Inferring the dense nuclear matter equation of state with neutron star tides

During the late stages of a neutron star binary inspiral finite-size effects come into play, with the tidal deformability of the supranuclear density matter leaving an imprint on the gravitational-wave signal. As demonstrated in the case of GW170817—the first direct detection of gravitational waves from a neutron star binary—this can lead to strong constraints on the neutron star equation of state. As detectors become more sensitive, effects which may have a smaller influence on the neutron star tidal deformability need to be taken into consideration. Dynamical effects, such as oscillation mode resonances triggered by the orbital motion, have been shown to contribute to the tidal deformability, especially close to the neutron star coalesence, where current detectors are most sensitive. We calculate the contribution of the various stellar oscillation modes to the tidal deformability and demonstrate the (anticipated) dominance of the fundamental mode. We show what the impact of the matter composition is on the tidal deformability, as well as the changes induced by more realistic additions to the problem, e.g. the presence of an elastic crust. Finally, based on this formulation, we develop a simple phenomenological model describing the effective tidal deformability of neutron stars and show that it provides a surprisingly accurate representation of the dynamical tide close to merger.

Transport Model Approach to Λ and Λ¯ Polarization in Heavy-Ion Collisions

This paper investigates the symmetry breaking between the polarizations of Λ and Λ¯ hyperons in relativistic collisions of heavy ions at intermediate and low energies. The microscopic transport model UrQMD is employed to study the thermal vorticity of hot and dense nuclear matter formed in non-central Au + Au collisions at center-of-mass energies 7.7≤sNN≤62.4 GeV. The whole volume of an expanding fireball is subdivided into small cubic cells. Then, we trace the final Λ and Λ¯ hyperons back to their last interaction point within a certain cell. Extracting the bulk parameters, such as energy density, net baryon density, and net strangeness of the hot and dense medium in the cell, one can obtain the cell temperature and the chemical potentials at the time of the hyperon emission. To do this, the extracted characteristics have to be fitted to the statistical model (SM) of ideal hadron gas. After that, the vorticity of nuclear matter and polarization of both hyperons are calculated. We found that the polarization of both Λ and Λ¯ increases with decreasing energy of heavy-ion collisions. The stronger polarization of Λ¯ is explained by (i) the slightly different freeze-out conditions of both hyperons and (ii) the different space–time distributions of Λ and Λ¯.

The axion-baryon coupling in SU(3) heavy baryon chiral perturbation theory

In the past, the axion-nucleon coupling has been calculated in the framework of SU(2) heavy baryon chiral perturbation theory up to third order in the chiral power counting. Here, we extend these earlier studies to the case of heavy baryon chiral perturbation theory with SU(3) flavor symmetry and derive the axion coupling to the full SU(3) baryon octet, showing that the axion also significantly couples to hyperons. As studies on dense nuclear matter suggest the possible existence of hyperons in stellar objects such as neutron stars, our results should have phenomenological implications related to the so-called axion window.

Neutron Stars and the Nuclear Matter Equation of State

Neutron stars provide a window into the properties of dense nuclear matter. Several recent observational and theoretical developments provide powerful constraints on their structure and internal composition. Among these are the first observed binary neutron star merger, GW170817, whose gravitational radiation was accompanied by electromagnetic radiation from a short γ-ray burst and an optical afterglow believed to be due to the radioactive decay of newly minted heavy r-process nuclei. These observations give important constraints on the radii of typical neutron stars and on the upper limit to the neutron star maximum mass and complement recent pulsar observations that established a lower limit. Pulse-profile observations by the Neutron Star Interior Composition Explorer (NICER) X-ray telescope provide an independent, consistent measure of the neutron star radius. Theoretical many-body studies of neutron matter reinforce these estimates of neutron star radii. Studies using parameterized dense matter equations of state (EOSs) reveal several EOS-independent relations connecting global neutron star properties. Expected final online publication date for the Annual Review of Nuclear and Particle Science, Volume 71 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Topology change, emergent symmetries and compact star matter

Topology effects have being extensively studied and confirmed in strongly correlated condensed matter physics. In the limit of large number of colors, baryons can be regarded as topological objects—skyrmions—and the baryonic matter can be regarded as a skyrmion matter. We review in this paper the generalized effective field theory for dense compact-star matter constructed with the robust inputs obtained from the skyrmion approach to dense nuclear matter, relying on possible “emergent” scale and local flavor symmetries at high density. All nuclear matter properties from the saturation density n0 up to several times n0 can be fairly well described. A uniquely novel—and unorthdox—feature of this theory is the precocious appearance of the pseudo-conformal sound velocity $v^{2}_{s}/c^{2} \approx 1/3$ v s 2 / c 2 ≈ 1 / 3 , with the non-vanishing trace of the energy momentum tensor of the system. The topology change encoded in the density scaling of low energy constants is interpreted as the quark-hadron continuity in the sense of Cheshire Cat Principle (CCP) at density $\gtrsim 2n_{0}$ ≳ 2 n 0 in accessing massive compact stars. We confront the approach with the data from GW170817 and GW190425.

First Multimessenger Observations of a Neutron Star Merger

We describe the first observations of the same celestial object with gravitational waves and light. ▪ GW170817 was the first detection of a neutron star merger with gravitational waves. ▪ The detection of a spatially coincident weak burst of gamma-rays (GRB 170817A) 1.7 s after the merger constituted the first electromagnetic detection of a gravitational wave source and established a connection between at least some cosmic short gamma-ray bursts (SGRBs) and binary neutron star mergers. ▪ A fast-evolving optical and near-infrared transient (AT 2017gfo) associated with the event can be interpreted as resulting from the ejection of ∼0.05 M⊙ of material enriched in r-process elements, finally establishing binary neutron star mergers as at least one source of r-process nucleosynthesis. ▪ Radio and X-ray observations revealed a long-rising source that peaked ∼[Formula: see text] after the merger. Combined with the apparent superluminal motion of the associated very long baseline interferometry source, these observations show that the merger produced a relativistic structured jet whose core was oriented ≈20 deg from the line of sight and with properties similar to SGRBs. The jet structure likely results from interaction between the jet and the merger ejecta. ▪ The electromagnetic and gravitational wave information can be combined to produce constraints on the expansion rate of the Universe and the equation of state of dense nuclear matter. These multimessenger endeavors will be a major emphasis for future work. Expected final online publication date for the Annual Review of Astronomy and Astrophysics, Volume 59 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Extracting Hypernuclear Properties from the (e, e′K+) Cross Section

Experimental studies of hypernuclear dynamics, besides being essential for the understanding of strong interactions in the strange sector, have important astrophysical implications. The observation of neutron stars with masses exceeding two solar masses poses a serious challenge to the models of hyperon dynamics in dense nuclear matter, many of which predict a maximum mass incompatible with the data. In this paper, it is argued that valuable new insight can be gained from the forthcoming extension of the experimental studies of kaon electro production from nuclei to include the 208Pb(e,e′K+)Λ208Tl process. A comprehensive framework for the description of kaon electro production, based on factorization of the nuclear cross section and the formalism of the nuclear many-body theory, is outlined. This approach highlights the connection between the kaon production and proton knockout reactions, which will allow us to exploit the available 208Pb(e,e′p)207Tl data to achieve a largely model-independent analysis of the measured cross section.