1,720,998 research outputs found
Fastest spinning millisecond pulsars: indicators for quark matter in neutron stars?
No full textWe 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 diquark couplings in the quark matter phase, we obtain different hybrid star sequences with varying extension of the quark matter core, ensuring consistency with astrophysical constraints from mass, radius, and tidal deformability measurements. As a result, we demonstrate the impact of an increasing rotational frequency on the maximum gravitational mass, the central energy density of compact stars, the appearance of the quasiradial oscillations, and nonaxisymmetric instabilities. We demonstrate that for the most favorable parameter sets with a strong vector coupling, hybrid star configurations with a color superconducting quark matter core can describe the fastest spinning and heaviest galactic neutron star PSR J0952-0607, while it is out of reach for the purely hadronic hypernuclear star configuration. We also revise the previously proposed empirical relation between the Kepler frequency, gravitational mass, and radius of nonrotating neutron stars, obtained based on the assumption that all neutron stars, up to the heaviest, are hadronic. We show how the phase transition to quark matter alters this relation and, consequently, the constraints on the dense matter equation of state. Our findings reveal that incorporating the hybrid equation of state has significant implications for the constraints on the properties of strongly interacting matter and neutron stars, placing the upper limit on R1.4≤14.90 and R0.7<11.49 km (considering 716 Hz frequency limit from PSR J1748+2446ad) and R1.4≤11.90 km (for 1000 Hz).</p
Aproximação do campo médio relativístico para matéria nuclear com volume excluído de nucleões
No full textDissertação de Mestrado em Astrofísica e Instrumentação para o Espaço apresentada à Faculdade de Ciências e TecnologiaNeutron stars are among the most captivating astrophysical objects, not only because of their unique properties but also because they are the protagonists of some of the most enigmatic phenomena observed in the Universe. Their strong gravitational field and compactness make them the ideal laboratories for studying the properties of strongly interacting matter.This work aims to develop a relativistic mean-field hadronic model that accounts for the finite size of nucleons, which play a crucial role at densities found in the interiors of neutron stars. To achieve this, the relativistic mean-field Walecka model is generalized by incorporating a short-range repulsion between nucleons, derived from the virial expansion applied to a multi-component mixture of particles with a hard-core repulsion. This approach includes the surface tension contribution induced by interparticle interaction, extending the model's causality limit. Moreover, this model is formulated within the grand-canonical ensemble, allowing for direct generalization to an arbitrary number of hadronic species.The equation of state formulated for npe-matter at vanishing temperature is in full agreement with known properties of normal nuclear matter, i.e. the incompressibility factor, symmetry energy, and its slope at normal nuclear density. It agrees with the chiral effective field theory results, provides a high-quality description of the proton flow constraint, and is equally consistent with astrophysical data from neutron star observations and binary neutron star mergers, GW170817 and GW190425. The results of this work will further our understanding of neutron star properties and strongly interacting matter at high density.Estrelas de neutrões estão entre os objetos astrofísicos mais cativantes, não só pelas suas propriedades únicas, mas também porque são protagonistas de alguns dos fenômenos mais enigmáticos observados no Universo. Por serem objetos tão compactos e com um forte campo gravitacional, tornam-se os laboratórios ideais para estudar as propriedades da matéria que interage fortemente. Este trabalho tem como objetivo desenvolver um modelo hadrônico do campo médio relativístico que leva em conta o tamanho finito dos nucleões, que desempenham um papel crucial nas densidades encontradas no interior de estrelas de neutrões. Para isso, é usado uma generalização do modelo relativístico do campo médio de Walecka, no qual é incorporado uma repulsão de curto alcance entre nucleões, derivada da expansão de virial aplicada a uma mistura multicomponente de partículas com uma repulsão de núcleo duro. Esta abordagem inclui a contribuição de uma tensão superficial induzida pela interação entre as partículas, ampliando o limite de causalidade do modelo. Além disso, este modelo é formulado dentro do ensemble-grand canônico, permitindo a generalização direta para um número arbitrário de espécies hadrônicas.A equação de estado formulada para a matéria de npe no regime de temperatura igual a zero está em plena concordância com as propriedades conhecidas da matéria nuclear normal, como o fator de incompressibilidade, a energia de simetria e o seu declive na densidade nuclear de saturação. O modelo concorda com os resultados que advêm da teoria de campo efetivo quiral, fornece uma descrição de alta qualidade da experiência do fluxo de protões e é igualmente consistente com os dados astrofísicos de observações de estrelas de neutrões e fusões binárias de estrelas de neutrões, GW170817 e GW190425. Os resultados deste trabalho irão aprofundar a nossa compreensão das propriedades das estrelas de neutrões e da matéria que interage fortemente a altas densidades
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Density functional approach to quark matter with confinement and color superconductivity
Get PDFWe present a novel relativistic density-functional approach to modeling quark
matter with a mechanism to mimic confinement. The quasiparticle treatment of
quarks provides their suppression due to a large quark selfenergy already at
the mean-field level. We demonstrate that our approach is equivalent to a
chiral quark model with medium-dependent couplings. The dynamical restoration
of the chiral symmetry is ensured by construction of the density functional.
Supplemented with the vector repulsion and diquark pairing the model is applied
to construct a hybrid quark-hadron EoS of cold compact-star matter. We study
the connection of such a hybrid EoS with the stellar mass-radius relation and
tidal deformability. The model results are compared to various observational
constraints including the NICER radius measurement of PSR J0740+6620 and the
tidal deformability constraint from GW170817. The model is shown to be
consistent with the constraints, still allowing for further improvement by
adjusting the vector repulsion and diquark pairing couplings.Comment: Version accepted for publication in Physical Review D, 16 pages, 16
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Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
A new class of hybrid EoS with multiple critical endpoints for simulations of supernovae, neutron stars and their mergers
No full textWe introduce a family of equations of state (EoS) for hybrid neutron star
(NS) matter that is obtained by a two-zone parabolic interpolation between a
soft hadronic EoS at low densities and a stiff quark matter EoS with color
superconductivity at high densities within a finite region of baryonic chemical
potentials . We consider two scenarios corresponding
to a cross-over and a strong first-order transition between quark and hadron
phases considered at finite and zero temperatures. This allows us to analyze
the effects of finite entropy on the EoS and mass-radius relation of NS. We
demonstrate that the formation of a color superconducting state of quark matter
drives the evolution of matter in supernovae explosions under the condition of
entropy conservation to higher temperatures than in the case of deconfinement
to normal quark matter. Within the presented hybrid EoS scenario, regions of
the QCD phase diagram may be accessible to supernovae and NS mergers that can
be reached also in terrestrial experiments with relativistic heavy ion
collisions.Comment: 10 pages, 6 figure
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