arXiv subjects

Constança Providência

Publications and source records attributed to Constança Providência.

At least 73 records · Page 4Linked to original sources

arXiv2023

Spanning the full range of neutron star properties within a microscopic description

The high-density behavior of nuclear matter is analyzed within a relativistic mean-field description with non-linear meson interactions. To assess the model parameters and their output, a Bayesian inference technique is used. The Bayesian setup is limited only by a few nuclear saturation properties, the neutron star maximum mass larger than 2 M$_\odot$, and the low-density pure neutron matter equation of state (EOS) produced by an accurate N$^3$LO calculation in chiral effective field theory. Depending on the strength of the non-linear scalar vector field contribution, we have found three distinct classes of EOSs, each one correlated to different star properties distributions. If the non-linear vector field contribution is absent, the gravitational maximum mass and the sound velocity at high densities are the greatest. However, it also gives the smallest speed of sound at densities below three times saturation density. On the other hand, models with the strongest non-linear vector field contribution predict the largest radii and tidal deformabilities for 1.4 M$_\odot$ stars, together with the smallest mass for the onset of the nucleonic direct Urca processes and the smallest central baryonic densities for the maximum mass configuration. These models have the largest speed of sound below three times saturation density, but the smallest at high densities, in particular, above four times saturation density the speed of sound decreases approaching approximately $\sqrt{0.4}c$ at the center of the maximum mass star. On the contrary, a weak non-linear vector contribution gives a monotonically increasing speed of sound. A 2.75 M$_\odot$ NS maximum mass was obtained in the tail of the posterior with a weak non-linear vector field interaction. We found that pQCD favors models with a large non-linear vector field contribution or hyperons.

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arXiv2022

Hybrid stars with large strange quark cores

The possible existence of hybrid stars is studied using several multi-quark interaction channels. The hadronic phase consists of an equation of state (EoS) with presently accepted nuclear matter properties and the quark model is constrained by the vacuum properties of several light mesons. The dependence of several NS properties on the different quark interactions is analyzed. We show that the present constraints from neutron star observations allow for the existence of hybrid stars with large strangeness content and large quark cores.

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arXiv2022

Extracting nuclear matter properties from the neutron star matter equation of state using deep neural networks

The extraction of the nuclear matter properties from neutron star (NS) observations is nowadays an important issue, in particular, the properties that characterize the symmetry energy which are essential to describe correctly asymmetric nuclear matter. We use deep neural networks (DNNs) to map the relation between cold $β$-equilibrium NS matter and the nuclear matter properties. Assuming a quadratic dependence on the isospin asymmetry for the energy per particle of homogeneous nuclear matter and using a Taylor expansion up to fourth order in the iso-scalar and iso-vector contributions, we generate a dataset of different realizations of $β$-equilibrium NS matter and the corresponding nuclear matter properties. The DNN model was successfully trained, attaining great accuracy in the test set. Finally, a real case scenario was used to test the DNN model, where a set of 33 nuclear models, obtained within a relativistic mean field approach or a Skyrme force description, were fed into the DNN model and the corresponding nuclear matter parameters recovered with considerable accuracy, in particular, the standard deviations $σ(L_{\text{sym}})= 12.85$ MeV and $σ(K_{\text{sat}})= 41.02$ MeV were obtained, respectively, for the slope of the symmetry energy and the nuclear matter incompressibility at saturation.

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arXiv2022

Machine-Learning Love: classifying the equation of state of neutron stars with Transformers

The use of the Audio Spectrogram Transformer (AST) model for gravitational-wave data analysis is investigated. The AST machine-learning model is a convolution-free classifier that captures long-range global dependencies through a purely attention-based mechanism. In this paper a model is applied to a simulated dataset of inspiral gravitational wave signals from binary neutron star coalescences, built from five distinct, cold equations of state (EOS) of nuclear matter. From the analysis of the mass dependence of the tidal deformability parameter for each EOS class it is shown that the AST model achieves a promising performance in correctly classifying the EOS purely from the gravitational wave signals, especially when the component masses of the binary system are in the range $[1,1.5]M_{\odot}$. Furthermore, the generalization ability of the model is investigated by using gravitational-wave signals from a new EOS not used during the training of the model, achieving fairly satisfactory results. Overall, the results, obtained using the simplified setup of noise-free waveforms, show that the AST model, once trained, might allow for the instantaneous inference of the cold nuclear matter EOS directly from the inspiral gravitational-wave signals produced in binary neutron star coalescences.

astro-ph.IM↗

arXiv2022

Bayesian inference of signatures of hyperons inside neutron stars

The possible signatures of the presence of hyperons inside neutron stars are discussed within a Bayesian inference framework applied to a set of models based on a density-dependent relativistic mean-field description of hadronic matter. Nuclear matter properties, hypernuclei properties, and observational information are used to constrain the models. General properties of neutron stars such as the maximum mass, radius, tidal deformability, proton fraction, hyperon fraction, and speed of sound are discussed. It is shown that the two solar mass constraint imposes that neutron stars described by equations of state that include hyperons have on average a larger radius, $\gtrsim 0.5$km, and a larger tidal deformability, $\gtrsim 150$, than the stars determined from a nucleonic equation of state, while the speed of sound at the center of the star is more than 25\% smaller. If a 1.4 M$_\odot$ star with a radius $\lesssim 12.5$ km is measured it is quite improbable that a massive star described by the same model contains hyperons. A similar conclusion is drawn if a two solar mass star with a radius $\lesssim 11.5$ km or a neutron star with a mass above 2.2 M$_\odot$ is observed: the possible hyperon content of these stars is ruled out or very reduced. The hyperon presence inside neutron stars is compatible with the present NICER mass-radius observations of the pulsars PSR J0030+0451 and PSR J0740+6620 and the gravitational wave detection GW170817. {It is shown that if the polytropic index $γ=\partial\ln p/\partial\lnε$ takes values of the order of 1.75 at not too large densities, it may indicate the onset of some kind of exotic matter, but not necessarily of deconfined quark matter.

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arXiv2022

Inferring the nuclear symmetry energy at supra saturation density from neutrino cooling

An ambitious goal of the astrophysical community is not only to constrain the equation of state (EOS) of neutron star (NS) matter by confronting it with astrophysics observations, but ultimately also to infer the NS composition. Nevertheless, the composition of the NS core is likely to remain uncertain unless we have an accurate determination of the nuclear symmetry energy at supra saturation density ($ρ>ρ_0$). We investigate how the nucleonic direct Urca (dUrca) processes can be used as an effective probe to constraint the high density nuclear symmetry energy. A large number of minimally constrained EOSs has been constructed by applying a Bayesian approach to study the correlations of the symmetry energy at different densities with a few selected properties of a NS. The nuclear symmetry energy above the baryon density 0.5 fm$^{-3}$ ($\sim 3 ρ_0$) is found to be strongly correlated with NS mass at which the onset of nucleonic dUrca neutrino cooling takes place in the core. This allows us to constrain the high density behavior of nuclear symmetry energy within narrow bounds. {The pure neutron matter pressure constraint from chiral effective field theory rules out the onset of nucleonic dUrca in stars with a mass $\lesssim$ 1.4 $M_\odot$.} The onset of dUrca inside 1.6 M$_\odot$ to 1.8 M$_\odot$ NS implies a slope of the symmetry energy $L$ at $\sim 2.5~ρ_0$, respectively, between 54 and 48 MeV.

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arXiv2022

Identification of Binary Neutron Star Mergers in Gravitational-Wave Data Using YOLO One-Shot Object Detection

We demonstrate the application of the YOLOv5 model, a general purpose convolution-based single-shot object detection model, in the task of detecting binary neutron star (BNS) coalescence events from gravitational-wave data of current generation interferometer detectors. We also present a thorough explanation of the synthetic data generation and preparation tasks based on approximant waveform models used for the model training, validation and testing steps. Using this approach, we achieve mean average precision ($\text{mAP}_{[0.50]}$) values of 0.945 for a single class validation dataset and as high as 0.978 for test datasets. Moreover, the trained model is successful in identifying the GW170817 event in the LIGO H1 detector data. The identification of this event is also possible for the LIGO L1 detector data with an additional pre-processing step, without the need of removing the large glitch in the final stages of the inspiral. The detection of the GW190425 event is less successful, which attests to performance degradation with the signal-to-noise ratio. Our study indicates that the YOLOv5 model is an interesting approach for first-stage detection alarm pipelines and, when integrated in more complex pipelines, for real-time inference of physical source parameters.

astro-ph.IM↗

arXiv2022

Delta baryons in neutron stars

By applying a relativistic mean-field description of neutron star matter with density dependent couplings, we analyse the properties of two different matter compositions: nucleonic matter with delta baryons and nucleonic matter with hyperons and delta baryons. The delta-meson couplings are allowed to vary within a wide range of values obtained by experimental data, while the hyperon-meson couplings are fitted to hypernuclear properties. Neutron star properties with no deconfinement phase transition are studied. It is verified that many models are excluded because the effective nucleon mass becomes zero before the maximum mass configuration is attained. Hyperon-free with delta-dominated composition compact stars are possible, the deltic stars. It is found that with a convenient choice of parameters the existence of deltic stars with 80% of delta baryons at the center of the star is possible. However, the presence of hyperons lowers the delta baryon fraction to values below 20% at the center and below 30% at 2-3 saturation densities. It is discussed that in the presence of delta baryons, the hyperon softening is not so drastic because deltas couple more strongly to the $ω$-meson, and the stiffness of the equation of state is determined by the $ω$-dominance at high densities. The speed of sound reflects very well this behavior. The compactness of the pulsar RX J0720.4-3125 imposes $x_{σΔ}>x_{ωΔ}>1$ and favors $x_{ρΔ}>1$.

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arXiv2022

Heavy baryons in hot stellar matter with light nuclei and hypernuclei

The production of light nuclei and hypernuclei together with heavy baryons, both hyperons and $Δ$-baryons, in low density matter as found in stellar environments such as supernova or binary mergers is studied within relativistic mean-field models. Five light nuclei were considered together with three light hypernuclei. The presence of both hyperons and $Δ$-baryons shift the dissolution of clusters to larger densities and increase the abundance of clusters. This effect is larger the smaller the charge fraction and the higher the temperature. The couplings of the $Δ$-baryons were chosen imposing that the nucleon effective mass remains finite inside neutron stars.

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arXiv2022

Relativistic description of dense matter equation of state and compatibility with neutron star observables: a Bayesian approach

The general behavior of the nuclear equation of state (EOS), relevant for the description of neutron stars (NS), is studied within a Bayesian approach applied to a set of models based on a density dependent relativistic mean field description of nuclear matter. The EOS is subjected to a minimal number of constraints based on nuclear saturation properties and the low density pure neutron matter EOS obtained from a precise next-to-next-to-next-to-leading order (N$^{3}$LO) calculation in chiral effective field theory ($χ$EFT). The posterior distributions of the model parameters obtained under these minimal constraints are employed to construct the distributions of various nuclear matter properties and NS properties such as radii, tidal deformabilites, central energy densities and speeds of sound etc. We found that 90% confidence interval (CI) for allowed NS mass - radius relationship and tidal deformabilites are compatible with GW170817 and recent NICER observations, without invoking the exotic degrees of freedom. A central speed-of-sound of the order of $\sqrt{2/3}$ $c$ is obtained. The maximum neutron star mass allowed by the model is 2.5 $M_\odot$.

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arXiv2022

Pasta phases in neutron stars under strong magnetic fields

In the present work, we consider nuclear matter in the innermost crust of neutron stars under the presence of a strong magnetic field within the framework of a relativistic mean-field description. Two models with a different slope of the symmetry energy are considered in order to discuss the density-dependence of the equation of state on the crust structure. The non-homogeneous matter in $β$-equilibrium is described within the coexisting phases method, and the effect of including the anomalous magnetic moment is discussed. Five different geometries for the pasta structures are considered. It is shown that strong magnetic fields cause an extension of the inner crust of the neutron stars, with the occurrence of a series of disconnected non-homogeneous matter regions above the one existing for a null magnetic field. Moreover, we observed that in these disconnected regions, for some values of the magnetic field, all five different cluster geometrical shapes occur, and the gas density is close to the cluster density. Also, the pressure at the neutron star crust-core transition much larger than the pressure obtained for a zero magnetic field. Another noticeable effect of the presence of strong magnetic fields is the increase of the proton fraction, favoring the appearance of protons in the gas background.

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arXiv2022

Landau parameters and entrainment matrix of cold stellar matter: effect of the symmetry energy and strong magnetic fields

Nuclear matter properties based on a relativistic approach suitable for the description of multi-component systems are calculated. We use a set of nuclear relativistic mean-field models that satisfy acceptable nuclear matter properties and neutron star observations. The effects of the density dependence of the symmetry energy and of the Landau quantization due to the presence of a strong external magnetic field are discussed. Properties such as the proton fraction, the Landau mass, Landau parameters and entrainment matrix, the adiabatic index and speed of sound are calculated for cold $β$-equilibrium matter. A large dispersion on the calculated properties is obtained at two to three times saturation density $ρ_0 $. The proton Landau mass can be as low as one third of the vacuum nucleon mass at 2-3$~ρ_0 $. Similar effects are obtained for the Landau parameters, in particular, the ones involving protons, where the relative dispersion of $F^0_{pp}$ and $F^1_{pp}$ is as high as 30\% to 50\% at 2-3$~ρ_0 $. These parameters are particularly sensitive to the symmetry energy. The effect of the magnetic field on the nuclear properties is small for fields as high as 10$^{18}$G except for a small range of densities just above the crust-core transition. Tables with the EoS, and the parameters, are provided in the Supplementary Material section.

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arXiv2022

Determination of the symmetry energy from the neutron star equation of state

We analyze the uncertainties introduced in the determination of the neutron star matter proton fraction, in a range of densities close to the saturation density, if the cold $β$-equilibrium neutron star matter equation of state (EoS) is known. In particular, we discuss the effect of neglecting the muon contribution and of considering that the energy density of nuclear matter is well described by taking only terms until second order in the proton-neutron asymmetry. It is shown that two types of uncertainties may be associated with the extraction of the symmetry energy from the $β$-equilibrium equation of state: an overestimation if terms above the parabolic approximation on the asymmetry parameter are neglected, or an underestimation if the muon contribution is not considered. The effect of the uncertainty on the symmetric nuclear matter EoS on the determination of the proton fraction is discussed. It could be shown that the neutron star mass-radius curve is sensitive to the parabolic approximation on the asymmetry parameter.

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arXiv2021

Hybrid stars from a constrained equation of state

We determine, within a meta-model, the properties of the nuclear matter equation of state (EoS) that allow for a phase transition to deconfinement matter. It is shown that the properties that define the isoscalar channel are the ones that are affected, in particular, a phase transition implies much larger values of the skewness and kurtosis. The effect of multi-quark interaction channels in the description of the quark phase in hybrid stars is also studied. NS properties, such as the mass and radius of the quark core, show an interplay dependence between the 8-quark vector and the 4-quark isovector-vector interactions. We show that low mass NS, $M\sim 1.4 M_\odot$, may already contain a quark core, and satisfy all existing NS observational constraints. We discuss the strangeness content of the quark core and its influence on the speed of sound.

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arXiv2021

Constraints on high density equation of state from maximum neutron star mass

The low density nuclear matter equation of state is strongly constrained by nuclear properties, however, for constraining the high density equation of state it is necessary to resort to indirect information obtained from the observation of neutron stars, compact objects that may have a central density several times nuclear matter saturation density, $n_0$. Taking a meta-modelling approach to generate a huge set of equation of state that satisfy nuclear matter properties close to $n_0$ and that do not contain a first order phase transition, the possibility of constraining the high density equation of state was investigated. The entire information obtained from the GW170817 event for the probability distribution of $\tildeΛ$ was used to make a probabilistic inference of the EOS, which goes beyond the constraints imposed by nuclear matter properties. Nuclear matter properties close to saturation, below $2n_0$, do not allow us to distinguish between equations of state that predict different neutron star (NS) maximum masses. This is, however, not true if the equation of state is constrained at low densities by the tidal deformability of the NS merger associated to GW170817. Above $3n_0$, differences may be large, for both approaches, and, in particular, the pressure and speed of sound of the sets studied do not overlap, showing that the knowledge of the NS maximum mass may give important information on the high density EOS. Narrowing the maximum mass uncertainty interval will have a sizeable effect on constraining the high density EOS.

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arXiv2021

The Neutron Star Outer Crust Equation of State: A Machine Learning approach

Constructing the outer crust of the neutron stars requires the knowledge of the Binding Energy (BE) of the atomic nuclei. Although the BE of a lot of the nuclei is experimentally determined and can be obtained from the AME data table, for the others we need to depend on theoretical models. There exist a lot of physical theories to predict the BE, each with its own strengths and weaknesses. In this paper, we apply Machine Learning (ML) algorithms on AME2016 data set to predict the Binding Energy {of atomic nuclei}. The novel feature of our work is that it is model independent. We do not assume or use any nuclear physics model but use only ML algorithms directly on the AME2016 data set. Our results are further refined by using another ML algorithm to train the errors of the first algorithm, and repeating this process iteratively. Our best algorithm gives $σ_{\rm rms} \approx 0.58$ MeV for Binding Energy on randomized testing sets. This is comparable to all physics models or ML improved physics models studied in literature till date. Using the predictions of our Machine Learning algorithm, we construct the outer crust equation of state (EoS) of a neutron star and show that our model is comparable to existing models. This work also demonstrates the use of various ML algorithms and a detailed analysis on how we arrived at our best algorithm. It will help the physics community in understanding how to choose an ML algorithm which would be suited for their data set. Our algorithms and best fit model is also made publicly available for the use of the community.

physics.comp-ph↗

arXiv2021

Crust-core transition of a neutron star: effect of the temperature under strong magnetic fields

The effect of temperature on the crust-core transition of a magnetar is studied. The thermodynamical spinodals are used to calculate the transition region within a relativistic mean-field approach for the equation of state. Magnetic fields with intensities $5\times 10^{16}$ G and $5\times 10 ^{17}$ G are considered. It is shown that the effect on the extension of the crust-core transition is washed away for temperatures above $10^{9}$ K for magnetic field intensities $ \lesssim 5\times 10^{16}$ G but may still persist if a magnetic field as high as $5\times 10 ^{17}$G is considered. For temperatures below that value, the effect of the magnetic field on crust-core transition is noticeable and grows as the temperature decreases and, in particular, it is interesting to identify the existence of disconnected non-homogeneous matter above the $B=0$ crust core transition density. Models with different symmetry energy slopes at saturation show quite different behaviors. In particular, a model with a large slope, as suggested by the recent results of PREX-2, predicts the existence of up to four disconnected regions of non-homogeneous matter above the zero magnetic field crust-core transition density.

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arXiv2021

Unveiling the nuclear matter EoS from neutron star properties: a supervised machine learning approach

We explore supervised machine learning methods in extracting the non-linear maps between neutron stars (NS) observables and the equation of state (EoS) of nuclear matter. Using a Taylor expansion around saturation density, we have generated a set of model independent EoS describing stellar matter constrained by nuclear matter parameters that are thermodynamically consistent, causal, and consistent with astrophysical observations. From this set, the full non-linear dependencies of the NS tidal deformability and radius on the nuclear matter parameters were learned using two distinct machine learning methods. Due to the high accuracy of the learned non-linear maps, we were able to analyze the impact of each nuclear matter parameter on the NS observables, identify dependencies on the EoS properties beyond linear correlations and predict which stars allow us to draw strong constraints.

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