the equation of state, and strange quark matter stars. Challenges in directly measuring neutron star radius have led to reliance on spectroscopic and timing techniques, with uncertainties addressed through careful source selection and theoretical modeling. Pulsar glitches reveal insights into the equation of state through angular momentum transfer within the neutron star. Tidal deformability is crucial in gravitational-wave astronomy, exemplified by the GW170817 event. Fast pulsars, instrumental in astrophysical testing, are classified into ordinary pulsars, millisecond pulsars, and magnetars. The EOS is vital for understanding neutron star internal structure, explored through various models. The chapter delves into the theoretical framework for rotating neutron stars, addressing uniform and differential rotation scenarios and their impacts on mass and radius. Additionally, the intriguing concept of quark stars and strange dwarfs is investigated. The various topics discussed in this book chapter contribute to a broader understanding of dense matter physics, astrophysical phenomena, and the potential for transformative discoveries through advanced observational techniques and technologies like gravitational wave detectors, radio telescopes, and X-ray telescopes.

astro-ph.HE↗

arXiv2023

What is the nature of the HESS J1731-347 compact object?

Once further confirmed in future analyses, the radius and mass measurement of HESS J1731-347 with $M=0.77^{+0.20}_{-0.17}~M_{\odot}$ and $R=10.4^{+0.86}_{-0.78}~\rm km$ will be among the lightest and smallest compact objects ever detected. This raises many questions about its nature and opens up the window for different theories to explain such a measurement. In this article, we use the information from Doroshenko et al. (2022) on the mass, radius, and surface temperature together with the multimessenger observations of neutron stars to investigate the possibility that HESS J1731-347 is one of the lightest observed neutron star, a strange quark star, a hybrid star with an early deconfinement phase transition, or a dark matter-admixed neutron star. The nucleonic and quark matter are modeled within realistic equation of states (EOSs) with a self-consistent calculation of the pairing gaps in quark matter. By performing the joint analysis of the thermal evolution and mass-radius constraint, we find evidence that within a 1$σ$ confidence level, HESS J1731-347 is consistent with the neutron star scenario with the soft EOS as well as with a strange and hybrid star with the early deconfinement phase transition with a strong quark pairing and neutron star admixed with dark matter.

astro-ph.HE↗

arXiv2021

Thermal Relaxation and Cooling of Quark Stars with a Strangelet Crust

In this article, we explore the cooling of isolated quark stars. These objects are structured of a homogeneous quark matter core and crusted by matter. To do this, we adopt two kinds of crust: (i) a crust made of purely nuclear matter following the Baym-Pethick-Sutherland (BPS) equation of state (EoS) and (ii) a crust made of nuggets of strange quark matter (strangelets). Both models have the same quark matter core described by the MIT bag model EoS. Our main purpose is to quantify the effects of a strangelet crust on the cooling and relaxation times of these strange stars. We also perform a thorough study of the thermal relaxation of quark stars, in which we have found that objects with a strangelet crust have a significantly different thermal relaxation time. Our study also includes the possible effects of color superconductivity in the quark core.

hep-ph↗

arXiv2020

Short-range correlation effects on the neutron star cooling

Short range correlations (SRC) have been known to be an important aspect of nuclear theory for some time. Recent works have re-ignited interest on this topic, particularly due to the fact that it has recently been demonstrated that SRC may be responsible for breaking pairing gaps in nuclear matter. In this work we revisit the concept of SRC for beta equilibrated matter in neutron stars. We construct two equivalent models, with and without SRC and proceed to investigate the thermal evolution of stars described by such models. We show that SRC play a major role in the thermal evolution of neutron stars. It will be shown that while the SRC largely leaves the macroscopic properties of the star unaltered, it significantly alters the proton fraction, thus leading to an early onset of the direct Urca (DU) process, which in turns leads to stars exhibiting much faster cooling.

nucl-th↗

arXiv2020

Revisiting the thermal relaxation of neutron stars

In this work we revisit the thermal relaxation process for neutron stars. Such process is associated with the thermal coupling between the core and the crust of neutron stars. The thermal relaxation, which takes place at around 10 -- 100 years, is manifested as a sudden drop of the star's surface temperature. Such drop is smooth for slow cooling objects and very sharp for those with fast cooling. In our study we focus particularly on the cooling of neutron stars whose mass is slightly greater than the value above which the direct Urca (DU) process sets in. Considering different mechanisms for neutrino production in each region of the star, and working with equations of state with different properties, we solve the thermal evolution equation and calculate the thermal relaxation time for ample range of neutron star masses. By performing a comprehensive study of neutron stars just above the onset of the direct Urca process we show that stars under these conditions exhibit a peculiar thermal relaxation behavior. We demonstrate that such stars exhibit an abnormally late relaxation time, characterized by a second drop of its surface temperature taking place a later ages. We qualify such behavior by showing that it is associated with limited spatial distribution of the DU process is such stars. We show that as the star's mass increase, the DU region also grows and the start exhibits the expected behavior of fast cooling stars. Finally we show that one can expect high relaxation times for stars in which the DU process takes place in a radius not larger than 3 km.

astro-ph.HE↗

arXiv2020

Orbital Properties and Gravitational Wave Signatures of Strange Crystal Planets

In this paper we consider the possibility that strange quark matter may be manifested in the form of strangelet crystal planets. These planet-like objects are made up of nuggets of strange quark matter (SQM), organized in a crystalline structure. We consider the so--called strange matter hypothesis proposed by Bodmer, Witten and Terazawa, in that, strange quark matter may be the absolutely stable state of matter. In this context, we analyze planets made up entirely of strangelets arranged in a crystal lattice. Furthermore we propose that a solar system with a host compact star may be orbited by strange crystal planets. Under this assumption we calculate the relevant quantities that could potentially be observable, such as the planetary tidal disruption radius, and the gravitational waves signals that may arise from potential star-planet merger events. Our results show that strangelet crystal planets could potentially be used as an indicator for the the existence of SQM.

astro-ph.HE↗

arXiv2019

Neutron star cooling and GW170817 constraint within quark-meson coupling models

In the present work we used five different versions of the quark-meson coupling (QMC) model to compute astrophysical quantities related to the GW170817 event and to neutron star cooling process. Two of the models are based on the original bag potential structure and three versions consider a harmonic oscillator potential to confine the quarks. The bag-like models also incorporate the pasta phase used to describe the inner crust of neutron stars. We show that the pasta phase always play a minor or negligible role in all studies. Moreover, while no clear correlation between the models that satisfy the GW170817 constraints and the slope of the symmetry energy is found, a clear correlation is observed between the slope and the fact that the cooling is fast or slow, i.e., fast (slow) cooling is related to higher (lower) values of the slope. We did not find one unique model that can describe, at the same time, GW170817 constraints and give a perfect description of the possible cooling processes.

astro-ph.HE↗

arXiv2019

The Equation of State and Cooling of Hyperonic Neutron Stars

We present two recent parametrizations of the equation of state (FSU2R and FSU2H models) that reproduce the properties of nuclear matter and finite nuclei, fulfill constraints on high-density matter stemming from heavy-ion collisions, produce 2$M_{\odot}$ neutron stars, and generate neutron star radii below 13 km. Making use of these equations of state, cooling simulations for isolated neutron stars are performed. We find that two of the models studied, FSU2R (with nucleons) and, in particular, FSU2H (with nucleons and hyperons), show very good agreement with cooling observations, even without including nucleon pairing. This indicates that cooling observations are compatible with an equation of state that produces a soft nuclear symmetry energy and, thus, generates small neutron star radii. Nevertheless, both schemes produce cold isolated neutron stars with masses above $1.8 M_{\odot}$.

astro-ph.HE↗

arXiv2018

Cooling of Small and Massive Hyperonic Stars

We perform cooling simulations for isolated neutron stars using recently developed equations of state for their core. The equations of state are obtained from new parametrizations of the FSU2 relativistic mean-field functional that reproduce the properties of nuclear matter and finite nuclei, while fulfilling the restrictions on high-density matter deduced from heavy-ion collisions, measurements of massive 2$M_{\odot}$ neutron stars, and neutron star radii below 13 km. We find that two of the models studied, FSU2R (with nucleons) and in particular FSU2H (with nucleons and hyperons), show very good agreement with cooling observations, even without including extensive nucleon pairing. This suggests that the cooling observations are more compatible with an equation of state that produces a soft nuclear symmetry energy and, hence, generates small neutron star radii. However, both models favor large stellar masses, above $1.8 M_{\odot}$, to explain the colder isolated neutron stars that have been observed, even if nucleon pairing is present.

astro-ph.HE↗

arXiv2015

Growth of the Magnetic Field in Young Neutron Stars

Rotation Powered-Pulsars are subjected to long-term changes in their period of rotation, which are measured by timing observations of their rotation frequency and its derivatives ($Ω$, $\dotΩ$, $\ddotΩ$). If the spin-down is solely due to dipolar radiation, the braking index should be $n=3$. To date, only a handful of braking indices have been estimated for young pulsars, and in all cases one observes that $n<3$. These observations suggest that there are complex spin-down processes taking place in the pulsar that are not fully well understood. In the present work we revisit the spin-down of young pulsars by considering a possible magnetic field growth due ohmic diffusion. In order to perform such study we perform calculations with phenomenological growth functions for the magnetic field. With that we are able to calculate the spin evolution of the neutron star with all relevant quantities. We show that such approach could explain the low values of $n$ in very young neutron stars and may be relevant to explain why a small group of neutron stars (including central compact objects) exhibit no evidence of a standard magnetic field. We find that the possibility of magnetic field growth are relevant to the spin evolution of young neutron stars, and therefore should not be neglected.

astro-ph.HE↗

arXiv2013

Thermal Evolution of Rotating Neutron Stars

In this work we consider the thermal evolution of rigidly rotating neutron stars. In order to perform such study we first calculate the structure of rotating objects, which is considerably more complicated than that of spherical objects. The structure of rotating neutron stars is obtained by solving Einstein's equation for a rotationally deformed fluid distributions. The numerical method used is based on the the KEH. The equation of state used for computing the neutron star structure and composition is a simple relativistic mean field model, with parameter set G300. With the structure of rotating neutron stars computed we calculate the thermal evolution of these objects. In order to do so, we re-derive the thermal evolution equations to account for the metric of a rotating object. The cooling of neutron stars with different frequencies is then calculated. We show that the cooling of the star strongly depends on the frequency of the object, with higher frequencies stars showing a substantial temperature difference between the equator and poles.

astro-ph.HE↗